The Human Brain Book: An Illustrated Guide to Its Structure, Function, and Disorders

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Smith′s Textbook of Endourology [2 Volume Set] Wiley-Blackwell Arthur D. Smith, Glenn Preminger, Gopal H. Badlani, Louis R. Kavoussi Year: 2019 Language: english File: PDF, 70.19 MB

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Non-archimedean tame topology and stably dominated types Princeton University Press Ehud Hrushovski, François Loeser Year: 2016 Language: english File: PDF, 10.14 MB

HUMAN
BRAIN

T H E

B O O K

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R I TA  C A RT E R
S U S A N  A L D R I D G E  

M A R T Y N  PA G E 
S T E V E  PA R K E R

CONSULTANTS    Professor Chris Fr ith,  
Professor Uta Fr ith,  and Dr.  Melanie Shulman

HUMAN
BRAIN

T H E

B O O K

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NO ORDINARY ORGAN 

INVESTIGATING THE BRAIN

LANDMARKS IN NEUROSCIENCE

SCANNING THE BRAIN

A JOURNEY THROUGH THE BRAIN

THE BRAIN AND THE BODY

BRAIN FUNCTIONS

THE NERVOUS SYSTEM

THE BRAIN AND THE 
   NERVOUS SYSTEM

BRAIN SIZE, ENERGY USE, 
   AND PROTECTION

EVOLUTION

BRAIN ANATOMY 

BRAIN STRUCTURES

BRAIN ZONES AND PARTITIONS

THE NUCLEI OF THE BRAIN

THE THALAMUS, HYPOTHALAMUS,  
   AND PITUITARY GLAND

THE BRAIN STEM AND CEREBELLUM

THE LIMBIC SYSTEM

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THE CEREBRAL CORTEX

BRAIN CELLS

NERVE IMPULSES

BRAIN MAPPING  
   AND SIMULATION 

THE SENSES

HOW WE SENSE THE WORLD

THE EYE

THE VISUAL CORTEX

VISUAL PATHWAYS

VISUAL PERCEPTION

SEEING

THE EAR

MAKING SENSE OF SOUND

HEARING

SMELL

PERCEIVING SMELL

TASTE

TOUCH

THE SIXTH SENSE

PAIN SIGNALS

EXPERIENCING PAIN

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CONTENTS

THIRD EDITION

DK DELHI

SENIOR EDITOR   Rupa Rao

ART EDITOR   Sonakshi Singh

MANAGING EDITOR   Rohan Sinha

MANAGING ART EDITOR   Sudakshina Basu

DTP DESIGNER   Bimlesh Tiwary

PICTURE RESEARCHER   Sumedha Chopra

PICTURE RESEARCH MANAGER   Taiyaba Khatoon

PREPRODUCTION MANAGER   Balwant Singh

PRODUCTION MANAGER   Pankaj Sharma

DK LONDON

SENIOR EDITOR   Peter Frances

PROJECT EDITOR   Ruth O’Rourke-Jones

PROJECT ART EDITOR   Francis Wong

US EDITOR   Jennette ElNaggar

US EXECUTIVE EDITOR   Lori Cates Hand

MANAGING EDITOR   Angeles Gavira Guerrero

MANAGING ART EDITOR   Michael Duffy

JACKET DESIGN DEVELOPMENT  

MANAGER   Sophia MTT

PRODUCER, PREPRODUCTION   Gillian Reid

SENIOR PRODUCER   Meskerem Berhane

ASSOCIATE PUBLISHER   Liz Wheeler

ART DIRECTOR   Karen Self

DESIGN DIRECTOR   Phil Ormerod

PUBLISHING DIRECTOR   Jonathan Metcalf

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MOVEMENT AND CONTROL

REGULATION

THE NEUROENDOCRINE  
   SYSTEM

PLANNING A MOVEMENT

EXECUTING A MOVEMENT

UNCONSCIOUS ACTION

MIRROR NEURONS

EMOTIONS AND FEELINGS

THE EMOTIONAL BRAIN

CONSCIOUS EMOTION

DESIRE AND REWARD

THE SOCIAL BRAIN 

SEX, LOVE, AND  
   SURVIVAL

EXPRESSION

THE SELF AND OTHERS

THE MORAL BRAIN

LANGUAGE AND  
COMMUNICATION

GESTURES AND BODY  
   LANGUAGE

THE ORIGINS OF  
   LANGUAGE

THE LANGUAGE AREAS

A CONVERSATION

READING AND WRITING

MEMORY

THE PRINCIPLES OF MEMORY

THE MEMORY WEB

LAYING DOWN A MEMORY

RECALL AND RECOGNITION

UNUSUAL MEMORY

THINKING

INTELLIGENCE

CREATIVITY AND HUMOR 

BELIEF AND SUPERSTITION

ILLUSIONS

CONSCIOUSNESS 

WHAT IS CONSCIOUSNESS?

LOCATING CONSCIOUSNESS

ATTENTION AND  
   CONSCIOUSNESS

THE IDLING BRAIN

ALTERING CONSCIOUSNESS

SLEEP AND DREAMS

TIME

THE SELF AND  
   CONSCIOUSNESS

THE INDIVIDUAL BRAIN

NATURE AND NURTURE

INFLUENCING THE BRAIN

PERSONALITY

BRAIN MONITORING  
   AND STIMULATION

STRANGE BRAINS

DEVELOPMENT AND AGING

THE INFANT BRAIN

CHILDHOOD AND ADOLESCENCE

THE ADULT BRAIN

THE AGING BRAIN

THE BRAIN OF THE FUTURE

DISEASES AND DISORDERS

THE DISORDERED BRAIN

DIRECTORY OF DISORDERS 

GLOSSARY

INDEX

ACKNOWLEDGMENTS

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SENIOR EDITOR   Peter Frances

SENIOR ART EDITOR   Maxine Lea

PROJECT EDITORS   Nathan Joyce,  

Ruth O’Rourke, Miezan van Zyl

EDITORS   Salima Hirani, Katie John,  

Rebecca Warren

PROJECT ART EDITORS   Alison Gardner,  

Siân Thomas, Francis Wong

DESIGNER   Riccie Janus 

EDITORIAL ASSISTANT   Elizabeth Munsey

INDEXER   Hilary Bird

PROOFREADER   Polly Boyd

PICTURE RESEARCHER   Liz Moore

JACKET DESIGNER   Duncan Turner

SENIOR PRODUCTION CONTROLLER  

Inderjit Bhullar

PRODUCTION EDITOR   Tony Phipps

CREATIVE TECHNICAL SUPPORT  

Adam Brackenbury, John Goldsmid

MANAGING EDITOR   Sarah Larter

SENIOR MANAGING ART EDITOR   Phil Ormerod

PUBLISHING MANAGER   Liz Wheeler

REFERENCE PUBLISHER   Jonathan Metcalf

ART DIRECTOR   Bryn Walls

ILLUSTRATORS   Medi-Mation, Peter Bull Art Studio

This American Edition, 2019  
First American Edition, 2009  

Published in the United States by DK Publishing  
345 Hudson Street, New York, New York 10014

Copyright © 2009, 2014, 2019 Dorling Kindersley Limited  
DK, a Division of Penguin Random House LLC

 
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All rights reserved.  
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above, no part of this publication may be reproduced, stored  
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any form, or by any means (electronic, mechanical, 

FIRST EDITION photocopying, recording, or otherwise), without the  
prior written permission of the copyright owner.  

Published in Great Britain by Dorling Kindersley Limited.

A catalog record for this book is available from the  
Library of Congress. 

ISBN 978-1-4654-7954-9

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The Human Brain Book provides information on a wide 
range of medical topics, and every effort has been made to 
ensure that the information in this book is accurate. The 
book is not a substitute for medical advice, however, and 
you are advised always to consult a doctor or other health 

professional on personal health matters.

Printed in China

A WORLD OF IDEAS: 
SEE ALL THERE IS TO KNOW

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NO ORDINARY ORGAN
The human brain is like nothing else. As organs go, it is not especially 
prepossessing—3lb (1.4kg) or so of rounded, corrugated flesh with  
a consistency somewhere between jelly and cold butter. It doesn’t 
expand and shrink like the lungs, pump like the heart, or secrete 
visible material like the bladder. If you sliced off the top of someone’s 
head and peered inside, you wouldn’t see much happening at all.  

SEAT OF CONSCIOUSNESS
Given this, it is perhaps not surprising that for centuries the contents 
of our skulls were regarded as relatively unimportant. When they 
mummified their dead, the ancient Egyptians scooped out the brains 
and threw them away, yet carefully preserved the heart. The Ancient 
Greek philospher, Aristotle, thought the brain was a radiator for 
cooling the blood. René Descartes, the French scientist, gave it a 
little more respect, concluding that it was a sort of antenna by which 
the spirit might commune with the body. It is only now that the full 
wonder of the brain is being realized.
 The most basic function of the brain is to keep the rest of the 
body alive. Among your brain’s 100 billion neurons, some regulate 
your breathing, heartbeat, and blood pressure and others control 
hunger, thirst, sex drive, and sleep cycle. In addition to this, the brain 

generates the emotions, perceptions, and thoughts that guide your 
behavior. Then it directs and executes your actions. Finally, it is 
responsible for the conscious awareness of the mind itself.

THE DYNAMIC BRAIN
Until about 100 years ago, the only evidence that brain and mind 
were connected was obtained from “natural experiments”—accidents 
in which head injuries created aberrations in their victims’ behavior. 
Dedicated physicians mapped out areas of the cerebral landscape by 
observing the subjects of such experiments while they were alive— 
then matching their deficits to the damaged areas of their brains. It 
was slow work because the scientists had to wait for their subjects to 
die before they could look at the physiological evidence. As a result, 
until the early 20th century, all that was known about the physical 
basis of the mind could have been contained in a single volume.
 Since then, scientific and technological advances have fueled a 
neuroscientific revolution. Powerful microscopes made it possible  
to look in detail at the brain’s intricate anatomy. A growing 
understanding of electricity allowed the dynamics of the brain to  
be recognized and then, with the advent of electroencephalography 
(EEG), to be observed and measured. Finally, the arrival of 

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functional brain imaging machines allowed scientists to look inside 
the living brain and see its mechanisms at work. In the last 20 years, 
positron emission tomography (PET), functional magnetic resonance 
imaging (fMRI), and, most recently, magnetic encephalography 
(MEG) have among them produced an ever more detailed map of  
the brain’s functions.

LIMITLESS LANDSCAPE
Today we can point to the circuitry that keeps our vital processes 
going, the cells that produce our neurotransmitters, the synapses 
where signals leap from cell to cell, and the nerve fibers that convey 
pain or move our limbs. We know how our sense organs turn light 
rays and sounds waves into electrical signals, and we can trace the 
routes they follow to the specialized areas of cortex that respond to 
them. We know that such stimuli are weighed, valued, and turned 
into emotions by the amygdala—a tiny nugget of tissue that punches 
well above its weight. We can see the hippocampus retrieve a memory, 
or watch the prefrontal cortex make a moral judgment. We can 
recognize the nerve patterns associated with amusement, empathy—
even the thrill of schadenfreude at the sight of an adversary suffering 
defeat. More than just a map, the picture emerging from imaging 

studies reveals the brain to be an astonishingly complex, sensitive 
system in which each part affects almost every other. “High level” 
cognition performed by the frontal lobes, for instance, feeds back to 
affect sensory experience—so what we see when we look at an object 
is shaped by expectation as well as by the effect of light hitting the 
retina. Conversely, the brain’s most sophisticated products can 
depend on its lowliest mechanisms. Intellectual judgments, for 
example, are driven by the body reactions that we feel as emotions, 
and consciousness can be snuffed out by damage to the humble 
brainstem. To confuse things further, the system doesn’t stop at the 
neck but extends to the tips of your toes. Some would argue it even 
goes beyond—to encompass other minds with which it interacts. 
 Neuroscientific investigation of the brain is very much a work  
in progress and no one knows what the finished picture will look  
like. It may be that the brain is so complicated that it can never 
understand itself entirely. So this book cannot be taken as a full 
description of the brain. It is a single view, from bottom to top,  
of the human brain as we know it today—in all its beauty and 
complexity. Be amazed.

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USING RATS
The brains of rats 
are very similar  
to human brains.  
Until imaging 
techniques were 
developed, the 
only way scientists 
were able to look 
directly at brain 
tissue was by using 
the brains of rats 
and other non- 
human animals.

IN
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THE BRAIN IS THE LAST OF THE HUMAN ORGANS TO GIVE UP ITS SECRETS. FOR A LONG TIME, PEOPLE 
WERE NOT EVEN ABLE TO UNDERSTAND WHAT THE BRAIN IS FOR. THE DISCOVERY OF ITS ANATOMY, 
FUNCTIONS, AND PROCESSES HAS BEEN A LONG AND SLOW JOURNEY ACROSS THE MILLENNIA, AS 
HUMAN KNOWLEDGE ABOUT THIS MYSTERIOUS ORGAN HAS DEVELOPED AND ACCUMULATED.

INVESTIGATING THE BRAIN

EXPLORING THE BRAIN
The brain is particularly difficult to investigate because its structures 
are minute and its processes cannot be seen with the naked eye. The 
problem is compounded by the fact that its most interesting product, 
consciousness, does not feel like a physical process, so there was no 
obvious reason for our distant ancestors to associate it with the brain. 
Nevertheless, over the centuries, philosophers and physicians built 
up an understanding of the brain and, in the last 25 years with the 
advent of brain-imaging techniques, neuroscientists have created  
a detailed map of what was once an entirely mysterious territory.

1700 BCE
Egyptian papyrus  

gives a careful  
description of the  

brain, but Egyptians do  
not rate this organ highly; 

unlike other organs, it is 
removed and discarded 
before mummification, 

suggesting that it was not 
considered to be of any 

use in future incarnations.

2500 BCE 
Trepanation (boring 

holes into the skull) is a 
common surgical 

procedure across many 
cultures, possibly used 

for relieving brain 
disorders such as 

epilepsy, or for ritual  
or spiritual reasons.

450 BCE 
Early Greeks begin 

to recognize the 
brain as the seat of  

human sensation. 

387 BCE 
The Greek 

philosopher Plato 
teaches at Athens; 

he believes the 
brain is the seat of 
mental processes.

335 BCE 
Greek philosopher 

Aristotle restates the 
ancient belief that 

the heart is the 
superior organ; the 
brain, a radiator to 

stop the body from 
overheating.

170 BCE
Roman physician Galen theorizes 
that human moods and 
dispositions are due to the four 
“humors” (liquids that are held in 
the brain’s ventricles). The idea 
persists for more than 1,000 years.  
Galen’s anatomical descriptions, 
used by generations of physicians, 
were based mainly on work  
on monkeys and pigs.

1543 
Andreas Vesalius,  
a European physician, 
publishes the first 
“modern” anatomy,  
with detailed drawings 
of the human brain.

1649
French philosopher René 

Descartes describes the 
brain as a hydraulic system 

that controls behavior. 
“Higher” mental functions 

are generated by a 
spiritual entity, however, 
which interacts with the 

body via the pineal gland.

1664    
Oxford physiologist Thomas 
Willis publishes the first brain 
atlas, locating various 
functions in separate  
brain “modules.”

1774  
German physician 
Franz Anton Mesmer 
introduces “animal 
magnetism,” later 
called hypnosis.

1791
Luigi Galvani, an 
Italian physicist, 
discovers the 
electrical basis  
of nervous activity 
by making frogs’  
legs twitch.

4000 BCE 
Early Sumerian 
writing notes the 
euphoric effect 
of poppy seeds.

DRAWING THE BRAIN BRAIN ATLAS

PAPYRUS

PLATO 

LUIGI GALVANI

GALEN AT WORK

RENÉ DESCARTES

ARISTOTLE

TREPANNING

1848
Phineas Gage 

has his brain 
pierced by  
an iron rod  
(see p.141).

1849
German physicist 

Hermann von 
Helmholtz measures 

the speed of nerve 
conduction and  

subsequently 
develops the idea 

that perception 
depends upon  
“unconscious 

inferences.”

8

4000 BCE 1500 1600 1700 18003000 BCE 2000 BCE 1000 BCE

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IN
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Circa 1900 
Sigmund Freud abandons an  
early career in neurology to study 
psychodynamics. The success of 
Freudian psychoanalysis eclipsed 
physiological psychiatry for half  
a century.

1906 
Santiago 
Ramón y Cajal 
describes how 
nerve cells 
communicate.

1906
Alois  

Alzheimer 
describes 
presenile 

degeneration 
(see p.231).

1909 
Korbinian Brodmann describes 
52 discrete cortical areas based 
on neural structure. These areas 
are still used today (see p.67).

1924   
The first electroencephalograms 
are produced by Hans Berger.

1934 
Portuguese neurologist Egas Moniz 
carries out the first leucotomy operations 
(later known as lobotomies, see p.11).  
He also invented angiography, one of  
the first techniques that allowed scientists 
to make images of the brain.

1957 
W. Penfield and  
T. Rasmussen devise  
a motor and sensory 
homunculus  
(see pp.10 and 103).

1970–80 
Brain scanning 
is developed:  

PET, SPECT, 
MRI, and MEG 

all emerge 
during this 

decade.

1973
Timothy Bliss 
and Terje Lomo 
describe 
long-term 
potentiation  
(see p.158).

1981 
Roger Wolcott Sperry is 
awarded the Nobel Prize for his 
work on the different functions 
of the two brain hemispheres 
(see pp.11 and 205).

1983 
Benjamin Libet writes on  
the timing of conscious 
volition (see p.11).

1862–74  
Broca and Wernicke 
(see p.10) discover the 
two main language 
areas of the brain.

1850  
Franz Joseph Gall 
founds phrenology 
(see p.10), which 
attributes different 
personality traits  
to specific areas  
of the head.

NERVE CELLS  
IN RODENT  
HIPPOCAMPUS

SIGMUND FREUD

EGAS MONIZ

ELECTROENCEPHALOGRAPHY

NERVE CELLS EARLY MAGNETIC IMAGINGCORTICAL MAP

1889
Santiago Ramón y Cajal proposes 

that nerve cells are independent  
elements and the basic units of 

the brain in The Neuron Doctrine. 
He wins the Nobel Prize in 1906.

1953
Brenda Milner 
describes 
patient HM 
(see p.157), 
who suffers 
memory  
loss after 
hippocampal 
surgery.

1919 
Irish neurologist 
Gordon Morgan 
Holmes  
localizes vision  
to the striate 
cortex (the 
primary visual 
cortex).

1859
Charles 
Darwin 

publishes  
On the 

Origin of 
Species.

1874
Carl Wernicke 
publishes on 
aphasia (language 
disorders after  
brain damage).

1914
British physiologist 
Henry Hallett Dale 
isolates acetylcholine, 
the first  of the neuro- 
transmitters (see p.73) to 
be discovered. He wins 
the Nobel Prize in 1936.

1991
Mirror neurons are 

discovered by 
Giacomo Rizzolatti  

in Parma (see 
pp.11 and 

122–23).

2013
The United States and European 

Union start human brain simulation 
projects. The Connectome, a 
global cooperative endeavor, 

delivers its first charts of the 
connections between neurons.

Scientists were unable to find out much about the 
workings of the brain until relatively recently. The only 
way they were able to match functions such as sight, 
emotion, or speech to the locations in the brain in which 
they are controlled was to find a person in whom a 
faculty was disturbed due to injury, and then wait until 
they were dead in order to look at the location and 
extent of the brain damage. Otherwise, scientists could 
only guess at what was happening to the brain by 
observing people’s behavior. Today, modern imaging 
techniques such as functional MRI and EEG (see p.12) 
allow neuroscientists to see the electrical activity in the 
brain as a person carries out various tasks or thought 
processes. This allows them to link types of actions, 
emotions, and so on, to specific types of activity in the 
brain. The freedom to observe the brain that imaging 
techniques have afforded has allowed for an explosion 
of knowledge within neuroscience, and has deepened 
our understanding of the brain and how it works.

THE ADVENT OF IMAGING TECHNIQUES

MAGNETIC RESONANCE IMAGING
Brain scans can reveal damaged tissue—
the red area in the MRI scan above 
indicates damage caused by a stroke.

ELECTRODES
Neural activity can 
be measured by 
attaching electrodes 
to the scalp. These 
pick up electrical 
activity in the brain 
and transform it into 
a digital record.

Electrode 
“cap”

1873
Italian scientist 
Camillo Golgi 

publishes the silver 
nitrate method, 

making it possible  
to see nerves in  

their entirety. He 
wins the Nobel  

Prize in 1906.

9

1900 2000

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MOST OF THE KNOWLEDGE WE HAVE ABOUT THE BRAIN HAS BEEN GATHERED BY SLOW, PAINSTAKING 
RESEARCH INVOLVING LARGE TEAMS OF PEOPLE. HOWEVER, OCCASIONALLY THE HISTORY OF 
NEUROSCIENCE HAS BEEN PUNCTUATED BY DRAMATIC DISCOVERIES OR IDEAS, OFTEN ARISING  
FROM THE WORK OF A SINGLE SCIENTIST. SOME OF THESE SUBSEQUENTLY PROVED TO BE  
VALUABLE BREAKTHROUGHS WHILE OTHERS, ALTHOUGH INFLUENTIAL, PROVED TO BE DEAD ENDS.

LANDMARKS IN NEUROSCIENCE

PHRENOLOGY
Franz Joseph Gall
Gall thought that personality could be read by feeling  
the contours of the skull. He theorized that various 
faculties were localized in the brain and that the strongest 
were correspondingly large, making the skull bulge 
measurably. It was hugely popular in nineteenth-century 
America and Europe—nearly every town had a phrenology 
institute. Although nonsense, Gall’s idea that brain functions 
are localized has turned out 
to be largely true. Imaging 
research aimed at locating 
brain functions is often 
called “modern phrenology.”

THE MAN WHO LOST HIMSELF
Phineas Gage
This polite, well-liked American railroad foreman 
changed dramatically, becoming “grossly 
profane,” after an accident destroyed part  
of his brain (see p.141). His 
case was the first to show 
that faculties such as social 
and moral judgment can be 
localized to the frontal lobes.

FATEFUL INJURY
This reconstruction of Phineas Gage’s 
skull shows how an iron rod damaged 
the frontal lobes of his brain.

PHRENOLOGY HEAD
Models such as this claimed  
to show the bulges on the  
skull that revealed a person’s 
character. Categories included 
“blandness” or “benevolence.”

LANGUAGE AREAS 
Broca and Wernicke
In 1861, French physician Paul Broca described a patient who he named “Tan,” as it  
was the only word “Tan” could say. When Tan died, Broca examined his brain and found 
damage to part of the left frontal cortex. This part of the brain became “Broca’s Area” (see 
p.148). In 1876, German neurologist Carl Wernicke found that damage to a different part 
of the brain (which became known as “Wernicke’s Area”) also caused language problems. 
These two scientists were the first to clearly define functional areas of the brain.CARL WERNICKEPAUL BROCA

MAPPING THE BRAIN
Wilder Penfield
The first detailed maps of human brain 
function were made by Canadian brain 
surgeon Wilder Penfield. He worked with 
patients undergoing surgery to control 
epilepsy. While the brain was exposed, 
and the patient conscious, Penfield 
probed the cortex with an electrode  
and noted the responses of the patient  
as he touched each part. 
Penfield’s work was  
the first to reveal the 
role of the temporal 
lobe in recall and  
map the areas of the 
cortex that control 
movement and provide 
bodily sensations.

EARLY BRAIN IMPLANT
José Delgado
Spanish neurologist Dr. José Delgado invented 
a brain implant that could be remotely 
controlled by radio waves. He found that 
animal and human behavior could be 
controlled by pressing a button. In a famous 
experiment, conducted in 1964, Delgado faced 
a charging bull, bringing it to a halt at his feet 
by activating the implant in its brain. In another, 

he put a device in 
the brain of a chimp 
that was bullying its 
mate. He put the 
control in the cage 
where the victim 
chimp used it to 
“turn off” the bully’s 
bad behavior.

MODERN MAPPING
Today advanced imaging (see 
above) allows neural activity 
to be matched to mental 
tasks. However, much of the 
basic map was established by 
Penfield half a century earlier.

CANADIAN 
STAMPDELGADO AND THE BULL

1
0

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CONSCIOUS DECISIONS
Benjamin Libet
A series of ingenious experiments by US 
neuroscientist Benjamin Libet (see p.191) 
in the early 1980s demonstrated that what 
we think are conscious “decisions” to act 
are actually just recognition of what the 
unconscious brain is already doing. Libet’s 
experiments have profound philosophical 
implications because, on the face of it,  
the results suggest that we do not have a 
conscious choice about what we do, and 
therefore cannot consider ourselves to 
have free will.

SPLIT-BRAIN 
EXPERIMENTS
Roger Sperry
Neurobiologist Roger Sperry 
conducted the split-brain 
experiments (see p.204) on 
people whose brain hemispheres 
were surgically separated in the 
course of treatment for epilepsy. 
They showed that, under certain 
conditions, each hemisphere 
could hold different thoughts 
and intentions. This raised the 
profound question of whether  
a person has  
a single “self.”

MAKING MEMORIES
Henry G. Molaison
In 1953, aged 27, “HM” underwent an operation in the US, to 
stem severe epilepsy. The surgeons, then unaware of the functions 
of the hippocampus, took out a large area of that part of his brain 

(see p.159). When he came round, 
he was unable to lay down new 
memories and remained so for the 
rest of his life. The tragic accident 
demonstrated the crucial role of the 
hippocampus in recall.

FROZEN IN TIME
Henry G. Molaison—generally known only 
as “HM” —was one of the most studied 
patients in the history of modern medicine.

MIRROR NEURONS 
Mirror neurons (see pp.122–23) were discovered in 1991—by 
accident. A group of researchers in Italy, led by Giacomo Rizzolatti, 
were monitoring neural activity in the brains of monkeys as they 
made reaching movements. One day a researcher inadvertently 
mimicked the monkey’s movement while it was watching, and 
found that the neural activity in the monkey’s brain that sparked 
up in response to the sight was identical to the activity that 
occurred when the monkey 
made the action itself. Mirror 
neurons are thought by some to 
be the basis of theory of mind, 
mimicry, and empathy.

MIMICKING MACAQUE
Mirror neurons produce automatic 
mimicry by producing a similar state  
in an observer’s brain to the state of 
the person they are watching.

INVESTIGATING  
FREE WILL

LOBOTOMY
The first lobotomies were performed in the 1890s, but they only took off in the 
1930s when the Portuguese neurosurgeon Egas Moniz found that cutting  
the nerves from the frontal cortex to the thalamus relieved psychotic symptoms 
in some patients. Moniz’s work was picked up by US surgeon Walter Freeman, 
who invented the “ice pick lobotomy.” From 1936 until the 1950s, he advocated 
lobotomy to cure for a range of problems, and 40,000 to 50,000 patients 

were lobotomized. The operation was overused 
and is now thought abhorrent. However, in many 
cases it eased suffering: a follow-up of patients 
in the UK found 41 percent were “recovered”  
or “greatly improved,” 28 percent “minimally 
improved,” 25 percent had “no change,” 4 
percent had died, and 2 percent were worse off.

“ICE PICK” LOBOTOMY
Walter Freeman, above, found he could perform  
a lobotomy under local anesthetic by hammering  
an ice pick above each eye of a patient and swishing  
the device back and forth like a windshield wiper.ICE PICK

TREPANATION
The practice of drilling holes in the head has been 
used since prehistoric times as a treatment for a vast 
array of illnesses. The modern equivalent, craniotomy, 
is carried out to relieve pressure within the skull.

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BRAIN IMAGING TECHNIQUES CAN BE DIVIDED INTO TWO DIFFERENT  
TYPES: ANATOMICAL IMAGING, WHICH GIVES INFORMATION ABOUT THE 
STRUCTURE OF THE BRAIN, AND FUNCTIONAL SCANNING, WHICH ALLOWS 
RESEARCHERS TO SEE HOW THE BRAIN WORKS. USED TOGETHER, THESE 
TECHNIQUES HAVE REVOLUTIONIZED NEUROSCIENCE.

SCANNING THE BRAIN

A WINDOW ON THE BRAIN
The structure of the brain is well known, but until 
recently the way it created thoughts, emotions, and 
perceptions could only be guessed at. Imaging 
technology has now made it possible to look inside  
a living brain and see it at work. The brain works  

by generating tiny 
electrical charges. 
Functional imaging 
reveals which areas are 
most active. This may 
be done by measuring 
electrical activity 
directly (EEG), picking 
up magnetic fields 
created by electrical 
activity (MEG), or 
measuring metabolic 
side effects such as 
alterations in glucose 
absorption (PET) and 
blood flow (fMRI).

PET SCANNER
Positron emission tomography 
(PET) scanners detect signals from 
radioactive markers in tissues to 
show activity in the brain.

BRAIN WAVES
Electroencephalo-
graphs (EEGs) show 
electrical activity 
caused by nerve 
cells firing. They 
record distinct 
“brain waves,” 
which reflect the 
speed of firing in 
different states  
of mind.

FUNCTION
The brain is composed of modules that are  
specialized to do specific things. Functional brain  
imaging is largely about identifying which ones are  
most concerned with doing what. This has allowed 
neuroscientists to build a detailed map of brain functions. 
We now know where perceptions, language, memory, 
emotion, and movement occur. By showing how various 
functions work together, imaging also gives us a glimpse into 
some of the most sophisticated aspects of human psychology. 
For example, observing a person’s brain making a decision, 
we see that apparently rational decisions are driven by the 
emotional brain. Imaging the brains of master chess players 
shows why expertise depends on practice. Watching the 
brain of a person seeing a frightened face shows that 
emotion is contagious.

STRUCTURAL DETAILS
These CT images show different tissues in detail. The image  
on the left shows the cerebellum and eyeballs in red, the bones 
in blue and green, and the sinuses and ear cavities in bright 
yellow. The image on the right shows a healthy brain (front  
at bottom). The black areas are the fluid-filled ventricles.

3-D BRAIN
CT allows pictures of 
brains to be displayed 
in three dimensions, 
and “sliced” to reveal 
the inner workings. 
Here, the front right 
quarter of the brain’s 
coverings and surface 
are cut away to reveal 
the tissues beneath.

ANATOMY
The brain looks very different according to how it is viewed. 
Computed tomography (CT) imaging combines the use of a 
computer and fine X-rays to produce multiple “slices” of the 
body. It allows you to see normally obscured body tissues,  
such as the inside of the brain, from any angle or level, with  
the delicate inner structures thrown into clear relief. Artificial 
coloring of the areas further distinguishes one part from 
another. CT scans are purely structural: they show the form of 
the organ but not how it works. They are very good at showing 
contrast between soft tissues and bone, and are therefore useful 
in diagnosing tumors and blood clots.

PET SCANS
These scans 
involve injecting a 
volunteer with a 
radioactive marker 
that attaches to 
glucose in the 
brain. Areas of 
high activity (red) 
attract glucose for 
fuel. The marker 
dye shows which 
parts of the brain 
are firing. 

REAL-TIME ACTIVITY
Magnetoencephalography (MEG) picks up 
magnetic traces of brain activity. It is poor 
at showing where activity occurs, but good 
at pinpointing timing. Here, a brain plans a 
finger movement, then 40 milliseconds later 
its activity shifts as the movement is made.

MOVEMENT

BEFORE 
MOVEMENT

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Sensory 
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Motor 
area 

Three-
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Computer-
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Inner tissue

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COMBINED IMAGING
Each type of imaging has its advantages. MRI is good on  
detail, for example, but is too slow to chart fast-moving  
events. EEG and MEG are fast but are not as good at 
pinpointing location. To get scans that show both fast  
processes and location, researchers use two or more methods  
to produce a combined image. Here (right), for example, 
high-resolution MRI, taking about 15 minutes to acquire, is 
combined with a low-resolution fMRI, which takes seconds to 
produce and shows the location of activity in the brain areas 
used in hearing language. The 
areas shift during a task like 
this that involves many 
aspects, and they have to  
work fast and in concert.  
The areas used in a task  
vary from person to person,  
so studies often combine  
data from volunteers to  
give an average.

STUDYING LANGUAGE
In most people, the main 
language areas of the brain 
are located in the left 
hemisphere, so this area shows 
greater activity when a person 
listens to spoken words. The 
right hemisphere is also 
required for complete hearing, 
and for distinguishing tone 
and rhythm.

INNER 
STRUCTURES
This MRI scan  
is set within  
an X-ray of the 
neck and skull. 
The MRI reveals 
the intricate 
folds of the 
brain tissue.

FIBER DETAIL
This diffusion 
tensor image 
shows another 
view of the nerve 
fibers. The green 
fibers link the 
various parts  
of the limbic 
system. The blue 
fibers run from the 
cerebellum, which 
joins onto the 
spine. The red 
fibers connect the 
two hemispheres.

MOVEMENT
FMRI is very good  
at localizing brain 
activity. In this image 
(bottom of brain at 
top), the red area 
shows activity in the 
part responsible for 
moving the right 
hand. Each side  
of the body is 
controlled by  
the opposite 
hemisphere  
of the brain.

SLICED TOGETHER
Here, a combined CT and MRI scan  
shows the surface folds of the brain. It also 
reveals the skull bones and the top vertebrae.

MAGNETIC RESONANCE IMAGING
Magnetic resonance imaging (MRI) provides a better contrast between tissue  
types than CT. Instead of using X-rays, it uses a powerful magnetic field, which  
causes hydrogen atoms in the body to realign. The nuclei of the atoms produce  
a magnetic field that is “read” by the scanner and turned into a three-dimensional 
computerized image. The brain is scanned at a rapid rate (typically once every 2–3 
seconds) to produce “slices” similar to those in CT scans. Increases in neural activity 
cause changes in the blood flow, which alter the amount of oxygen in the area, 
producing a change in the magnetic signal. Functional MRI (fMRI) involves showing 
differing levels of electrical activity in the brain, overlaid on the anatomical details. 

NERVE PATHWAYS IN THE BRAIN
A refinement of MRI called diffusion tensor 
imaging picks up the passage of water along 
nerve fibers. Here, the blue fibers run from 
top to bottom, the green from front to back, 
and the red between the two hemispheres.

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US_014-015_b_atlas_intro.indd   14 01/08/18   5:23 PM



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THE BRAIN IS THE MOST COMPLEX ORGAN IN THE BODY AND IS PROBABLY THE MOST COMPLEX SYSTEM 
KNOWN TO HUMANKIND. OUR BRAIN CONTAINS BILLIONS OF NEURONS THAT ARE CONSTANTLY SENDING 
SIGNALS TO EACH OTHER, AND IT IS THIS SIGNALING THAT CREATES OUR MINDS. WITH THE HELP OF 
MODERN SCANNING TECHNOLOGY, WE NOW KNOW ABOUT BRAIN STRUCTURE IN GREAT DETAIL. 

A JOURNEY THROUGH THE BRAIN

In the nineteenth century, much was learned about the structure of 
the brain by removing it from the body after death. Knowledge of the 
workings of the living human brain could only be gained by studying 
people with damaged brains, for example Phineas Gage (see p.141), 
but the precise location of this damage could not be known while the 
patient was still alive. Everything changed with the invention of brain 
scanners at the end of the twentieth century. In the following pages, 
we shall undertake a journey through the brain of a healthy, 55-year-
old man revealed by magnetic resonance imaging (MRI). In these 
images, we can see the many components of the brain. We are 

starting to understand the function of some of these, but we  
are only at the very beginning of this journey of understanding.  
 The captions that accompany the scans indicate the most likely 
function of various brain regions. But these regions often have  
many functions, and these functions depend upon interactions with 
other brain regions. Most structures in the brain are paired, with 
identical counterparts in the left and right hemispheres, so structures 
identified in one hemisphere are mirrored in the opposite one. The 
scans themselves have been colored, so that the cerebrum appears  
in red, the cerebellum in light blue, and the brainstem in green. 

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1 THE FRONTAL-POLAR CORTEX The frontal-polar cortex is the most recently evolved part 
of the prefrontal cortex in the frontal lobe and is concerned 
with forward planning and the control of other brain regions. 
This slice, right at the front of the brain, also reveals other 
features of the skull, including the eyes, nasal cavity, maxillary 
sinus, and tongue.

Frontal-polar cortex

Eye

Orbitofrontal 
gyrus

Maxillary sinus

Nasal cavity

Frontal lobe

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Frontal-polar cortex

Olfactory bulb

Orbitofrontal 
gyrus

Nasal septum

Optic nerve

Tongue

2 THE FRONTAL LOBE The frontal lobe, of which the prefrontal cortex is the 
front part, is the largest of the brain’s lobes and the latest  
to  evolve. The frontal lobe is devoted to the control of 
action—precise control of muscles at the back, high-level 
planning at the front. In this slice, the optic nerve can also  
be seen carrying visual information from the eye to the brain.

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Optic nerve

Nasal septum

Tongue
Masseter muscle

Temporalis 
muscle

Orbitofrontal gyrus

Inferior frontal gyrus

Middle frontal gyrus

Superior frontal 
gyrus

3     THE CORTEX The cortex, which appears on these scans as yellow lines,  
is heavily folded, creating a large surface area. The major 
ingoing folds (sulci, singular sulcus) are used as landmarks to 
define brain regions. The bulges between the ingoing folds are 
known as gyri (singular, gyrus). The major components of the 
frontal lobe are the superior, middle, and inferior frontal gyri.

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Nasal septum

Tongue

Masseter muscle

Temporalis 
muscle

Orbitofrontal 
gyrus

Inferior frontal 
gyrus

Middle frontal 
gyrus

Superior 
frontal gyrus

4     THE ORBITOFRONTAL GYRI The orbitofrontal gyri, located at the bottom of the 
brain, receive signals about smell and taste. Like the rest  
of the prefrontal cortex, this area is concerned with  
predicting the future, but specializes in predictions about 
rewards and punishments and therefore emotions. This  
area is connected with the amygdala (see slice 9, p.24).

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5 THE ANTERIOR CINGULATE CORTEX Here we see the beginning of the anterior cingulate 
cortex, which lies between the two hemispheres. This  
sits alongside the limbic system. It is involved in linking 
emotions to actions and predicting the consequences  
of actions. The back part of the anterior cingulate cortex  
has direct connections with the motor system.

Orbitofrontal gyrus

Inferior frontal gyrus

Middle frontal gyrus

Superior frontal 
gyrus

Anterior cingulate 
cortex

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6 THE TEMPORAL LOBES In this slice, the temporal lobes come into view for  
the first time. At the very front of the temporal lobes (the 
temporal poles), knowledge acquired from all the senses  
is combined, along with emotional tone. We can also see  
the lateral ventricles in the middle of the slice. These are parts 
of a system of fluid-filled spaces in the middle of the brain.

Orbitofrontal 
gyrus

Inferior frontal gyrus

Middle frontal gyrus

Superior frontal 
gyrus

Anterior cingulate 
cortex

Fusiform gyrus

Middle temporal 
gyrus

Lateral ventrical

Temporal 
lobe

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7     THE INSULA The insula is a fold of cortex hidden deep in the brain 
between the frontal and temporal lobes. Signals about the 
internal state of the body—such as heart rate, temperature, 
and pain—are received here. Also visible in this slice is the 
corpus callosum, the band of nerve fibers that joins the  
brain’s left and right hemispheres.

Putamen

Nucleus 
accumbens

Fusiform gyrus

Insula

Inferior frontal 
gyrus

Middle frontal 
gyrus

Superior frontal 
gyrus

Corpus callosum

Lateral ventrical

Head of caudate

Anterior cingulate 
cortex

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior temporal 
gyrus

Optic chiasm

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8     THE BASAL GANGLIA Located in the middle of the brain, the basal ganglia 
include the caudate, putamen, and globus pallidus. Also known 
as nuclei, ganglia are clumps of gray matter (or nerve-cell 
bodies) surrounded by white matter. The basal ganglia are 
linked to the cortex, the thalamus, and the brainstem and  
are concerned with motor control and decision making.

Putamen

Fusiform gyrus

Insula

Inferior frontal 
gyrus

Middle frontal 
gyrus

Superior frontal 
gyrus

Corpus callosum

Lateral ventrical

Head of caudate

Anterior cingulate 
cortex

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior temporal 
gyrus

Internal globus 
pallidus

External globus 
pallidus

Internal capsule

Third ventrical

Amygdala

Hippocampus

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9 THE AMYGDALA AND HIPPOCAMPUS This slice includes the amygdala and the front part  
of the hippocampus. Both structures lie in the inner part  
of the temporal lobe. The amygdala is involved in learning  
to approach or avoid things and hence with emotion. The 
hippocampus has a critical role in spatial navigation and 
memory of past experiences, including routes between places.

Putamen

Fusiform gyrus

Inferior frontal 
gyrus

Middle frontal 
gyrus

Superior frontal 
gyrus

Corpus callosum

Lateral ventrical

Head of caudate

Anterior cingulate 
cortex

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior temporal 
gyrus

Internal globus 
pallidus

External globus 
pallidus

Internal capsule

Third ventrical

Amygdala

Hippocampus

Insula

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10    BROCA’S AREA Here we approach the back of the frontal lobe.  
The bottom of the inferior frontal gyrus in the left 
hemisphere, just above the insula, contains Broca’s area,  
which has a critical role in speech and language. At the  
bottom of the slice, we see the front of the brainstem,  
the pons, which joins the brain to the spinal cord.

Putamen

Fusiform gyrus

Inferior frontal 
gyrus

Middle frontal 
gyrus

Superior frontal 
gyrus

Corpus callosum

Lateral ventrical

Head of caudate

Anterior cingulate 
cortex

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior temporal 
gyrus 

Internal globus 
pallidus

External globus 
pallidus

Internal capsule

Third ventrical

Amygdala

Hippocampus

Spine

Pons

Ear

Temporal horn of 
lateral ventrical

Insula

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11      THE THALAMUS This slice includes the thalamus, which lies between  
the cerebrum and the brainstem. A complex structure, the 
thalamus is made up of more than 20 nuclei (see p.60).  
The thalamus acts as a relay station, taking in information 
from all of the senses (except smell) and sending them  
on to different parts of the cerebral cortex.

Putamen

Fusiform gyrus

Insula

Inferior frontal 
gyrus

Middle frontal 
gyrus

Superior frontal 
gyrus

Corpus callosum

Lateral 
ventrical

Precentral gyrus

Anterior cingulate 
cortex

Inferior temporal 
gyrus

Middle 
temporal gyrus

Superior 
temporal gyrus

Body of fornix

Thalamus

Third ventrical

Hippocampus

Pyramidal tract

Ear

Pons

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12    THE BRAINSTEM The brainstem (in green) joins the brain to the spinal 
cord and contains a number of structures such as the pons. 
The brainstem has a special role in the control of basic body 
functions, including the control of heart rate and breathing.  
It also relays signals from the brain to the muscles and from 
senses in all parts of the body to the brain. 

Putamen

Fusiform gyrus

Insula

Middle frontal gyrus

Superior frontal gyrus

Corpus callosum

Lateral 
ventrical

Precentral gyrus

Anterior cingulate 
cortex

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior 
temporal gyrus

Body of fornix

Thalamus
Third 
ventrical

Hippocampus

Pyramidal tract

Pons

Ear

Cerebellum

Temporal horn of 
lateral ventrical

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13    THE PARIETAL LOBE The parietal lobe includes the supramarginal gyrus and  
the angular gyrus (see slices 14–20, pp.29–35). The parietal lobe 
integrates signals from many of the senses (including visual 
information that arrives via the dorsal route, see pp.84–85)  
to estimate the position of the body and the limbs in space.  
This information is critical when we reach for and grasp objects. 

Fusiform gyrus

Insula

Middle frontal 
gyrus

Superior frontal 
gyrus

Corpus callosum

Lateral ventrical

Postcentral 
gyrus

Precentral 
gyrus

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior temporal 
gyrus

Entorhinal cortex

Ear

Cerebellum

Temporal horn of 
lateral ventrical

Posterior cingulate 
cortex 

Pulvinar of the 
thalamus

Parietal lobe

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14    THE PRECENTRAL AND POSTCENTRAL GYRUS The last part of the frontal cortex is the precentral gyrus.
This contains the motor strip, where different regions send 
signals to control different parts of the body. The immediately 
adjacent part of the parietal cortex (the postcentral gyrus)  
has a corresponding sensory strip, where sensory signals are 
received from different parts of the body. 

Fusiform gyrus

Middle frontal 
gyrus

Superior frontal 
gyrus 

Corpus callosum

Lateral ventrical

Postcentral 
gyrus

Precentral 
gyrus

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior temporal 
gyrus

Cerebellum

Posterior cingulate 
cortex 

Supramarginal 
gyrus

Vermis

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15 THE PRIMARY AUDITORY CORTEX The primary auditory cortex, where signals from the  
ears reach the cortex via the thalamus, lies along the very  
top of the superior temporal gyrus, in the fissure between  
the temporal lobe and the parietal lobe. Adjacent to the 
primary auditory cortex is Wernicke’s area, where incoming 
sounds are turned into words. 

Lateral ventrical

Postcentral 
gyrus

Precentral 
gyrus

Inferior temporal 
gyrus

Middle temporal 
gyrus

Superior 
temporal gyrus

Cerebellum

Supramarginal 
gyrus

Fusiform gyrus

Posterior  
cingulate cortex 

Vermis

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16 THE FUSIFORM GYRUSThe inferior temporal gyrus and the fusiform gyrus  
at the bottom of the temporal lobe are two areas concerned  
with recognition of objects. Part of the fusiform gyrus, known  
as the face-recognition area, is specialized for recognizing faces. 
It not only identifies facial features but also scrutinizes them  
for meaning, so it plays an important part in social interaction. 

Lateral 
ventrical

Postcentral 
gyrus

Precentral 
gyrus

Inferior temporal 
gyrus

Middle temporal 
gyrus

Cerebellum

Supramarginal 
gyrus

Vermis

Posterior cingulate 
cortex

Fusiform gyrus

Occipital gyrus

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17 THE CEREBELLUMThe cerebellum (colored light blue) is the highly 
convoluted “little brain” that sits at the back and below  
the main brain (also known as the cerebrum). The cerebellum  
is concerned with fine motor control and the timing of 
movements. There are many connections between the 
cerebellum and the motor cortex.

Lateral ventrical

Postcentral 
gyrus

Inferior temporal 
gyrus

Middle temporal 
gyrus

Cerebellum

Supramarginal 
gyrus

Posterior  
cingulate cortex

Occipital gyrus

3
2

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A
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IN

 

18 THE OCCIPITAL LOBEThe occipital lobe is concerned with vision. In the 
forward-most areas, signals from the primary visual cortex  
(see slice 20, p.35) are analyzed in terms of features such as 
shape and color. This information is then sent forward to the 
inferior temporal cortex (see slice 16, p.31), along a pathway 
called the ventral route, and used for object recognition.

Lateral ventrical

Postcentral 
gyrus

Inferior temporal 
gyrus

Angular gyrus

Cerebellum

Supramarginal 
gyrus

Occipital gyrus

Precuneus

Occipital lobe

3
3

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A
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E

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IN

Angular gyrus 

Postcentral 
gyrus

Lateral ventrical

Cerebellum

Superior 
parietal lobule

Occipital gyrus

Precuneus

19 THE PRECUNEUS AND THE POSTERIOR  CINGULATE CORTEX
The precuneus in the back part of the parietal lobe and 
posterior cingulate cortex (see slice 17, p.32) lie between 
the two hemispheres. These remain some of the more 
mysterious regions of the brain. They probably have a role 
in memory, especially memories about the self.

3
4

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A
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Angular gyrus

Cerebellum

Occipital gyrus

Precuneus

Superior 
parietal lobule

Cuneus

Primary visual cortex

20 THE PRIMARY VISUAL CORTEX The primary visual cortex is right at the back of the 
brain and lies mostly on the inside of the two hemispheres. 
This is the first point in the cortex where signals arrive from 
the eyes via the thalamus. These signals are retinotopically 
mapped—that is, a signal from a particular point on the retina 
is sent to a corresponding point on the primary visual cortex.

3
5

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THE HUMAN BRAIN KEEPS US PRIMED TO RESPOND TO THE 

WORLD AROUND US. IT IS AT THE HUB OF A VAST AND 

COMPLEX COMMUNICATIONS NETWORK THAT CONSTANTLY 

SEEKS AND COLLECTS INFORMATION FROM THE REST OF THE 

BODY AND THE OUTSIDE WORLD. AS THE BRAIN INTERPRETS 

THIS INFORMATION, IT GENERATES EXPERIENCES—SIGHTS 

AND SOUNDS, EMOTIONS AND THOUGHTS. BUT ITS PRIMARY 

FUNCTION IS TO PRODUCE CHANGES IN THE BODY. THESE 

INCLUDE LIFE-SUSTAINING BASICS SUCH AS THE REGULAR 

CONTRACTIONS OF THE HEART THROUGH TO THE COMPLEX 

ACTIONS THAT CONSTITUTE BEHAVIOR.

THE BRAIN AND  
THE BODY

US_036-037_brainbody_op.indd   36 01/08/18   5:23 PM



THE BRAIN AND  
THE BODY

US_036-037_brainbody_op.indd   37 01/08/18   5:23 PM



KEY FEATURES OF THE BRAIN

FEATURE DESCRIPTION

Processing 
information

Sending signals

Modules and 
connections

Individuality

Plasticity

The brain registers a vast amount of information. However, only 
a very small amount of this is actually selected for processing to 
the point at which it enters our consciousness and can be 
reported. Experience that cannot be reported is not conscious. 
Unconscious brain processing nevertheless guides and 
sometimes initiates actions (see p.116 and p.191).

The brain consists of about 1000 billon cells. Roughly 10 percent 
are specialized electrical cells called neurons, which send signals  
to one another; this signal transmission makes brain function 
different from any other bodily process. Although the signals are 
electrical, the mode of transmission between cells is chemical—
the signals are passed on by substances called neurotransmitters.

The brain is modular—different parts do different things. The 
modules are densely interconnected, however, and none works 
without the support of many others (and the rest of the body). 
Generally, lower-level functions, such as registering sensations,  
are strongly localized, but higher-level functions, such as memory  
and language, result from interconnections between brain areas.

The basic “blueprint” of the brain is dictated by our genes. As 
with any other body feature, brains share a basic anatomy, but 
each one is also unique. Even identical twins have visibly different 
brains, right from the time they are born, because the brain is 
exquisitely sensitive to its environment. The differences between 
individual brains result in each person having a unique personality.

Brain tissue can be “strengthened” and built up like a muscle, 
according to how much it is exercised. So, if a person learns and 
practices a skill, such as playing a musical instrument or doing 
mathematics, the part of the brain concerned with that task will 
grow physically bigger. It also becomes more efficient and 
enables the person to perform the task more skillfully. 

THE PRIMARY TASK OF THE BRAIN IS TO HELP MAINTAIN THE 
WHOLE BODY IN AN OPTIMAL STATE RELATIVE TO THE 
ENVIRONMENT, IN ORDER TO MAXIMIZE THE CHANCES OF 
SURVIVAL. THE BRAIN DOES THIS BY REGISTERING STIMULI 
AND THEN RESPONDING BY GENERATING ACTIONS. IN THE 
PROCESS, IT ALSO GENERATES SUBJECTIVE EXPERIENCE.

BRAIN FUNCTIONS

WHAT THE BRAIN DOES
The brain receives a constant stream of information 
as electrical impulses from neurons in the sense 
organs. The first thing it does is determine whether 
the information warrants attention. If it is irrelevant 
or just confirmation that everything is staying the 
same, it is allowed to fade away and we are not 
conscious of it. But if it is novel or important,  
the brain amplifies the signals, causing them to  
be represented in various regions. If this activity  
is sustained for long enough, it will result in a 

conscious experience. In some 
cases, thoughts are taken one 
step further, and the brain 
instructs the body to act  
on them, by sending signals  
to the muscles to make  
them contract.

HOW THE BRAIN DOES IT 
No one knows exactly how electrical activity turns into experience. 
That remains a famously hard problem, which has yet to be cracked 
(see p.179). However, much is now known about the brain processes 
that turn incoming information into the various components of 
subjective experience, such as thoughts or emotions. Much depends on 
where the information comes from. Each sense organ is specialized to 
deal with a different type of stimulus—the eyes are sensitive to light, 
the ears to sound waves, and so on. The sense organs respond to these 
stimuli in much the same way—they generate electrical signals, which 
are sent on for further processing. But the information from each organ 
is sent to a different part of the 
brain, and then processed along 
a different neural pathway. 
Where information is processed 
therefore determines what sort 
of experience it will generate.TH

E
 B

R
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I 
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THE BRAIN AND BODY
The brain and spinal cord 
constitute the central 
nervous system, which  
is the body’s main control 
center, responsible for 
coordinating all of the 
processes and movement 
in the body.

ACTIONS  
Certain brain areas are specialized 

to produce body movement. Brainstem 
modules control automatic internal 
actions, such as the lung and chest 
movements needed for breathing,  
the beating of the heart, and the 
constriction or dilation of blood vessels  
to control blood pressure. In conscious 
activities, the primary motor cortex 
sends messages (via the cerebellum  
and basal ganglia) to the muscles of  
the limbs, trunk, and head to create 
gross movements.

LANGUAGE
Language involves both 

producing speech and analyzing  
what others say to understand  
the meaning. It depends on the 
brain’s ability to link objects  
with abstract symbols and then to 
convey the symbols—and thus the 
ideas they represent—to others via 
words. In addition to facilitating 
communication between people, 
language enables individuals to 
reflect on their own ideas. 

MEMORIES
Some of the experiences we 

have change brain cells in such a  
way that the pattern of neural activity 
that produced the original experience 
can be replicated later in time.  
This process gives rise to recall, or 
memory, which enables us to use  
past experiences as a guide to how  
to behave in the present.

F E
E L

I N G

TA S
T E 

3
8

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FEATURE FACT

Structure

Connectivity

Growth

Signaling speed

Utilizing the 
whole brain

Pain-free 
zone

Regeneration

The brain is highly compact. If you 
smoothed out all the wrinkles in the 
cortex, the brain would cover an area  
of about 2 1/2 square ft (2,300 square cm).

The brain has around 100 billion neurons. 
There are more potential connections 
between the neurons than there are 
atoms in the universe.

A fetus grows neurons at the rate of 
250,000 a minute. A person is born with 
nearly all the neurons of an adult, but  
the neural networks are not mature yet.

Information travels at different speeds 
within different types of neurons.
Transmission speeds range from 3 to  
330 feet/sec (1 to 100 meters/sec).

The claim that we only use 10 percent of 
our brains is false—we use all of it. Some 
complex functions, such as memory, 
involve many areas at once.

Brain tissue has no pain receptors, so 
despite the fact it registers pain from all 
parts of the body, it does not actually  
feel pain itself. 

You do not “lose” brain cells as you age, 
although some functions may decline. 
You can maintain the networks or even 
form new ones by exercising your brain.

B
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IN

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BRAIN FACTS

THOUGHTS
The brain uses sensations, perceptions, 

and emotions to generate action plans. 
Some of the plans give rise to internalized 
brain activity, or thoughts. “Inner speech,” 
for example, is actually generated by the 
motor areas, but has no visible sign. Some 
activity occurs in the hippocampus, which  
we experience as recollection. 

PERCEPTIONS
Most of the time we are 

receiving information from many 
sensory areas at once, as with the 
combination of auditory and visual 
signals at a fireworks display. These 
signals may be communicated to 
association areas, which bind all of 
this information together. If these 
items of “bound” information 
become conscious, they form  
what is known as a multisensory 
perception. There is a great deal  
of current neuroscientific research 
on how the binding process formsa 
unified perception, because  
it is still not fully understood. 

SENSATIONS 
Information from the environment enters  

the brain via the different sense organs and is 
transmitted to specific areas of the cerebral cortex 
called the primary sensory areas. This information 
includes some input from the body itself. In the 
absence of external stimuli, the sensory areas 
continue to be active and are thought to generate 
the experiences that we know as dreams, 
hallucinations, and imagination.

EMOTIONS 
Certain stimuli (including  

some thoughts and imaginings) 
cause changes in the body by 
activating areas in the limbic system, 
especially the amygdala. Conscious 
“feelings” occur when signals from 
the limbic system are sent on to 
“association areas” in the prefrontal 
cortex that support consciousness. 
During adolescence, the amygdala 
is relied heavily upon for processing 
emotional information, because the 
prefrontal cortex only matures when 
a person reaches their late 20s.

F E
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3
9

US_038-039_functions.indd   39 01/08/18   5:23 PM



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0

US_040-041_Nervous_system.indd   40 02/08/18   2:32 PM



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4
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US_040-041_Nervous_system.indd   41 02/08/18   2:32 PM



Meninges
Three layers of 
connective tissues 
that protect spinal 
cord; cerebrospinal 
fluid fills space 
under middle layer

T
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THE BRAIN SITS AT THE TOP OF THE BODY, DIRECTING AND COORDINATING ALL 
ACTION AND ACTIVITY THROUGHOUT ITS ENTIRETY. IT DOES SO VIA THE SPINAL 
CORD, AND THE NERVES THAT STEM FROM IT AT VARIOUS POINTS ALONG ITS 
LENGTH AND BRANCH OUT INTO A NETWORK THAT SPANS THE WHOLE BODY.

THE BRAIN AND THE NERVOUS SYSTEM

THE SPINAL CORD
The spinal cord carries information to and from the brain and all parts  
of the body except the head, which is served by the cranial nerves. The 
signals that travel along the spinal cord are known as nerve impulses. 
The cord itself comprises a bundle of nerve fibers, which are the long 
projections of nerve cells. They extend from the base of the brain to the 
lower region of the spine. The cord is roughly the width of a pencil, 
tapering at its base to a narrow bunch of fibers. Data from the sensory 
organs in different parts of the body is collected via  
the spinal nerves and transmitted along the spinal 
cord to the brain. The spinal cord also sends motor 
information, such as movement commands,  
from the brain out to the body, again transmitted 
via the spinal nerve network.

EXTENT OF THE SPINAL CORD
The spinal cord extends from the 
brainstem down to the first lumbar 
vertebra, where it forms a filament, 
known as the filum terminale, that 
extends to the coccyx.

SPINAL CORD ANATOMY
The core of the spinal cord is gray matter, which 
is composed of nerve cells (neurons). The outer 
layer of white matter insulates the long fibers 
(axons) that extend from the nerve cells.

Subarachnoid space

Spinal nerve
Carries both sensory  

and motor information 
between brain and body

Nerve fibers
Bundles of nerve fibers carry signals to and from 
spinal cord and specific areas of the brain

White matter

Gray matter

Sensory root 
ganglion
Cluster of nerve 
cell bodies on 
each spinal nerve; 
partially processes 
incoming signals

Motor nerve rootlet
Individual nerve fiber 

that emerges from front 
of spinal cord; carries 

signals to muscles

FRONT OF BODY

Sensory nerve root
Nerve splits into 
rootlets that enter 
spinal cord at rear, 
carrying incoming 
signals about touch 
sensations to brain

Pia mater

Arachnoid

Dura mater

Anterior fissure
Deep groove along front of  

spinal cord

cortex

Central canal
Filled with cerebrospinal fluid, 
which provides nourishment

REAR OF 
BODY

Spinal nerve 
root Spinal nerve

Spinal cord

Sacral region
Six pairs of sacral nerves 
connect to legs, feet, and 
anal and genital areas

Cervical region
Eight pairs of cervical nerves 
serve chest, head, neck, 
shoulders, arms, and hands

Lumbar region
Five pairs of lumbar nerves 
form network to serve lower 
abdomen, thighs, and legs

Thoracic region
12 pairs of thoracic nerves 
connect to back and abdominal 
muscles and intercostal muscles

HOW SPINAL NERVES ATTACH
There are gaps in the vertebrae of 
the backbone through which spinal 
nerves enter the spinal cord. The 
nerves divide into spinal nerve 
roots, each made up of tiny 
rootlets that enter the back and 
front parts of the cord.Vertebra

Spinal nerves contain a special fiber, 
the dorsal root, that sends sensory 
information from the skin to the 
brain. All but one pair of spinal 
nerves serves a specific area of the 
body, or dermatome. Nerve fibers in 
contact with skin receptors join up 
along the network of fibers in one 
dermatome to form the relevant 
dorsal root, which enters the spinal 
cord and conveys sensory impulses 
from that dermatome to the brain.

SPINAL NERVES
There are 31 pairs of spinal 
nerves. These branch out from 
the spinal cord, dividing and 
subdividing to form a network 
connecting the spinal cord to 
every part of the body. The spinal 
nerves carry information from 
receptors around the body to the 
spinal cord. From here the 
information passes to the brain 
for processing. Spinal nerves also 
transmit motor information from 
the brain to the body’s muscles 
and glands so that the brain’s 
instructions can  
be carried out swiftly.

SPINAL REGIONS
Each of the 31 pairs of nerves belong 
to one of four spinal regions—
cervical, thoracic, lumbar, or sacral.

MAP OF DERMATOMES
This map shows the 30 dermatomes  
of the body. Each zone is served by  
a corresponding pair of spinal nerves.

lumbar region

brainstem

coccyx

spinal cord

filum 
terminale

DERMATOMES

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US_042-043_nervous_system.indd   42 02/08/18   2:32 PM



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CRANIAL NERVES
There are 12 pairs of cranial nerves that are linked directly 
to the brain and do not enter the spinal cord. They allow 
sensory information to pass from the organs of the head, 
such as the eyes and ears, to the brain and also convey 
motor information from the brain to these organs—for 
example, directions for moving the mouth and lips in 
speech. The cranial nerves are named for the body part  
they serve, such as the optic nerve for the eyes, and are also 
assigned Roman numerals, following anatomical convention.

Spinal accessory 
nerve (XI, mixed)
Motor functions 
responsible for 
muscles and 
movements of 
head, neck, and 
shoulders; also 
stimulates muscles 
of larynx and 
pharynx, which  
are involved in 
swallowing; sensory 
functions unknown

CRANIAL NERVE CONNECTIONS
The cranial nerves I and II connect to the 
cerebrum, while cranial nerves III to XII 
connect to the brainstem. The fibers of 
sensory cranial nerves each project from a 
cell body that is located outside the brain 
itself, in sensory ganglia or elsewhere 
along the trunks of sensory nerves.

Vagus nerve (X, mixed)
Longest and most branched of all cranial 
nerves, with autonomic, sensory, and 
motor fibers; serves lower part of head, 
throat, neck, chest, and abdomen, and 
plays role in many functions, including 
swallowing, breathing, heartbeat, and 
production of stomach acid

Facial nerve (VII, mixed)
Sensory fibers collect information from  
taste buds at front two-thirds of tongue; 
motor fibers are predominantly responsible 
for muscle movements controlling facial 
expression and also function of salivary gland 
and lacrimal gland, which secretes tears and 
lubricates the surface of the eye and 
conjunctiva of the eyelid

Trigeminal nerve  
(V, two sensory and one mixed branch)
Ophthalmic and maxillary branches of  
this nerve convey signals from eyes, teeth, 
and face, and other sensory fibers carry 
impulses from lower jaw; motor fibers 
control muscles involved with chewing

Optic nerve (II, sensory)
Visual information from retina is conveyed  
to brain by optic nerve at back of eye; optic 
nerves from both eyes meet at point known  
as optic chiasm, then signals from both visual 
fields are sent to opposite sides of brain

Glossopharyngeal and hypoglossal 
nerves (IX, XII, both mixed)
Motor fibers of these nerves control  
most of the muscles involved with tongue 
movement and swallowing; sensory fibers 
convey information on taste, touch, and 
temperature from tongue and pharynx 
and can trigger gag reflex if stimulated

Vestibulocochlear nerve 
(VIII, sensory)
Vestibular branch of this 
nerve collects information 
from inner ear about head 
orientation and balance; 
cochlear branch is 
concerned with sound and 
hearing signals from ear

Oculomotor, trochlear, 
and abducens nerves 
(III, IV, VI, motor)
Three nerves regulating 
voluntary movements  
of eye muscles, allowing 
movement of eyeball 
and eyelids; oculomotor 
nerve also allows for 
pupil constriction

Olfactory nerve  
(I, sensory)
Smell molecules in 
nasal cavity trigger 
nerve impulses that 
pass along this 
nerve to olfactory 
bulb, then on to 
limbic areas (see 
pp.64–65) of brain

II

VII

XI X

VIII

IX

XII

III

IV

VI

I

V

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US_042-043_nervous_system.indd   43 01/08/18   5:23 PM



THE BRAIN ACCOUNTS FOR AROUND 2 PERCENT OF TOTAL BODY WEIGHT, BUT CONSUMES  
A DISPROPORTIONATE AMOUNT OF FUEL TO SUPPORT ITS MANY ACTIVITIES. IT HAS SEVERAL 
FORMS OF PROTECTION—THE LAYERS OF MEMBRANE SURROUNDING IT, A BONY SKULL, 
AND FLUID PRODUCED IN ITS CHAMBERS (VENTRICLES) TO ABSORB THE IMPACT OF SHOCKS. 

BRAIN SIZE, ENERGY USE, AND PROTECTION

WEIGHT AND VOLUME
The average adult human brain weighs about 31/4 lb (1.5kg).  
Its volume and shape are similar to those of an average-sized 
cauliflower, and the consistency of its tissues is similar to stiff  
jelly. The size of a person’s brain bears little relation to his or her 
intelligence, and every brain, whatever its weight and volume, has 
roughly the same number of neurons and synapses. After the age  
of 20 or so, brain mass decreases by about 1/32 oz (1g) per year. New 
neurons are made throughout life, but not enough to replace those 
that die off with age. This is generally no cause for concern, as there 
are plenty of neurons left to carry out the brain’s functions.

COMPOSITION OF THE BRAIN
The brain consists mainly of water, which 
occurs in the cytoplasm of neurons and glial 
cells, as well as being a major constituent of 
blood. The brain is also rich in lipids—fatty 
molecules that make up cell membranes. 

BRAIN WEIGHT
The brain’s weight increases from birth and reaches its maximum during adolescence. The 
number of neurons is fixed in infancy but, as the body grows, they grow in size and form 
new connections. The male brain is consistently heavier than the female brain from birth.

AGE  (IN YEARS)

BRAIN WEIGHT AND BODY WEIGHT
This graph shows brain weight as a percentage of total body weight 
over the course of a lifetime. Proportionally, a baby’s brain is around 
six times larger than an adult’s. Despite being lighter than the male 
brain overall, the female brain after the age of 13 is actually heavier 
than the male brain as a proportion of the entire body’s weight.

B
R

A
IN

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E

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H

T 
A

S 
A

 %
   

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FEMALE

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 O

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1.5

1

0.5

0
0.25

0.75

1.25

0 10 20 30 40 50 60 70 80

8

4

12

0
2

6

10

14

0 10 20 30 40 50 60 70 80

 10% blood

 10–12% lipids 
(fatty molecules)

10% CSF

8% protein

2% soluble organic 
substances

1% carbohydrate
1% inorganic salts

77–78% water

cerebellar 
degeneration

A recent study linked alcohol consumption to brain 
shrinkage. Participants disclosed their drinking habits and 
MRI scanning was used to measure each person’s ratio of 
brain volume to skull size. It was found that abstainers had 
greater brain volumes than former drinkers, light drinkers, 
moderate drinkers, or heavy drinkers. On average, 
abstainers had 1.6 percent greater brain volume than 
heavy drinkers. Interestingly, the effects were most marked 
among elderly women. In another study, participants 
between the ages of 60 and 79 took up either regular 
aerobic exercise or toning and stretching exercises for six 
months. MRI scans of each participant taken both before 
and after the six-month period showed an increase in the 
brain volumes of those doing aerobic exercise, suggesting 
that aerobic exercise can help maintain the health of the 
brain in older adults.

BRAIN VOLUME AND LIFESTYLE

BRAIN OF A NORMAL MALE

BRAIN OF AN ALCOHOLIC

ALCOHOLISM AND BRAIN ATROPHY
Alcoholism can lead to cerebellar degeneration as 
shown above. The low quality of the scan was due  
to the man’s withdrawal symptoms, preventing him 
from sitting still.

The brain is housed within the 
intracranial cavity, so measurements 
of the skull effectively relate to the 
size of the brain. The actual length, 
width, and height of an individual 
human brain can be measured using 
MRI scanning. There is considerable 
variation in the size of the adult 
human brain, but the average 
dimensions are given against the 
diagrams below. Bear in mind that, 
because of the numerous complex 
folds within the cerebrum, the brain 
has a much larger surface area than 
is apparent from its overall shape. 

LENGTH, WIDTH,  
AND HEIGHT

51/2 IN (140MM)

3
1/2

  IN
 (93M

M
)

61/2 IN (167MM)

LEFT HEMISPHERE

INTRACRANIAL CONTENT
Brain tissue comprises gray and white 
matter, which consist of neurons and 
supporting glial cells respectively. A series 
of ventricles is filled with cerebrospinal 
fluid (CSF) and the brain is also richly 
supplied with blood vessels.

FRONT

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80% brain tissue

AGE  (IN YEARS)

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US_044-045_size_weight.indd   44 02/08/18   2:32 PM



OXYGEN AND GLUCOSE SUPPLY
Glucose is the brain’s sole fuel, except under 
conditions of starvation, when it breaks down 
protein. The brain is by far the body’s  
hungriest organ. Although it accounts  
for just 2 percent of the body’s  
weight, it requires a staggering 20 
percent of its total glucose supplies.  
This is obtained from dietary 
carbohydrate, which is transported 
to the brain via the bloodstream.  
It consumes roughly 4oz (120g) of 
glucose (about 420kcal) per day. 
Because the brain cannot store 
glucose, it must be readily 
available at all times via the 
blood supply. Without oxygen  
or glucose, the brain can last  
for only about 10 minutes before 
irreparable damage occurs. This is 
why prompt resuscitation is needed 
in cases of cardiac arrest.

PROTECTING THE BRAIN
The brain has several defense mechanisms to 
protect it from damage. The bony skull acts as  
a box, containing the brain and buffering it  
against blows. The meninges are three layers  
of membranes that line the skull, enclosing the 
brain and providing extra layers of protection 
between the skull and the brain. Cerebrospinal 
fluid circulates within the brain, nourishing brain 
tissue and working as a shock absorber to reduce 
the impact of knocks.

CEREBROSPINAL FLUID FLOW
Brain tissue floats in cerebrospinal fluid (CSF) within the 
skull. CSF absorbs shocks from blows to the brain. It is 
produced in a series of connected chambers within the 
brain known as the ventricles, and is renewed four to five 
times per day. It contains proteins and glucose to nourish 
brain cells, as well as white blood cells to protect 
against infection. It moves through the 
ventricles, propelled by the pulsation 
of the cerebral arteries.

3 Circulation around spinal cordHelped by vertebral movement, 
fluid travels downward along the back  
of the spinal cord, into the central canal, 
and upward along the front of the cord. 

4 Site of reabsorption (arachnoid granulations)
After traveling around the brain, 
the fluid is finally reabsorbed  
into the bloodstream through  
tiny arachnoid granulations 
(projections from the arachnoid 
layer of the meninges into the 
sagittal sinus).

2 Direction of flowCSF flows from the lateral 
ventricles into the third and fourth 
ventricles. It then continues up 
the back of the brain, down 
around the spinal cord, and to  
the front of the brain, as indicated 
by the arrows.

1 Site of fluid production  (choroid plexus)
CSF is produced in the clusters  
of thin-walled capillaries (the 
choroid plexus) that line the  
walls of the ventricles.

Third ventricle

Fourth ventricle

Spinal cord

Cerebellum

Central canal

Lateral ventricle

Dura matter

Sagittal sinus

Skull

Dura mater

Cerebral vein Skull

Blood 
vesselCerebrum

Arachnoid

Pia mater

THE CIRCLE OF WILLISZThe 
angiogram above and the 
illustration to the left show  
the Circle of Willis, a ring  
of communicating arteries 
encircling the base of the brain. 
It provides the brain with supply 
routes for glucose and oxygen. 
If one route becomes blocked, 
another one compensates for it.

THE MENINGES
The outermost layer, the dura mater, 
contains blood vessels; the arachnoid 
consists of connective tissue; and the 
pia mater lines the brain’s contours. 

Anterior inferior cerebellar artery

Posterior cerebral artery

Optic nerve

Anterior spinal artery

Anterior 
communicating 
artery

Anterior  
cerebral artery

Superior 
cerebellar artery

Basilar artery

Vertebral artery

Labyrinthine artery

B
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Internal  
carotid artery

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US_044-045_size_weight.indd   45 02/08/18   2:32 PM



CIRCLE OF WILLIS
The major arteries  
of the brain can be 
seen in this MRI scan. 
They include the Circle 
of Willis (below center) 
at the base of the  
brain, where arteries 
from the neck meet  
before branching.

US_046-047_blood_flow_dps.indd   46 01/08/18   5:23 PM



OXYGEN SUPPLY
This arteriograph  
shows arteries carrying 
oxygen-rich blood  
to the brain. The 
arrangement of the 
arteries allows blood to 
be supplied by another 
route if one of the 
pathways is blocked.

US_046-047_blood_flow_dps.indd   47 01/08/18   5:23 PM



BRAINS EVOLVED TO ENABLE ANIMALS TO RESPOND TO ENVIRONMENTAL CHANGES. 
THE HUMAN BRAIN HAS EVOLVED TO ITS PRESENT COMPLEXITY THROUGH SEVERAL 
STAGES, MANY OF WHICH ARE COMMON TO ALL ANIMALS. ITS ORIGINS CAN BE SEEN  
IN THE BRAINS OF OTHER SPECIES, IN WHICH MORE PRIMITIVE STRUCTURES REMAIN.

EVOLUTION

EVOLUTION OF THE INVERTEBRATE BRAIN
All animals have to respond to changes in their internal and external environment in 
order to survive. To do this, they have evolved cells that are sensitive to stimuli such  
as light and to vibrations. The sensory cells are, in turn, connected to other cells that 
can move the organism or change its state in response to the stimulus. This system of 
interconnected nervous tissue is a crude form of brain. In invertebrates, such as worms, 
the nervous system is distributed throughout the creature’s 
body, as a loose network of reactive fibers. Some of these 
networks contain small masses of nerves, known as ganglia. 
These are the forerunners of the structures that, in some 
species, have become the central nervous system or brain.

EVOLUTION OF THE VERTEBRATE BRAIN
Through the course of evolution, the brain has undergone 
considerable changes. Compared to the primitive nervous 
systems of invertebrates, the brain of vertebrates is a 
well-developed, highly interconnected organ. The central 
nervous system is connected to the rest of the body by a 
peripheral nervous system that includes the fibers running 
to and from the sensory organs. The basic vertebrate 
brain—also sometimes referred to as the “reptilian brain”—

EARTHWORM BRAIN
The earthworm has a crude brain, the 
cerebral ganglion, which is connected 
to a cord of nervous tissue (the ventral 
nerve cord) that runs the length of  
its body. Nerve fibers from the cord 
extend into each segment, so muscle 
contraction along the body can be 
coordinated to produce movement  
in response to stimuli.

consists of the cluster of nuclei that lies just above the 
brainstem in humans. They include the modules that 
produce arousal, sensation, and reaction to stimuli. It is 
unlikely, however, that these nuclei alone are sufficient  
to produce consciousness. This basic vertebrate brain  
does not include more advanced features, such as the  
limbic system or cerebral cortex, which exist only in  
the brains of mammals.

PRIMITIVE NERVOUS SYSTEM
The simplest system, as seen in this 
hydra (a tiny aquatic invertebrate), 
consists of a loose network of 
sensory cells with clumps of 
interconnected cells called ganglia.   

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FISH FROG TURTLE

Ventral nerve cord

Brain

Ganglia

Oesophagus

Cerebellum

Optic lobe

Cerebrum

Pituitary gland

Medulla 

Olfactory bulb

KEY TO VERTEBRATE 
BRAIN AREAS

The amphibian brain resembles the fish brain except 
that the cerebrum is roofed over with nervous tissue. 
The main function of this region is to perceive smell, 
as reflected by the large olfactory bulb. The forebrain 
is much larger than the cerebellum.

AMPHIBIANS 
A fish’s cerebrum receives sensory signals from the 
sense organs and combines them with information 
from the internal organs and nerves to guide 
action. Fish have a large cerebellum in order  
to coordinate movement and gauge pressure.

FISH
Modern reptiles show greater development in the 
basal parts of the forebrain, and the cerebrum is 
much larger than the optic lobe. The olfactory 
bulb is large in comparison with the other 
structures of the brain and is well developed.

REPTILES  

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MAMMAL BRAINS
The mammalian brain comprises a cluster of structures that evolved on top  
of the basic vertebrate brain, known as the limbic system, and a wrinkled 
covering called the cortex, which interconnects with the limbic structures 
beneath. The limbic system is the part of the brain that produces emotions. 
These are responses to stimuli that go beyond the basic “grab” or “avoid” reactions  
in the vertebrate brain, and produce subtle and complex actions that are not 
always predictable. The limbic system also contains structures that encode 
experiences as memories, to be recalled for use in guiding future actions. The 
emotional and memory faculties greatly increase the range and complexity of 
behavior that a mammal displays, because it is not governed purely by instinct. 

HOMINID BRAINS
The brains of hominids (modern humans and their ancestors) 
underwent a surge of evolutionary changes that left them, in some 
ways, distinctly different even from their near relatives, such as 
chimpanzees and gorillas. The main distinction between human 
and other mammalian brains is the size and density of the cortex, 
and particularly of the frontal lobe, which is responsible for 
complex thought, conscious judgement, and self-reflection. No  
one knows why the human brain evolved as it did—it may have 
been due to some change in diet forced by the environment, or  
the product of living in groups (see p.138) that depended on  
close interdependence for survival. 

BRAIN SIZE AND SHAPE
One striking aspect of 
mammalian brain evolution is 
the development of the cortex. 
This outer layer has evolved to 
serve the particular needs of 
each species, and therefore 
varies dramatically between  
one animal and another. A few 
mammals, such as humans, 
elephants, and dolphins, have a 
disproportionately large cortex 
compared to most mammals.

DOES SIZE MATTER?
The growth of the human brain  
over the course of evolution is 
thought to be the reason why we  
are so dominant. However, size is  
not the only factor that matters for 
intelligence or survival—the way 
brains are wired up may be more 
important. Neanderthals had  
bigger brains than humans, but  
were less innovative and were finally 
superseded by other hominids. 

THRUSH CAT HUMAN

ELEPHANT HUMAN

DOLPHIN CAT

WOLF

1400 1600

BRAIN VOLUME (CUBIC CENTIMETERS)

NEANDERTHAL SKULL

HOMO 
NEANDERTHALENSIS

MODERN HUMANS

0 200 400 600 800 1000 1200

CHIMPANZEE

GORILLA

HOMO HABILIS

HOMO ERECTUS

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Birds’ brains are similar to those of reptiles except 
that the cerebellum is highly developed to control 
balance and position in flight. Despite the size of 
the olfactory bulb, most birds have a poor sense  
of smell, with some exceptions, such as the kiwi.

BIRDS
In mammals, the cerebellum is relatively small 
compared to the forebrain. The cerebrum is covered 
in wrinkled cortex; these wrinkles allow a greater 
volume of cortex to fit into the skull, compared to 
the smooth surface of the reptilian brain.

MAMMALS
The human brain is completely dominated by the 
cerebrum, and the cortex is intricately folded to 
allow the maximum amount to be contained in  
the skull. The cerebellum remains large and active, 
however, to enable complex motor activity.

MAN

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BRAIN ANATOMY IS HIDDEN, SECRET, AND MORE COMPLEX 

THAN ANY OTHER PART OF THE BODY. THE BASIC BUILDING 

BLOCK OF THE BRAIN IS THE CELL. SIGNALING CELLS KNOWN 

AS NEURONS FORM LARGER STRUCTURES CALLED NUCLEI 

THAT CARRY OUT PARTICULAR FUNCTIONS. THEY ALSO 

CLUSTER TOGETHER TO FORM THE THICK, LAMINATED SHEET 

OF GRAY MATTER FORMING THE COVERING OF THE BRAIN 

CALLED THE CORTEX. DEEP FISSURES IN ITS SURFACE DIVIDE 

THE BRAIN INTO TWO HALVES (THE HEMISPHERES), EACH WITH 

FIVE LOBES. THESE MAJOR DIVISIONS “SPECIALIZE” IN 

DIFFERENT TASKS, BUT ALSO INTERCONNECT AND INTERACT.

BRAIN  
ANATOMY

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BRAIN  
ANATOMY

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THE BRAIN HAS A COMPLEX AND MANY-LAYERED ANATOMY. PEELING BACK THE DOMINANT CEREBRAL 
HEMISPHERES REVEALS A FURTHER SET OF STRUCTURES WITHIN. SOME ARE DISCRETE MASSES, SUCH AS  
THE CEREBELLUM AND THALAMUS, WHILE OTHERS ARE ZONES OF NERVE FIBERS OR NERVE CELLS WITHIN 
LARGER STRUCTURES, DISCERNIBLE ONLY BY MICROSCOPIC EXAMINATION.

BRAIN STRUCTURES

Caudate  
nucleus

Putamen

Corpus 
callosum

Amygdala

Cerebellum Hippocampus

Right 
hemisphere

EXPLODED HEAD
A whole head “exploded” sideways reveals the main brain 
regions or divisions. The central brainstem stands up like  
a fist on an arm, and the cerebrum wraps over and around 
it, dominating it both physically and mentally. The next 
largest structure after the cerebrum is the cerebellum  
at the lower rear, comprising about ten percent of the 
brain’s total volume. In common with standard anatomical 
terminology, right and left refer to the owner rather than 
the viewer. So here the right hemisphere of the cerebrum 
is on the left of the picture.

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THE BRAIN HIERARCHY
The brain’s major parts can be classified or categorized in several ways. 
In all of these systems, the dominant part is the cerebrum, the large 
pinky-gray wrinkled structure that forms more than three-quarters  
of the brain’s total volume. The cerebrum is divided into left and 
right hemispheres, which are linked by a “bridge” of nerve fibers, the 
corpus callosum. The cerebrum, which includes the hippocampus and 
amygdala, is also known as the telencephalon. Together with the parts 

it wraps around—the thalamus, hypothalamus, and associated parts, 
collectively known as the diencephalon—it comprises the major brain 
“division” known as the forebrain (prosencephalon). Below the 
forebrain is the midbrain (mesencephalon), a small division that 
includes groups of nerve-cell bodies known as nuclei, such as the 
basal ganglia. Below the midbrain is the hindbrain (rhombencephalon), 
with the pons as its uppermost part, and beneath it the cerebellum 
and the medulla, which tapers to merge with the spinal cord.

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Hypothalamus

Fornix

Thalamus

Interior globus 
pallidus

Exterior globus 
pallidus

Optic chiasm

Trigeminal 
nerve

Subthalamic 
nucleus

Pituitary gland

Olive (rounded 
protrusion that contains 
the olivary nuclei)

Pons

Superior 
colliculus

Pyramid (anterior medulla)

Medulla

Cervical spinal cord 
(in neck)

Thoracic spinal cord 
(in chest)

Geniculate 
nucleus

Midbrain

Maxilla (upper jaw  
bone of skull)

Nasal cavity

Sphenoid bone of skull

Foramen magnum (hole 
for spinal cord)

Occipital bone of skull

Left hemisphere

Cervical vertebra 
(neck backbone)

Spinal nerve

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SCALP SKIN
The skin of the scalp has only  
a thin underlying layer of 
subcutaneous fat and the 
hard skull is just beneath,  
so it wounds relatively easily 
and bleeds copiously. 

SCALP NERVES
Many small peripheral nerves 
branch through and under the 
scalp skin from cranial nerves 
II, III, and V. Even faint contact 
registers, allowing us to react 
quickly and avoid injury.

SKULL
The upper domed part of the 
skull, called the neurocranium, 
forms a “braincase” to shield 
against knocks and jolts. This 
function is aided by the 
meninges (see p.56).

Internal globus 
pallidus

External globus 
pallidus

Sulcus

Gyrus

Putamen

Caudate nucleus

Right 
thalamus

Right cerebral 
hemisphere

FRONTAL BONE
The neurocranium is composed of eight  

bones. Most prominent is the frontal bone 
under the forehead. The left and right parietals 
are behind it, the occipital below them at the 

lower rear, and the two temporals on the lower 
sides. The sphenoid and ethmoid bones are  

at the lower front, behind the nose area.

FACIAL BONES
Complicated in shape, the facial bones have 
gaps (foramina) in them. Some allow cranial 
nerves to pass from the brain within the 
neurocranium, out to the nasal epithelium in  
the nose cavity, the eyes in their sockets, the 
inner ear, and other sensory parts. Blood 
vessels have similar sets of skull foramina.

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LEFT AND RIGHT HEMISPHERES
An overhead view of the “exploded” brain shows how the two cerebral hemispheres 
can be neatly separated by cutting through the corpus callosum. Many other brain 
structures are symmetrically paired in this way, such as the thalamus, which is 
sometimes described as “two hen’s eggs sitting side by side.” The cerebellum at  
the lower rear of the brain is accommodated within a bowl-like cavity of the skull 
known as the posterior cranial fossa. The cranial nerves (numbered I to XII, see 
p.43) enter the brain directly rather than connecting to the spinal cord. 

CEREBELLUM
This name means “little brain,” referring to the pattern  
of grooves and bulges on the cerebellar surface, which 
reflects the external appearance of the cerebrum. The 
cerebellum is connected to the brainstem immediately  
in front of it by three pairs of thick, short, stalklike 
extensions, called the cerebellar peduncles.

CEREBRAL CORTEX
The thin grayish covering of  
each cerebral hemisphere is  
called the cerebral cortex. It has  
a characteristic pattern of bulges 
(gyri), shallower grooves (sulci),  
and deeper ones (fissures).

Fornix

Left thalamus

Pineal gland

Mamillary body

Hypothalamus

Midbrain

Subthalamic nucleus

Left olfactory tract 
(cranial nerve I)

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Hypothalamus
Situated under the thalamus, as its name 
implies (“hypo” means “under”), the 
sugar-cube-sized hypothalamus has many 
important functions, including temperature 
control and basic behavioral drives

Pituitary gland
“Master gland” of hormonal or 
endocrine system hangs by a stalk 
from hypothalamus above 

Cerebellum
Responsible 
for balance 

and posture

Neck 
vertebra

Spinal cord

Thalamus
Processes and sends  
on sensory information  
to higher brain areas

Pons
“Crossroads” area 
consisting mainly of  
nerve fibers

Corpus callosum
Main link between left and right cerebral 
hemispheres is a highway of more than 
200 million nerve fibers

Medulla
Regulates vital functions 
such as heartbeat and 
respiration

SLICED DOWN THE MIDDLE
A medial sagittal section (a cut through the 
brain from front to rear, exactly in the middle 
or center line between the eyes) shows the 
sliced corpus callosum and brainstem.  
The left cerebral hemisphere and thalamus 
are off-center, so they remain unsectioned.

Superior sagittal sinus
Around the brain’s midline is 
a shallow groove containing 
blood, which is part of the 
venous return to heart

Dura mater and arachnoid
The outer two meninges are the tough,  
strong dura mater attached to the inside  
of the skull, and the blood-rich arachnoid

Subarachnoid space
This gap between the 
arachnoid and pia 
mater is filled  
with cushioning 
cerebrospinal fluid

Pia mater
Innermost meninx

Specific names are given to various sections or slices of the brain, which show 
different views of the internal parts. For example, a sagittal section that is not 
medial (down the middle), misses the corpus callosum and cuts down through  
a cerebral hemisphere to reveal its intricate pattern of surface folds and grooves.

 
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HORIZONTAL CORONAL SAGITTAL MEDIAL

Skull

Scalp

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THE BRAIN’S PHYSICAL STRUCTURE BROADLY REFLECTS ITS 
MENTAL ORGANIZATION. IN GENERAL, HIGHER MENTAL 
PROCESSES OCCUR IN THE UPPER REGIONS, WHILE THE 
BRAIN’S LOWER REGIONS TAKE CARE OF BASIC LIFE SUPPORT.

BRAIN ZONES  
AND PARTITIONS

THE HOLLOW BRAIN
The brain has an internal system of chambers (ventricles), which  
are filled with a liquid—cerebrospinal fluid (CSF)—produced by  
the ventricle linings. The upper two chambers are the left and right 
lateral ventricles, one in each cerebral hemisphere, with hornlike 
forward- and side-facing projections. Small openings connect them 
to the third ventricle in the midbrain, which  
in turn links to the fourth ventricle in the 
pons and medulla. CSF flows slowly and 
continuously through the ventricles, then out 

via small openings into 
the subarachnoid 
space around the 
brain and the  
spinal cord.

VERTICAL ORGANIZATION
The uppermost brain region, the cerebral cortex, is mostly 
involved in conscious sensations, abstract thought processes, 
reasoning, planning, working memory, and similar higher mental 
processes. The limbic areas (see pp.64-65) on the brain’s innermost 
sides, around the brainstem, deal largely with more emotional  
and instinctive behaviors and reactions, as well as long-term 
memory. The thalamus is a preprocessing and relay center, 
primarily for sensory information coming from lower in the 
brainstem, bound for the cerebral 
hemispheres above. Moving down  
the brainstem into the