Bma Illustrated Medical Dictionary

bradycardia may cause a drop in blood

pressure that results in fainting (see

vasovagal attack).

BrailleA system of embossed dots,

now accepted for all written languages,

that enables blind people to read and

write. The system is based on 6 raised

dots, which can be combined in differ-

ent ways. There are 2 types of Braille. In

grade I, each symbol represents an indi-

vidual letter or punctuation mark. In

grade II, which is the more widely used,

symbols represent common letter com-

binations or words.

brainThe major organ of the nervous

system, located in the cranium(skull).

The brain receives, sorts, and interprets

sensations from the nerves that extend

from the central nervous system(brain

and spinal cord) to the rest of the body;

it initiates and coordinates nerve signals

involved in activities such as speech,

movement, thought, and emotion.

An adult brain weighs about 1.4 kg and

has 3 main structures: the largest part,

the cerebrum,consisting of left and

right hemispheres; the brainstem;and

the cerebellum. Each hemisphere in the

cerebrum has an outer layer called the

cortex, consisting of grey matter, which

is rich in nerve-cell bodies and is the

main region for conscious thought, sen-

sation, and movement. Beneath the

cortex are tracts of nerve fibres called

white matter, and, deeper within the

hemispheres, the basal ganglia. The sur-

face of each hemisphere is divided by

fissures (sulci) and folds (gyri) into dis-

tinct lobes (occipital, frontal, parietal,

and temporal lobes), named after the

skull bones that overlie them. A thick

band of nerve fibres called the corpus

callosum connects the hemispheres.

The cerebrum encloses a central group

of structures thatincludes the thalami

and the hypothalamus,which has close

connections with the pituitary gland.

Encircling the thalami is a complex of

nerve centres called the limbic system.

These structures act as links between

parts of the cerebrum and the brainstem

lying beneath the thalami.

The brainstem is concerned mainly

with the control of vital functions such as

breathing and blood pressure. The cere-

bellum at the back of the brain controls

BRAILLE BRAIN

B

Thalamus

Cerebrum

Corpus

callosum

BRAIN

Midbrain

Pons

Cerebellum

Medulla

Brainstem

Hypothalamus

Pituitary gland

Cerebrospinal fluid

Meninges

Spinal cord

Cerebral cortex

Skull