The Brain

The cerebrum is the largest part of the brain (Fig. 17-3). It is composed largely of white matter with a thin outer layer of gray matter, the cerebral cortex. It is within the cortex that the higher brain functions of memory, reasoning, and abstract thought occur. The cerebrum is divided into two hemispheres by a deep groove, the longitudinal fissure. Each hemisphere is further divided into lobes with specialized functions.

The diencephalon contains the thalamus, the hypothalamus, and the pituitary gland. The thalamus receives sensory information and directs it to the proper portion of the cortex. The hypothalamus controls the pituitary and forms a link between the endocrine and nervous systems.

The brainstem consists of the midbrain, the pons, and the medulla oblongata (see Fig. 17-3). The midbrain contains reflex centers for improved vision and hearing. The pons forms a bulge on the anterior surface

Medical Terminology Brain

of the brainstem. It contains fibers that connect different regions of the brain. The medulla connects the brain with the spinal cord. All impulses passing to and from the brain travel through this region. The medulla also has vital centers for control of heart rate, respiration, and blood pressure.

The cerebellum is under the cerebrum and dorsal to the pons and medulla (see Fig. 17-3). Like the cerebrum, it is divided into two hemispheres. It helps to control voluntary muscle movements and to maintain posture, coordination, and balance.

FIGURE 17-2. A motor neuron. The break in the axon denotes length. The arrows show the direction of the nerve impulse.

Direction Impulse Motor Neuron

Within the brain are four ventricles (cavities) in which cerebrospinal fluid (CSF) is produced. This fluid circulates around the brain and spinal cord, acting as a protective cushion for these tissues.

Covering the brain and the spinal cord are three protective layers, together called the meninges. The outermost and toughest of the three is the dura mater. The middle layer is the arachnoid. The thin, vascular inner layer, attached directly to the tissue of the brain and spinal cord, is the pia mater.

Twelve pairs of cranial nerves connect with the brain (see Fig. 17-1). These are identified by Roman numerals and also by name. See Display 17-1 for a summary chart of the cranial nerves.

Cranial Parts
FIGURE 17-3. External surface of the brain, showing the main parts and some lobes of the cerebrum.

DISPLAY 17-1 The Cranial Nerves

NUMBER

NAME

FUNCTION

I

olfactory ol-FAK-to-re

carries impulses for the sense of smell

II

optic OP-tik

carries impulses for the sense of vision

III

ocu_lomoto_r ok-u-lo-MO-tor

controls movement of eye muscles

IV

trochlea_r TROK-le-ar

controls a muscle of the eyeball

V

trigeminal tri-JEM-i-nal

carries sensory impulses from the face; controls chewing muscles

VI

abducens ab-DU-sens

controls a muscle of the eyeball

VII

fa_cial FA-shal

controls muscles of facial expression, salivary glands, and tear glands; conducts some impulses for taste

VIII

vestibuloco_chlear_ ves-tib-u-lo-KOK-le-ar

conducts impulses for hearing and equilibrium; also called auditory or acoustic nerve

DISPLAY 17-1 The Cranial Nerves, continued

NUMBER

NAME

FUNCTION

IX

glossopharyngeal glos-o-fa-RIN-je-al

conducts sensory impulses from tongue and pharynx; stimulates parotid salivary gland and partly controls swallowing

X

vagus VA-gus

supplies most organs of thorax and abdomen; controls digestive secretions

XI

spinal accessory ak-SES-o-re

controls muscles of the neck

XII

h_ypo_glossal hi-po-GLOS-al

controls muscles of the tongue

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