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Functions of the Various Parts of the Brain

motor, corpora, movements, impressions, fibres, nervous, sensory, cerebral and medulla

FUNCTIONS OF THE VARIOUS PARTS OF THE BRAIN.

The Functions of the cerebral hemi spheres have been already indicated. They are associated with mental phenomena—feeling, thinking, and willing, and with all conscious states. They are the seat of intelligence. An animal deprived of the cerebral hemispheres is only a very complicated mechanism, but a mechanism incapable of starting itself, though, when stimulated by some impulse from with out, capable of as great a variety of complex and co-ordinated movements as an animal en dowed with intelligence. But it is nevertheless only an elaborate mechanism, without conscious ness, and incapable of spontaneous action. From the cerebral hemispheres originate all voluntary movements, through the cells of the motor areas, and only when sensory impulses reach them do these impulses rise into consciousness.

The corpora striata are concerned in the transmission downwards of motor influences, that is, nervous impulses which excite muscular contraction. Injury or disease causes paralysis of motion on the opposite side of the body, the motor fibres crossing in the medulla. Motor impulses do not originate in the corpora striata, but these organs receive the impulse from the higher centres in the cerebrum and transmit it downwards. Again, they may be concerned in reflex movements without consciousness. Thus a nervous impression, starting from the eyes, reaching the corpora quadrigemina, is probably transmitted forwards to the corpora striata, and thence passes downwards to excite muscular contractions.

The optic thalami are sensory centres re ceiving impressions, probably those of touch, and transmitting them upwards to the cerebral hemispheres, where they may arouse conscious ness. Injury or disease of these organs on one side causes loss of sensibility, or disordered sensibility, on the opposite side of the body, as the majority of sensory fibres cross from one side to the other in some part of their course. Just as the corpora quadrigemina are the re cipients of visual impressions which may result in such complex muscular movements as those of walking without consciousness, so the optic thalami perform the same functions in relation to impressions of touch.

The corpora quadrigemina are connected with the sense of vision and with the move ments of the pupil. Disease or injury causes dilatation of the pupil and blindness. The corpora also receive visual impressions which may be transmitted upwards to the cerebral hemispheres, resulting in a sensation of light ; or they may cause co-ordinated or regulated movements without sensation. Thus visual impressions may cause well-regulated move ments without the person being actually con scious of what he sees, as happens in somnam bulism.

The pons Varolli contains motor and sen sory fibres passing downwards and upwards, and, in addition, it contains centres of gray matter which are connected with the roots of some of the cranial nerves (Fig. 85, p, 135).

Irritation of the polls causes very severe con vulsive spasms. The polls also contains, as already stated, transverse fibres connecting one half of the cerebellum with the other half, thus securing co-operation between the two portions, but the exact way in which this is done is unknown.

The cerebral peduncles or crura, seen immediately in front of the puns in Fig. 85, contain both sensory and motor fibres. The sensory are in the back part of the peduncles, and pass upwards to the corpora quadrigemina, optic thalami, and posterior lobes of the cere brum, whilst the motor fibres pass downwards from the corpora striata in the fore part of the peduncles. Injury to one peduncle sometimes causes an animal to move to the side opposite the injury, so as to describe a circle, somewhat in the manner of a horse in a circus.

The cerebellum is the regulator or co-ordi nator of muscular movements. The mechanism by which it does so has not yet been satisfac torily explained. It receives nervous impres. sions connected with sight from the corpora quadrigemina, connected with touch by nerve fibres coming from the skin and passing up the back part of the spinal cord, and also impres sions associated with the sense of equilibrium by the nerve of hearing from the semicircular canals in the ear. (See EAR.) Disease of the cerebellum in the human being is rare, but when it does exist there is usually blindness, a dilated pupil, giddiness, a tendency to move backwards, and a staggering irregular gait. There is no loss of consciousness or other dis turbance of the mental functions.

The medulla oblongata, connecting the spinal cord with the brain, may be regarded as a great nervous tract for transmitting sensory and motor impressions, and also as the seat of a number of centres for reflex actions of the highest importance to life. Motor transmission, that is the transmission of nervous influences from above that result in contractions of muscles in sonic part of the body, occurs chiefly at the fore part of the medulla. Here also many of the motor fibres cross from one side to the other, as already explained (p. 148). Through the medulla sensory impressions also pass, that is, impressions from below upwards, from the surface of the body to the brain, resulting in sensation or feeling; but their route is not accurately known.

The medulla is remarkable for being the seat of a number of reflex centres connected with the movements of the heart, the movements of respiration, the influence of the nervous system on the blood-vessels, the movements of swallow ing, and the secretion of saliva. In addition, it is at least partly the origin of many of the important cranial nerves (p. 135). - Conse quently, injury to this part of the nervous system is quickly fatal, and in nervous dis eases, or towards the close of life, when the power of swallowing is lost, we may infer that the medulla has become affected.