Page images

ing united, ascend higher and terminate in the paracentral lobule, the superior extremity of the ascending frontal and parietal convolutions. The sensory tract can be traced upward, through the posterior third, into the cerebrum, where they probably terminate in the hippocampus major and unciate convolution.

Functions.—The corpora striata are the centers in which terminate some of the fibers of the superficial or motor tract of the crura cerebri; others pass upward through the internal capsule, to be distributed to the cerebrum. It might be inferred, from their anatomical relations, that they are motor centers. Irritation by a weak galvanic current produces muscular movements of the opposite side of the body; destruction of their substance by a hemorrhage, as in apoplexy, is followed by a paralysis of motion of the opposite side of the body, but there is no loss of sensation. When the hemorrhagic destruction involves the fibers of the anterior twothirds of the posterior segment of the internal capsule, and thus separates them from their trophic centers in the cortical motor region, a descending degeneration is established, which involves the direct pyramidal tract of the same side and the crossed pyramidal tract of the opposite side.

Destruction of the posterior one-third of the posterior segment of the internal capsule is followed by a loss of sensation on the opposite side of the body, and a loss of the senses of smell and vision on the same side (Charcot). The precise function of the corpora striata is unknown, but they are in some way connected with motion.

The optic thalami receives the fibers of the tegmentum, the posterior portion of the crura cerebri. They are insensible and inexcitable to direct irritation. Removal of one opticthalamus, or destruction of its substance by disease or hemorrhage, is followed by a loss of sensibility of the opposite side of the body, but there is no loss of motion ; their precise function is also unknown, but in some way connected with sensation. In both cases their action is crossed.

CEREBELLUM. The Cerebellum is situated in the inferior fossæ of the occipital bone, beneath the posterior lobes of the cerebrum. It attains its maximum we which is about 5 ozs., between the twenty-fifth and fortieth years, the proportion between the cerebellum and cerebrum, being i to 84.

It is composed of two lateral hemispheres and a central elongated lobe, the vermiform process; the two hemispheres are connected with each other by the fibers of the middle peduncle forming the superficial portion of the pons Varolii. It is brought into connection with the medulla oblongata and spinal cord through the prolongation of the restiform bodies; with the cerebrum, by fibers passing upward beneath the corpora quadrigemina and the optic thalami, and then forming part of the diverging cerebral fibers.

Structure. It is composed of both white and gray matter, the former being internal, the latter external, and convoluted, for economy of space.

The White matter consists of a central stem, the interior of which is a dentated capsule of gray matter, the corpus dentatum. From the external surface of the stem of white matter processes are given off, forming the laminæ, which are covered with gray matter.

The Gray matter is convoluted and covers externally the laminated processes; a vertical section through the gray matter reveals the following structures :

1. A delicate connective tissue layer, just beneath the pia mater, containing rounded corpuscles, and branching fibers passing toward the external sursace.

2. The cells of Purkinje, forming a layer of large, nucleated; branched nerve cells sending off processes to the external layer.

3. A granular layer of small, but numerous corpuscles.
4. Nerve fiber layer, formed by a portion of the white matter.

Properties and Functions.—Irritation of the cerebellum is not followed by any evidences either of pain or convulsive movements; it is, therefore, insensible and inexcitable.

Co-ordination of Movements.-Removal of the superficial portions of the cerebellum in pigeons produces feebleness and want of harmony in the muscular movements; as successive slices are removed, the movements become more irregular, and the pigeon becomes restless; when the last portions are removed, all power of flying, walking, standing, etc., is entirely gone, and the equilibrium cannot be maintained, the power of coördinating muscular movements being entirely gone. The same results have been obtained by operating on all classes of animals.

The following symptoms were noticed by Wagner, after removing the whole or a large part of the cerebellum. 1. A tendency on the part of the animal to throw itself on one side, and to extend the legs as far as possible. 2. Torsion of the head on the neck. 3. Trembling of the muscles of the body, which was general. 4. Vomiting and occasionally liquid evacuations.

Forced Movements.-Division of one crus cerebelli causes the animal

to fall on one side and roll rapidly on its longitudinal axis. According to Schiff, if the peduncle be divided from behind, the animal falls on the same side as the injury; if the section be made in front, the animal turns to the opposite side.

Disease of the cerebellum partially corroborates the result of experi. ments; in many cases symptoms of unsteadiness of gait, from a want of coördination, have been noticed.

Comparative anatomy reveals a remarkable correspondence between the development of the cerebellum and the complexity of muscular actions. It attains a much greater development, relatively to the rest of the brain, in those animals whose movements are very complex and varied in character, such as the kangaroo, shark, and swallow.

The cerebellum may possibly exert some influence over the sexual function, but physiological and pathological facts are opposed to the idea of its being the seat of the sexual instinct. It appears to be simply a center for the coördination and equilibration of muscular movements.

CEREBRUM. The Cerebrum is the largest portion of the encephalic mass, constituting about four-fifths of its weight; the average weight in the adult male is from 48 to 50 ozs., or about three pounds, while in the adult female it is about five ozs. less. After the age of forty the weight of the cerebrum gradually diminishes at the rate of one ounce every ten years.

In idiots the brain weight is often below the normal, at times not amounting to more than twenty ounces.

The Blood Supply to the cerebrum is unusually large, considering its comparative bulk, nearly one-fifth of the entire volume of blood being distributed to it by the carotid and vertebral arteries. These vessels anastomose so freely, and are so arranged within the cavity of the cranium, that an obstruction in one vessel will not interfere with the regular supply of blood to the parts to which its branches are distributed. A diminished amount, or complete cessation, of the supply of blood is at once followed by a suspension of its functional activity.

The cerebrum is connected with the pons Varolii and medulla oblongata through the crura cerebri, and with the cerebellum through the superior peduncles. It is divided into two lateral halves, or hemispheres, by the longitudinal fissure running from before backward in the median line ; each hemisphere is composed of both white and gray matter, the former being

« PreviousContinue »