CHAPTER XIII.

THE CEREBELLUM.

Some points in the physiological anatomy of the cerebellum-Course of the fibres in the cerebellum-General properties of the cerebellum-Functions of the cerebellum-Extirpation of the cerebellum in animals-Incomplete extirpation of the cerebellum-Pathological facts bearing upon the functions of the cerebellum-Andral's cases-Other cases of disease of the cerebellum-Connection of the cerebellum with the generative functionDevelopment of the cerebellum in the lower animals-Paralysis from disease or injury of the cerebellum.

Ir is not necessary, in order to comprehend the functions of the cerebellum, as far as these are known, to enter into a full description of its anatomical characters. The points, in this connection, that are most interesting to us as physiologists are, the division of the substance of the cerebellum into gray and white matter; the connection between the cells and fibres; the connection of the fibres with the cerebrum, and with the prolongations of the columns of the spinal cord; and the passage of fibres between the two lateral lobes. These points, therefore, will be the only ones that will engage our attention.

Some Points in the Physiological Anatomy of the Cerebellum.

As we have seen, in treating of the general arrangement of the encephalon, the cerebellum, situated beneath the posterior lobes of the cerebrum, weighs about 5.20 ounces av. in the male, and 4.70 ounces in the female. The propor

tionate weight to that of the cerebrum is as 1 to 84 in the male, and as 1 to 8 in the female. It is separated from the cercbrum by a strong process of the dura mater, called the tentorium. Like the cerebrum, the cerebellum presents an external layer of gray matter, the interior being formed of white, or fibrous nerve-tissue. The amount of the gray substance is very much increased by numerous fine convolutions, and is farther extended by the penetration, from the surface, of arborescent processes of gray matter. Near the centre of each lateral lobe, embedded in the white substance, is an irregularly dentated mass of cellular matter, called the corpus dentatum. The cerebellar convolutions are more numerous, and the gray substance is deeper, than in the cerebrum; and these convolutions are present in many of the inferior animals in which the surface of the cerebrum is smooth.

The cerebellum consists of two lateral hemispheres, more largely developed in man than in the inferior animals, and a median lobe. The hemispheres are subdivided into smaller lobes, which it is unnecessary to describe. Beneath the cerebellum, bounded in front and below by the medulla oblongata and pons, laterally by the superior peduncles, and superiorly by the cerebellum itself, is a lozenge-shaped cavity, called the fourth ventricle. The crura, or peduncles will be described in connection with the direction of the fibres.

The structure of the gray substance of the convolutions. presents certain peculiarities. This portion is divided quite distinctly into an internal and an external layer. The internal layer presents an exceedingly delicate net-work of fine nerve-fibres, which pass to the cells of the external layer. In the plexus of anastomosing fibres, are found numerous bodies like free nuclei, called by Robin, myelocytes. The external layer is somewhat like the external layer of gray substance on the posterior lobes of the cerebrum, and is more or less sharply divided into two or more secondary

layers. The most external portion of this layer contains a few small nerve-cells and fine filaments of connective tissue; and the rest of the layer contains a great number of large cells, rounded or ovoid, with two or three, and sometimes, though rarely, four prolongations.' The mode of connection between the nerve-cells and the fibres has already been described under the head of the general structure of the nervous system."

Course of the Fibres in the Cerebellum.-Most anatomical writers give a very simple description of the course of the nerve-fibres in the cerebellum. From the gray substance of the convolutions and their prolongations, the fibres converge to form finally the three crura, or peduncles on each side. The superior peduncles pass forward and upward to the crura cerebri and the optic thalami. These connect the cerebellum with the cerebrum. Beneath the tubercular quadrigemina, some of these fibres decussate with the corresponding fibres upon the opposite side; so that certain of the fibres of the superior peduncles pass to the corresponding side of the cerebrum, and others pass to the cerebral hemisphere of the opposite side.

The middle peduncles arise from the lateral hemispheres of the cerebellum, pass to the pons Varolii, where they decussate, connecting together the two sides of the cerebellum.

The inferior peduncles pass to the medulla oblongata, and are continuous with the restiform bodies, which, in turn, are continuations chiefly of the posterior columns of the spinal cord.

According to Luys, the fibres from the cortical substance of the cerebellum all pass to the corpora dentata and there terminate, being connected with the cells. From the corpora dentata, new fibres arise, which go to form the cerebellar peduncles. Luys does not admit the existence of com1 KÖLLIKER, Éléments d'histologie humaine, Paris, 1868, p. 387, et seq. See page 50.

missural fibres connecting the two lateral halves of the cerebellum, and assumes that the decussation between the two sides takes place through a special system of decussating prolongations from the cells of the cortical substance, which he calls "intercortical commissural fibres." This view, however, is not adopted by the best anatomists; but nearly all agree that new fibres arise from the cells of the corpora dentata and contribute to the formation of the peduncles.

From the above sketch, the physiological significance of the direction of the fibres, as appears from the most reliable and generally-accepted anatomical investigations, is sufficiently evident. By the superior peduncles, the cerebellum is connected, as are all of the encephalic ganglia, with the cerebrum; by the middle peduncles, the two lateral halves of the cerebellum are intimately connected with each other; and by the inferior peduncles, the cerebellum is connected with the posterior columns of the spinal cord. We shall see, when we come to study the functions of the cerebellum, that its connection with the posterior white columns of the cord is a point of great interest and importance.

General Properties of the Cerebellum.-There is now no difference of opinion among physiologists, with regard to the general properties of the cerebellum. We may safely discard the observations of Zinn and Haller upon this point, for these experimenters, who conceived that irritation of the cerebellum produced convulsive movements,' undoubtedly stimulated portions of the medulla oblongata; at least, this must be assumed, if we accept the results of the more recent experiments of Flourens, Longet, and many others. Flourens, who made the first elaborate and entirely satisfactory observations upon the cerebellum in living animals, noted,

1 LUYS, Recherches sur le système nerveux cérébro-spinal, Paris, 1865, p. 126, et seq.

2 HALLER, Mémoires sur la nature sensible et irritable des parties du corps animal, Lausanne, 1756, p. 208.

n all of his experiments, that lesion or irritation of the cerebellum alone produced neither pain nor convulsions; and the same results have followed the observations of Longet" and of all modern physiologists who have investigated this question practically. We have ourselves frequently exposed and mutilated the cerebellum in pigeons, and have never observed any evidence of excitability or sensibility. From these facts, we must conclude that the cerebellum is inexcitable and insensible to direct stimulation, at least as far as has been shown by direct observations. It is not impossible, however, that future experiments may reverse this generally-received opinion; particularly in view of the recent ob servations of Fritsch and Hitzig, already cited,' which show that certain parts of the cerebrum are excitable, and that the excitability of the encephalic centres rapidly disappears in living animals, as the result of pain and hæmorrhage. We should note, also, the experiments of Budge, who observed movements in the testicles and vasa deferentia, in males, and in the cornua of the uterus and the Fallopian tubes, in females, following irritation of the cerebellum. Hammond noted movements of this kind in cats just killed, and also movements of the intestines and of the muscles of the abdomen, thigh, and back."

Functions of the Cerebellum.

There are still the widest differences of opinion among physiologists, with regard to the functions of the cerebellum, mainly for the reason that the experiments upon the lower

1 FLOURENS, Recherches expérimentales sur les propriétés et les fonctions du système nerveux, Paris, 1842, p. 18.

2 LONGET, Anatomie et physiologie du système nerveux, Paris, 1842, tome i., pp. 783, 734.

3 See page 323.

4 BUDGE, Lehrbuch der speciellen Physiologie des Menschen, Leipzig, 1862, S. 788.

5 HAMMOND, Physiology and Pathology of the Cerebellum.—Quarterly Journal of Psychological Medicine, New York, 1869, vol. iii., p. 223.