CEREBELLUM, s6r-e-bel'fitn (the little brain”, that portion of the brain situated be hind and beneath the cerebrum. It is connected with the main brain mass by means of two feet or stems, the cerebellar peduncles, and is sepa rated from the main brain mass in the cranial cavity by a thick layer of connective tissue, the tentorium cerebelk. It is also connected with the pons by a pair of middle peduncles, and with the medulla oblongata by the inferior peduncles. It thus forms a very integral por tion of the brain mass. In general the form of the cerebellum in human beings is a flattened ovoid measuring from 8 to 10 centimeters from side to side, five to six centimeters from before backward, and five centimeters vertically. Its average weight is about 140 grams, which is one-eighth of the weight of the whole cerebro spinal axis. It is larger and heavier in the male than in the female, and is relatively larger in the adult than in the child. Like the brain, it is divided up into a number of lobes, of which three are most prominent, the middle portion or vermis, and the two lateral lobes. The minute structure of the cerebellum is some what similar to that of the cerebrum, but there are certainly very marked differences, particu larly in the development of a layer of very characteristic cells, the Purkinje cells. The in
terior of the cerebellum contains masses of gray matter, or nuclei. These are the dentate nucleus, the nucleus emboliformis, nucleus glo bosus and the nucleus fastigii in the vermis. Through the inferior, middle and superior peduncles fibres pass to and from the cerebrum, pons, medulla and spinal cord, thus bringing the cerebellum into organic union with the rest of the nervous system. The functions of the cerebellum are not yet completely worked out, but it is certain that the cerebellum has a num ber of important functions, chief among which are those connected with locomotion and the act of balancing, i.e., the orientation of the body in space. It is termed by Sherrington the chief head ganglion for the proprioceptors, or those organs which give rise to the sense of position of the human body. The chief symptoms of cerebellar disease, either from maldevelopment or new growths, are: Cerebellar ataxia, a peculiar drunken-like gait; nystagmus (q.v.), adiadochokinesis, vertigo, hypotonus, asynergia, muscular asthenia, forced movements of the head and eyes, peculiar speech dysarthrias and occasionally cerebellar fits. Consult Jelliffe and White, (Diseases of the Nervous System' (2d ed., 1917). See BRAIN.