OP TUE C EREBELLUM.— (TragEyxrtpaxic, tivicreioc Ertzeprx?“5.; Fr. armlet ; Germ. Kleine Gchirn.) This remarkable portion of the ence phalon, so called from its general resemblance to the cerebrum, of which it is, as it were, the diminutive, is situate behind the mesocephale and medulla oblongata. It is lodged in a com partment of the cranium, the floor of which is formed by the fossw of the occipital bone, and which is separated from the cavity occupied by the cerebrum, by the horizontal process of the dura mater, previously described as the tento riunt eerebelli. This process forms a partition between the inferior surface of the posterior lobes and the superior surface of the cerebellum.
The cerebellum, like the cerebrum, is at its highest point of developement in the human subject. It exists as a very distinct portion of the encephalon in all the classes of vertebrate animals, and exhibits a marked gradation of increase from Fishes, through Reptiles and Birds, up to Mammals.
In Fishes and Reptiles it consists of a sin gle lobe, overhanging the posterior _surface of the medulla oblongata, and closing the fourth ventricle partially like a valve. It is, in ge neral in these classes, smooth on its surface, rind exhibits no complication of structure, no subdivision into laminw. But in the sharks a manifest increase in size and an incipient lamellar arrangement are distinctly observable, which shew that in them this organ is more highly developed than in any other fishes.
In birds a similar complication of structure takes place to a much greater extent, and a lateral lobe or appendage is added on each side to the single central organ which constitutes the cerebellum of fishes and reptiles. And in the mammiferous series, the lateral lobes along with the central portion experience a progres sive augmentation of size (proportionally to the body), and a corresponding complexity of structure up to the quadrumana and man.
The best and most obvious subdivision of the human cerebellum is into the median lobe and the /aterat tobes or hemispheres. The
former is the fundamental and primitive portion of the organ ; the latter, although each exceeds the median lobe in size, and therefore they con jointly form far the largest portion of the cerebellum, are appendages, which in man assume great physiological importance. The median lobe has likewise been called venni form process, the upper and lower laminte being distinguished as the superior and isaferior vermaform processes.
From the tables already given it would ap pear that the cerebrum is to the cerebellum in the proportion of 8 or 9 to I in the adult, and in the infant, according to Chaussier, as 16 or 18 to 1. The average weight of the cere bellum is, according to Professor Reid's re searches, 5 oz. 4 dr. in the male, and 4 oz. 12 dr. in the female.
The cerebellum seems to keep pace, in its developement, with that of the cerebrum. It attains its greatest size, both in male and fe male, at the same age as the cerebrum. At the most advanced ages, however, it seems to diminish with greater rapidity than that organ.
Some variety appears to occur as regards the relative developement of cerebellum to cere brum in the adult. Chaussier remarks that he had in some instances found the cerebellum equal to a seventh or a sixth part of the weight of the cerebrutn, but rarely the eleventh or twelfth.
There do not appear to be any good grounds for the assertion that the cerebellum is rnore developed in proportion to the brain in the female than in the male. Professor Reid's extensive series of researches show, beyond all question, that it maintains the same propor tionate bulk in both sexes.
It has also been asserted that castration, or disease of the genital organs, such as would destroy the generative instinct, causes wasting of the cerebellum. If both testicles be re moved, the whole cerebellum, it is said, dege nerates; if only one, the hemisphere of the opposite side is affected.