NERVOUS CENTRES. (HCMAN ANATOMY. THE ENCEPHALON.) 



687 



cessus cerebelli ad testes, all connect neigh- 

 bouring parts with the intrinsic matte/ of the 

 mesocephale. 



It is plain, then, that anatomy affords abun- 

 dant grounds for the conclusion, that the me- 

 socephale must be regarded as a distinct centre, 

 connected by numerous bonds of union with 

 the other segments of the brain. 



If further proof of this were wanting, it 

 would be found in the connexion of two im- 

 portant nerves with this segment. These are 

 the fifth and the fourth pair. The former pene- 

 trate between the superficial fibres of the pons 

 which spread out upon the crus cerebelli; the 

 latter are connected with the superior surface 

 of the mesocephale. 



OF THE CEREBELLUM. (tiafiyv.^a.\^ 

 oTno-Qio? syxEpaAK; Fr. ceruelet; Germ. Kleine 

 Geliirn.) 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 fossae 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- 

 rium cerebelli. 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, 

 and exhibits no complication of structure, no 

 subdivision into laminae. 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 lateral lobes 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 vermi- 



form process, the upper and lower laminae 

 being distinguished as the superior and inferior 

 vermiform 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 1 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 cerebrum, but rarely the eleventh or 

 twelfth. 



There do not appear to be any good grounds 

 for the assertion that the cerebellum is more 

 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. 



The most complete refutation of this assertion 

 is afforded by M. Leuret's series of observations 

 of the brains of geldings and entire horses. 

 These researches, indeed, shew that in stallions 

 the cerebellum is proportionally smaller than in 

 mares or geldings, and that in geldings it is 

 larger than in mares. It is very evident from 

 them that mutilation of the sexual organs does 

 not cause degeneration of the cerebellum. 



The shape of the cerebellum is that of " an 

 ellipsoid flattened from above downwards."* 

 Its principal diameter, which is transverse, is 

 from three-and-a-half to four inches in length; 

 the antero-posterior diameter is from two in- 

 ches to two inches and a half; the anterior part 

 is about two inches in thickness ; whilst near 

 its posterior edge it does not measure above 

 half an inch. 



At its anterior edge the cerebellum is notched, 

 and receives fibres by which it is connected to 

 the cerebrum and mesocephale. This notch is 

 of considerable transverse extent, and is semi- 

 lunar in shape. The greater portion of the 

 posterior part of the mesocephale corresponds 

 to it. By Reil this is called the semilunar Jin- 

 sure. In it we find several parts which the 

 anatomist should study; namely, on the highest 

 plane, the processus cerebelli ad testes, sepa- 



* Cruveilhier. 



