326 SPECIAL PHYSIOLOGY. 



apes, than it is in man. Speaking generally, when examined in a median 

 section, it becomes shorter from before backwards, and thinner, and, gradually 

 losing its horizontal position, is inclined upwards and backwards in the lower 

 Mammalia ; in the Rodents it becomes very short and thin, and nearly verti- 

 cal. In the Marsupialia, it is so rudimentary, and so limited to the anterior 

 part of the fissure between the hemispheres, as to have been described as ab- 

 sent, there being no transverse bridge of cerebral matter, connecting the two 

 hemispheres above the ventricular cavities ; there exists, however, a compact 

 transverse commissural mass, situated at the anterior part of the base of the 

 brain, and besides this, the so-called anterior commissure is largely developed. 

 In no Vertebrate animal lower than the Mammalia, is there any trace of a 

 corpus callosum, but there are merely transverse commissural fibres, crossing 

 at the base of the cerebrum, as is seen in Birds, Keptiles, Amphibia, and 

 Fishes. In the Mammalia, its size, and its development backwards, are 

 exactly proportional to the size, and extension backwards, of the cerebral 

 hemispheres, until they even overlap the cerebellum, as in the Quadrumana 

 and in Man. 



The gradual simplification, in the number and connections of the white 

 fibres of the cerebrum, must involve a less perfect physiological co-operation, 

 or combination of actions, between its several parts ; and it is obviously as- 

 sociated with retrogressively inferior psychical endowments. But the facts of 

 structure, and the observations on the powers and characters of the lower ani- 

 mals, which might throw light on the special physiology of the cerebral hemi- 

 spheres, remain for future inquirers to collect. 



The Cerebellum. Like the cerebrum, the cerebellum gradually diminishes 

 in size, as compared with the spinal cord, or with the weight of the body, in 

 passing from the highest to the lowest Vertebrata ; though, if compared with 

 the cerebrum, it is larger in many of the lower Vertebrata than in man, owing 

 to the extraordinary development of the cerebral hemispheres in him. The 

 principal anatomical feature of the highest forms of the cerebellum, is the 

 great development of its lateral masses, or hemispheres, which, however, are 

 still proportionally very large in the Quadrumana. Considered generally, the 

 lateral parts diminish rapidly in the lower Mammalia, until, at last, they are 

 represented only by the small portions named the flocculi. In Birds, the 

 hemispheres are represented by still smaller lateral appendages, the great 

 bulk of this organ, in them, being evidently composed of the central lobe or 

 vermiform processes; whilst in the Reptiles, Amphibia, and Fishes, this me- 

 dian portion is alone present, so that it would seem to be more fundamentally 

 important than the superadded lateral parts or hemispheres. 



The structure of the cerebellum, as well as its size, become also progressively 

 simplified. The number of its laminae diminish, until at last, as in the Birds, 

 they are comparatively few ; whilst, in the Reptiles, Amphibia, and Fishes, 

 its surface is commonly quite smooth, but still, however, consists of a thin 

 stratum of gray matter. In the frog, the cerebellum forms a simple smooth 

 curved band ; and in the lowest Fishes, it is reduced to a thin layer of nervous 

 substance, too small to conceal the back of the medulla oblongata ; but in some 

 cartilaginous Fishes, as in the sharks, for example, the otherwise simple me- 

 dian cerebellum is slightly notched, or laminated, upon its surface. In certain 

 Mammalia, as in the Carnivora and Ruminantia, the cerebellum, instead of 

 consisting of broad and comparatively smooth lateral hemispheres, joined by 

 a narrow median and much-divided portion, or vermiform process, is very 

 uneven upon its surface, apparently consisting of a cluster of many irregular 

 and deeply foliated lobules. 



The internal structure of the cerebellum also becomes simplified, in accord- 

 ance with the gradual diminution in the number of its lamina? ; so that the 

 appearance, on a section, named the arbor vitce, ceases to be distinguishable. 

 The corpora fantata exist in all Mammalia, but they are less plicated, and, in 

 the lowest forms, present on a section a smooth outline ; they have not been 

 seen in the oviparous Vertebrata. 



Physiologically considered, the size and complexity of structure of the cere- 

 bellum, appear to be in harmony with the degree of complication of the move- 

 ments capable of being executed by any given animal ; for example, it is 



