460 GENERAL ANATOMY. 



er number of reptiles. In the others, and especially in birds, 

 it is more complex. In them there are already perceived la- 

 minae and the commencement of lateral hemispheres; but in 

 no oviparous animal are there yet seen the prolongations des- 

 tined to form the annular protuberance, or that protuberance 

 itself. In all the mammifera we find the lamellated structure 

 of the cerebellum, lateral hemispheres, a ciliary body in the 

 peduncles, and a protuberance. These parts are the more de- 

 veloped the higher we rise in the class of mammifera towards 

 man. The prolongations of the cerebellum at the tubercula 

 quadrigemina also exist in all the mammifera. The ventricle 

 of the cerebellum is common to the four classes of vertebrate 

 animals. 



In some fishes there are observed encephalic lobes posterior 

 to the cerebellum. Such are those which correspond to the 

 origin of the nerves of the electric apparatus of the torpedo. 



The corpara quadrigemina, which are formed by the de- 

 velopment of the lateral or olivary cords of the spinal marrow, 

 appear to exist in all the vertebrate animals, although there 

 has been much diversity of opinion with respect to their de- 

 termination. In all they are the principal point of origin of 

 the optic nerves. In all they form, by their union in the mid- 

 dle line, the upper wall of a cavity situated between the ven- 

 tricle of the cerebellum and the third ventricle. They are so 

 much the larger in proportion to the encephalon in general, 

 the more simple it is. They are bigeminous only in the ovi- 

 para, and are quadrigeminous only in the mammifera. The 

 anterior pair is larger than the posterior in the ruminantia, so- 

 lipeda and rodentia. The reverse takes place in the carnivo- 

 ra. The two pairs are about equal in the quadrumana and in 

 man. 



The brain, properly so called, which results from the expan- 

 sion of the anterior or pyramidal cords of the spinal marrow, 

 which cross each other in all the mammifera and in the birds 

 of prey only, and not in the other animals, and are enlarged by 

 additional fibres from the optic thalami and the corpora stria- 

 ta, presents many differences in its volume and complication, 

 which are in general proportionate to the volume of these thala- 



