NERVOUS SYSTEM OF VERTEBRATA. 557 



discernible. They are very small in the Perca 

 fluviatilis, or common Perch (Fig. 45.3) ; but 

 more developed in Reptiles, as in the Testudo 

 mydas, or Green Turtle (Fig. 454), and in the 

 Crocodile (Fig. 455) ; and still more so in Birds, 

 as is seen in the brain of the Dove (Fig. 449) ; 

 but most of all in Mammalia, as is exemplified 

 in the brain of the Lion (Fig. 456). On the other 

 hand, the optic tubercles (t) are largest, com- 

 pared with the rest of the brain, in Fishes ; and 

 their relative size diminishes as we ascend to 

 Mammalia ; and the same observation applies 

 also to the olfactory lobes (o). 



The relative positions of the parts of the brain 



are much influenced by their proportional deve- 



lopement. This will be rendered manifest by 



the lateral views of the brains of the Perch, the 



Turtle, the Dove, and the Lion, presented in 



Figures 457, 458, 459, and 460, respectively, 



where the same letters are employed to designate 



the same parts as in the preceding figures. In 



Fishes, all the tubercles which compose this 



organ, are disposed nearly in a straight line, 



continuous with the spinal marrow, of which, as 



they scarcely exceed it in diameter, they appear 



to be mere enlargements. As the skull expands 



more considerably than the brain, this organ 



does not fill its cavity, but leaves a large space, 



filled with fluid. Some degree of shortening, 



however, may be perceived in the brain of the 



