500 THE SENSORIAL FUNCTIONS. 



understanding the physiology of this complicated 

 and wonderful organ. 



On comparing the relative proportions of the 

 brain and of the spinal cord in the four classes of 

 vertebrated animals, a progressive increase in the 

 size of the former will be observed, as we ascend 

 from Fishes to Reptiles, Birds, and Mammalia. 

 This increase in the magnitude of the brain arises 

 chiefly from the enlargement of the cerebral hemis- 

 pheres (h), which in the inferior orders of fishes, as 

 in the Trigla lyra, or Piper Gurnard (Fig. 451), 

 and in the Murcsna conger, or Conger Eel (Fig. 

 452), are scarcely discernible. They are very 

 small in the Perca fluviatilis, or common Perch 

 (Fig. 45')); but more developed in Reptiles, as in 

 the Testudo wi/clas, 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 JLio?i (Fig. 456). On the other 

 hand, the optic tubercles (t) are largest, compared 

 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 olfac- 

 tory 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 tuber- 

 cles which compose this organ, are disposed nearly 



