486 



COMPARATIVE PHYSIOLOaY. 



numerous connections through nerve-fibers, etc. But in this 

 respect we find great difference among vertebrates. We can 

 recognize, on passing upward from the Amphioxus, destitute 

 of a brain proper, to man, all gradations in the form, relative 

 size, multiplicity of connecting ties, etc. 



Sj^eaking generally, there is great difference in the weight 

 of the cerebrum, both relative and absolute. In all animals be- 

 low the primates (man and the apes) the cerebellum is either 

 not at all or but imperfectly covered by the cerebrum ; while 



Fig. 339. — Nervons syetem of medicinal leech (after Owen), a, double snpia-cesopha- 

 geaJ ganglion connected with rudimentary ocelli (&, b) by nerves; c, double infra- 

 ceaophageal ganglionic mass, which is continuous with double ventral cord, hav- 

 ing compound ganglia at regular intervals. 



in man, so great is the relative size of the latter, that the 

 cerebellum is scarcely visible from above. If we except the 

 elephant, in which the brain may reach the weight of ten 

 pounds, and the whale with its brain of more than five pounds 



Fig 340 —Brain and cranial nerves of perch, seen from the side (after Gegenbaur and 

 Cuvier) A cerebral lobe with olfactory ganglion in front; B, optic lobe; C, cere- 

 bellum- D, medulla oblongata; I— Till, nerves in usual order; K, lateral branch 

 of vai'us; I, upper twig of same; «i, dorsal branch of trigeminus, joined by re, dor- 

 sal branch of vagus; a, /3, v, three branches of trigemmus; it, facial nerve; A, 

 branchial branches of vagus. 



