Chap. IX.] 



AND OF BIRDS. 



131 



more completely roofed over than it is in Fishes or Rep- 

 tiles, by the under surface of the now larger cerebellum 

 (fig. 64). The Auditory nerves arise from about the middle 

 of the floor of the fourth ventricle, where, as in some 

 Reptiles, they are connected with a distinct ganglionic emi- 

 nence on each side of the middle line. The Trigeminus 

 is always large, and exceeds all the other cranial nerves in 

 size, with the exception of the Optic. 



The Cerebellum is much larger than we have hitherto 

 met with it — with the single exception of that of the 



Fig. 62. 



Fig. 



Fig. 64. 



Fig. 62. — Bi-nin of Pigeon. (Ferrier.) a, Cerebral hemispheres ; b, optic lobe ; 

 C, cerebellum with transverse furrows and very small lateral lobes. 



Fig. 63. — Brain and part of Spinal Cord of Chick 16 days old, showing the optic 

 lobes (/>) still in contact — at their inner borders. (Owen, after Anderson.) 



Fig. 64.— Biain and part of Spinal Cord of Chick, 20 days old, showing the optic 

 lobes (6) now widely scimrated, and cerebellum (c) greatly developed. (Owen, 

 after Anderson.) 



Shark. It now consists of a more or less ovoid median 

 lobe (deeply scored by transverse furrows), and of two 

 much smaller lateral portions, which project slightly be- 

 hind the optic lobes (fig. 62, c). 



These Optic Lobes are pushed aside and depressed so 

 that they are partly covered by the large cerebral hemi- 

 spheres (figs. 63, 64). In form they are rounded bodies, 

 showing no trace of a transverse division. Each contains 

 a cavity, opening below and internally into a subjacent 

 passage or canal, which serves to connect the fourth with 



