Comparative Anatomy of the Brain. 573 



bellum. The Tunny has the largest and most highly organized cerebel- 

 lum of all. This fish is very active, and is also nearly equal to the 

 warm-blooded animals in some of its physical make-up. In the shark, 



e' 6 



r V VI 



FIG. 280. 



vm 'X 



FIG. 279. Top view of the Brain of River Lamprey ( Petro- 

 myzonfluviatiiis.) 



FIG. 280. Side view of same. 



I. Olfactory nerves. A, . Olfactory lobes. 



II. Optic nerves. B, 6. Cerebral hemispheres. 



III. Motor oculi nerves. C, c. Thalainencephalon. 



IV. Pathetic nerves. . D,d Optic lobes. 



V. Trigeminal nerves. E, e. Medulla oblongata. 



VI. Abducens nerve. F,f. Fourth ventricle. 



VII. Facial & auditory nerves. 0. A narrow band between F, 

 VIII. Glosso pharyngeal and / and Z), d is the Cerebellum. 



Vagus nerves. G, g Spinal cord. 



IX. Hypoglossal nerves. c. Pineal gland. 



1,1,2, 2. Sensory and motor roots of the first two spinal nerves. 



FIG. 279. ns r g an covers not only the fourth ventricle but 



a good share of the optic lobes also. ( See fig. 286.) In the saw-fish it 

 covers the optic lobes and laps over upon the cerebrum. But in these 

 active fishes it is not merely largest, it is also the best developed ; for 



. FIG. 281. Brain of Sturgeon. 

 R. Olfactory lobes. 

 P. Cerebral hemispheres. 

 O. Optic lobes. 

 C. Cerebellum. 

 A. Fourth ventricle. 

 TV. Vagal columns origin of 



[ Nerves. 

 the Vagus 



FIG. 281. 



FIG. 282. Brain of Chimera Monstrosa, a 

 cartilaginous fish with complete cranial wall. 

 R. Ol factory lobes. 

 1. Olfactory nerves. 

 P. Cerebrum. 

 O. Optic lobes. 

 B. Cerebral Crura. 

 C. Cerebellum. 

 TV. Trigeminal lobes. 

 2. Optic nerves they do not decussate. 



(Busch.) 



its surface is increased by transverse 

 folds, as seen in fig. 286. It is variously 

 placed in different fishes, and its shape 

 greatly varies. In the cod it is flat 

 and tongue-shaped, and covers the 

 fourth ventricle. In the amblyopsis 

 it is a hemispherical bulb, extending 

 well over the optic lobes which is an 

 unusual arrangement, owing to the 



FIG. 282. 



small use this animal makes of eyes. There is generally some gray 



