GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



25 



lobes become divided by transverse furrows into anterior and 

 posterior parts and the four bodies thus formed receive the name 

 of corpora quadrigemina. The optic lobes vary in size in different 

 classes of vertebrates, being noticeably larger in those animals 

 in which the eyes are especially large and important (bony fishes, 

 some selachians, birds, etc.). In mammals, however, the size of 

 the eyes does not greatly affect the size of the corpora quadri- 



Rec.neur 



Figs. 8, 9 and 10. — The outline of the brain and brain ventricles of several ver- 

 tebrates as seen from above. The relative size of the brains is ignored in the figures 

 but the form of the brain and ventricles is accurately drawn from dissections or 

 microscopic sections. 



Fig. 8. — A, the brain of a cyclostome fish, Lampetra Wilder i. 



B, the brain of a selachian, Miistelus cants. The outline of the ventri- 

 cle in the optic lobes and cerebellum is drawn in dotted lines. 



C, the brain of a young specimen of a bony fish, Coregonus albus. On 

 the left side is shown in dotted line the form of the optic ventricle, on the right side 

 the outline and cavity of the inferior lobe of the dicncephalon. 



D, the brain of a tailed amphibian, Nectunts maciilatiis. 



gemina (compare Chapter XVI). The cephalic border of the 

 roof of the mid-brain is marked by the posterior commissure. 

 The lateral and ventral walls of the mid-brain are in general 

 comparable to the same portions of the medulla oblongata. From 

 the ventral surface of the mesencephalon arises the third cranial 

 or oculomotor nerve which supplies four of the eye muscles, rectus 

 superior, rectus inferior, rectus intemus and obliquus inferior. 

 The diencephalon or interbrain, although the smallest of the 



