124 THE ANATOMY OF VERTEBEATED ANIMALS. 



FiK. 31. 





Fig. 31.— Side and upper views of the brain of Petromijzon fltiviutiUs, 

 and an upper and inner view of the membranous labyrinth of 

 P. marinus. The following letters refer to the figures of the brain : — 

 I. the olfactory nerves, narrow anterior prolongations of the rhinen- 

 cephalon (.A.) ; B, the prosencephalon ; O, the thalamencephalon ; D, 

 the mesencephalon ; P., the medulla oblongata ; F, the fourth ven- 

 tricle ; e, the narrow band which is all that represents the cerebellum : 

 G, the spinal cord; II. the optic; III. the oculnmntorius; IV.thepathe- 

 ticus; V. the trigeminal; VI. the abducens; VII. the facial, and the 

 auditory; VIII. the glossopharyngeal and pneumogastric ; IX. the 

 hypoglossal nerves; 1, I', 2, 2', sensory and motor roots of the first 

 two spinal nerves. In the figure of the membranous labyrinth : h, 

 the auditory nerve; a, the vestibule ; c, the two semicircular canals, 

 which correspond with the anterior and posterior vertical canals of 

 other Vertebrata ; d, their union and common opening into the ves- 

 tibule ; 6, the ampullae. 



