NERVOUS SYSTEM. VERTEBRATA. 



65 



similar in many respects to that of a low Urodele or to 

 the embryonic stages of higher Vertebrates. It consists 

 of a slight enlargement of the anterior end of the cord 

 accompanied by a corresponding increase in the size of the 

 central canal and its partial transverse division into three 

 ventricles. Upon this foundation certain excrescences have 

 been developed in connection with the senses of sight and 

 smell. The dorsal wall is to a very large extent purely 

 epithelial, nervous matter being mainly confined to the 

 floor and side walls. The primitive condition of the brain 

 is also shown by the relatively large size of the ventricles, 



Fig. 19. 



U-EPIPH, 



OP.T.N. 



GANG.HAB. 

 OPT. LOBE 



NUC.V 



NUC.X. 



B 



Brain of Petromyzon marinus (enlarged). 

 [For list of abbreviations, see page 508.] 



and by the absence of any special thickenings of their 

 nervous walls. 



The medulla forms nearly half the brain (fig. 19, A). It 

 passes behind insensibly into the cord, and in front is 

 laterally expanded with separation of its walls in the 

 mid-dorsal line to enclose a large pyriform opening (fossa 

 rhomboidalis) covered by a thin vascular membrane. This 

 roof consists of an epithelium continuous with that lining 

 the fourth ventricle, of pia mater and blood-vessels ; it is 

 considerably larger than the fossa and in consequence 

 projects in all directions into the cranial cavity, and in 



VOL. II. F 



