THE NERVOUS SYSTEM 137 



inherited from a supposed invertebrate ancestor, which originally 

 had the form of a blind sac on the free surface of the head, close 

 to the olfactory organ. Scott and von Kupffer thus differ con- 

 siderably in their views ; [but whatever the original significance 

 of the hypophysis, all observers are agreed that it is the vestige 

 of an organ originally distinct from the present vertebrate mouth 

 and from the nose of at least the gnathostomata. With respect 

 to it, the Vertebrata collectively fall into two distinct and diversely 

 modified assemblages, viz. (i.) the Epicraniata (Lampreys and 

 IJags), in which it is carried up with the nose and perforates the 

 basis cranii from above ; and (ii.) the Hypocraniata (Fishes 

 proper, Amphibians, and Amniota), in which it is carried down 

 and inwards with the mouth, and perforates the basis cranii from 

 beneath.] 



We still have to consider those cases in which degeneration 

 of the brain is either beginning or has made some progress. 

 We find an instance of commencing degeneration in the lobus 

 olfactorius, to which we shall have to return when considering 

 the olfactory organs. A case of advanced degeneration is seen 

 in the roof of the fourth ventricle. This, in Man, as in all 

 Vertebrates, becomes almost entirely transformed in the course of 

 Ontogeny into a vascular membrane, overlying a simple epithelium, 

 and continuous laterally and anteriorly with the pia-mater. The 

 lining epithelium is continuous laterally and posteriorly with the 

 delicate structures bordering on the calamus scriptorius known as 

 the obex, ponticulus, and ligula (tsenia). These all consist of 

 nervous tissue, and are to be classed morphologically with the 

 epithelial layer just mentioned. The rudimentary character of 

 the series is evident, and the same applies to the velum medullare 

 posterius. 



In contrast to the degenerate portions of the brain, other 

 parts are found to be in course of progressive development ; these 

 more than compensate for the loss not only of the above 

 mentioned, but of all other degenerating parts. We have only 

 to mention the cerebrum, with its continually developing com- 

 plexity of the nerve tracts, especially the complex components of 

 the gray cortex, which, as the organs of the mental faculties, are 

 kept in constant touch with the surrounding world by means of 

 the centripetal and centrifugal tracts of the peripheral nervous 

 system. 



To this topic we shall have to return. It will here suffice 

 to mention one more portion of the brain in which variation in 



