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 1s 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 Hpicraniata (Lampreys and 
Hags), in which it is carried up with the nose and perforates the 
basis cranii from above; and (11.) the AHypocramata (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 
-im the root 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 
hning epithelium is continuous laterally and posteriorly with the 
delicate structures bordering on the calamus scriptorius known as 
the obex, ponticulus, and ligula (tenia). 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 apphes to the velum medulare 
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 
