128 



ANATOMY OP THE CENTRAL NEHVOUS SYSTEM. 



trace of it remains. The blunt, often enlarged end of the tube remains as 

 a little tubercle, — the pineal gland, — just anterior to the midbrain. 



The base of the thalamencephalon is, in the median line at least, 

 separated from the skull by only a thin membrane. Within it and under 

 it pass important systems of transverse fibers, and just at the middle line 

 the hypothalamus is thickened into a structure of varying form. Ventral 

 to the commissural region of the cerebrum the lamina terminalis has a 

 small evagination anterior to the chiasma: the Recessus prceopticus. It 

 then covers the basal wall over the chiasma, through which it is 

 strongly curved inward (Fig. 76), and then curves outward, forming the 

 Becessus postopticus. Still farther posterior follows always a deep recess, 



Fig. 77. — Sagittal section through the infundibular region of a shark: 

 Scyllium caniciila. 



which often ends in a narrow, often thread-like tube: the infundib- 

 ulum or Recessus infundihularis. The projection which this makes upon 

 the base of the brain is called the Tuber cinereum. To the terminal tube 

 of the infundibulum is applied the hypophysis: an outgrowth from the 

 oral epithelium to the base of the skull. In mammals it grows so fast to the 

 infundibulum that it is customary to refer to the end of the infundibulum 

 as the cranial part of the hypophysis. 



Kupffer made a discovery a few years ago that is destined to throw new 

 light on the significance of this structure. In the embryos of lower verte- 

 brates — Petromyzon, sturgeon, and others — there exists, for a period, a 

 peculiar evagination from the dorsal side of the primitive pharynx and 

 directed forward. He called this the preoral gut. 



