No. I.] DESMOGNATHUS FUSCA. 253 



horizontal, sections. Various functions have been ascribed to 

 this part. It has been thought to be concerned in vision and 

 audition. Mrs. Gage has also suggested that it may be a 

 trophic organ, concerned in the nourishment of the brain, 

 since it appears before the plexuses in a region which "has 

 need of a means of repairing waste." 



Diencephal. — This segment merges into the mesencephal 

 with no ectal sign of differentiation. The cavity (diacoele) is 

 narrow, but has a greater dorso-ventral diameter than has any 

 other cavity of the brain. On each side of the wall are two 

 sulci, beginning at the portas and extending in a caudal direc- 

 tion. Both are rather indistinct at their origin, but soon 

 become deeper; the ventral one extends, somewhat obliquely, 

 toward the base, and expands into the infundibulum ; the dorsal 

 sulcus (the sulcus of Monro or the aulix of Wilder [58]) takes 

 a more direct course, and expands into a fairly-wide mesocoele. 

 The infundibulum is, relatively, very large, and, from the 

 ventral aspect, almost entirely obscures the mesencephal. Its 

 cavity is not high, dorso-ventrally, but has considerable lateral 

 extension. The hypophysis is quite closely applied to the 

 caudal end of the infundibulum. It appears somewhat glan- 

 dular in structure, and is highly vascular. Immediately in front 

 of the chiasma is a slight lateral extension of the diacoele 

 along each optic nerve, representing the preoptic recess. Con- 

 siderable interest centers about this region on account of its 

 connection with the morphological front of the brain. Differ- 

 ent anatomists have taken different standards. If the axis pass 

 along the roof of the neural tube, then it will fall upon the 

 summit of the terma; if it pass midway between the floor and 

 the roof, it will terminate at the preoptic recess; if the floor be 

 chosen, then the axis will end in the infundibulum. His favors 

 the last view. 



In the roof of the diencephal, and just caudal to the supra- 

 plexus, are the habenae ; they are not conspicuous on the 

 surface of the brain, being overlapped by a caudal projection of 

 the plexus. In transections, the left habena seems to be larger 

 than the right (Fig. 28). The supracommissure is quite well 

 developed, and easily seen in frontal or transections. The 



