638 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



almost as far ventrally as the locus of the sulcus limitans. The 

 tuberculum posterius of the mid-brain extends considerably farther 

 rostrad than the level of the posterior commissure and is marked 

 by an external eminence immediately dorsally of the mammillary 

 body (fig. 2, t.p.). The rostral boundary of this eminence and 

 of the mammillary body is a well defined sulcus which separates 

 these structures from the large eminence formed by the optic 

 chiasma and preoptic nucleus above and the hypothalamus be- 

 low. Immediately above the attachment of the optic nerve is a 

 small, relatively depressed region within which is the nucleus 

 olfactorius medialis {nuc. ol.m.). 



At the ventral end of the chiasma ridge and immediately above 

 it is a narrow deep membranous evagination of the brain wall, 

 the preoptic recess (figs. 2, 3, r.po.). This projects slightly for- 

 ward to form the most rostral part of the brain except the ol- 

 factory bulb. There is a wider and shallower postoptic recess 

 below the chiasma ridge (fig. 3, r.o.) which, however, is not 

 externally evident in a lateral view of the brain. The postoptic 

 recess is bounded behind by a groove on the ventral surface of the 

 brain, which is not externally visible because it is covered by the 

 pars glandularis of the hypophysis. It is a conspicuous object 

 in the medial section of the brain (fig. 3, com.pri.), where its floor 

 is seen to be occupied by the commissura preinfundibularis (p. 

 653) . In the floor of the ventral groove which marks the rostral 

 boundary of the mammillary body there is also a commissure, 

 the commissura postinfundibularis (fig. 3, com.pi.). 



The corpus mammillare is a deep evagination of the massive 

 brain wall (fig. 2, c.mam.) directed ventralward and backward 

 and containing the recess mammillaris (figs. 3, 4, r.mam.). This 

 structure is unpaired, but asjinmetrical, the right side being 

 much larger than the left. 



Between the postoptic recess and the corpus mammillare the 

 ventral part of the third ventricle is widely dilated laterally to 

 form a thin-walled infundibular sac. This sac forms the most 

 ventral part of the brain, the rostral portion of its walls being 

 concealed in surface views of the brain by the glandular part of 

 the hypophysis. This portion is the pars nervosa of the hypophy- 



