282 ANATOMY OF THE CENTKAL NEETOUS, SYSTEM. 



nected with either the one or the other part of the hypophysis. The accompanying 

 sagittal section through the hypophysis of a human embryo of four months shows all 

 three parts very plainly. 



The optic tracts pass in a broad curve around the infundibulum and 

 over the peduncles in a direction toward the pulvinar of the thalamus. 

 Concealed on both sides by the lobus temporalis, they curve upward and 

 outward around the origin of the peduncles, the crura cerebri, to attain the 

 corpus geniculatum laterale and the pulvinar. 



Anteriorly, in front of the infundibulum, the tracts unite to form the 

 chiasma, from which, after the decussation of a part of their bundles, the 

 optic nerves proceed. 



In front of the optic tract and lateral to the chiasma there lies, just 

 under (above, when examining the brain from the base) the anterior part 

 of the corpus striatum, the suhstantia perforata anterior, a gray mass that 



jPflrj- post.. . Ifi'u:,',!i, Js'-/*>vof? 

 Pars intermf^l' 

 F-rs ant. 



Cr aniiim. 



Fig. 181. — Sagittal section through the brain-floor and the hypophysis of a 

 human embryo of four months. Combined from three consecutive sections. 



is pierced by numerous vessels from the pia. The region of the loTjus olfac- 

 torius begins in front of it. 



The substantia perforata anterior is nothing else but the olfactory field, 

 or area, which has become greatly atrophied in man. In Primates the lobus 

 olfactorius is also atrophied along with the entire olfactory apparatus. In 

 man only the most posterior part of it has been retained with its cortical 

 structure (Figs. 172 and 173); the anterior part has diminished to an insig- 

 nificant, gray cord, the tractus olfactorius, on which anteriorly is found the 

 small bulbus olfactorius. From the bulbus, however, there still arise, pre- 

 cisely as in the other vertebrates, the large fiber-tracts of the olfactory radi- 

 ation. Since these pass backward to the cortex of the lobulus olfactorius 

 and the region of the gyrus hippocampi, they must pass over the tractu= 

 olfactorius, giving a white appearance to its under side. After reach inu' 

 the olfactory field (the substantia perforata anterior) the tracts, whicli horc. 



