390 PHYSIOLOGICAL SERIES. 



convexity of the hemisphere. The principal sulcus on the 

 lateral convexity is a combination of the Sylvian fissure 

 with the intraparietal sulcus. (To this compound sulcus 

 greater apparent length is given by the appearance of the 

 SyJvian vallecula, which is a deep groove and thus differs 

 decidedly from the shallow form of vallecula met with in 

 Lemur-brains. Unlike the long furrow of the brain of 

 OkrysothrtXj it does not attain the mesial aspect.) 



Anteriorly to this compound fissure the following sulci 

 are found on the frontal part of the hemisphere, viz.: a 

 small depression in a situation similar to that occupied in 

 other Primate brains by the central sulcus, of which the 

 depression is probably a rudiment : near the frontal extremity 

 of the hemisphere is a sagitally-running shallow groove 

 which probably represents the sulcus rectus (inferior 

 frontal) of higher forms ; it is, however, interesting to note 

 that it is a little more mesially placed than is usually the 

 case with the s. rectus * : on the orbital surface of the 

 frontal lobe is a single, sagittally-directed, distinct orbital 

 sulcus ; and on the same surface, under the posterior part 

 of the olfactory peduncle, is a shallow sulcus olfactorius. 

 Behind the combined Sylvian and intraparietal fissures is a 

 distinct parallel sulcus ; above and behind this are one or 

 two indistinct depressions, but none can justifiably be 

 described as genuine sulci (compare fig. 229). 



The calcarine sulcus is Y-shaped rather than T-shaped, 

 the two posterior limbs meeting at an angle of about 60, 

 instead of being in the same straight line. The stem 

 of this Y-shaped sulcus almost reaches the hippocampal 

 fissure. Below the calcarine is a curved collateral sulcus. 

 There is another sulcus behind this; it is placed almost 

 coronally, though it does not run into the calcarine. 

 This somewhat unusual sulcus occurs in both hemispheres 

 of this specimen of N. trivirgatus, but in neither of the 

 hemispheres of the other specimen of N. trivirgatus (D. 560) 

 and in none of the six hemispheres of Aotusfelinus which 

 were available for examination. 



* In other words, the sulcus rectus occupies a progressively lower position 

 in the hemisphere in the evolution of the Ape-brain ; so that it seems to be 

 placed exceptionally high up in those forms where it first appears. 



