432 PHYSIOLOGICAL SERIES. 



gyms. The upper lip of this sulcus is the more prominent, 

 an indication of its conversion into an operciilum in a 

 higher phase of development. 



The Sylvian fissure has a T-shaped caudal extremity. 

 The long central (Rolando's) sulcus has a slight, genii : 

 it does not attain to the mesial surface. 



The inferior precentral (arcuate sulcus) is a deep irre- 

 gularly-triradriate sulcus, the inferior limb of which is 

 superficially joined to the Sylvian sulcus by the inferior 

 transverse sulcus. The gyrus which separates these sulci is 

 wholly submerged on the left, and only partially so on the 

 right hemisphere In calling this sulcus " arcuate " it must 

 be borne in mind that in the earliest phase of its history 

 (curiously reproduced again in the Gibbons) the arcuate 

 and straight (rectus) sulci are one, and later this single 

 sulcus becomes broken up into the two distinct elements, 

 which acquire a very questionable individuality, so that 

 one must not imagine that the sulci called respectively 

 arcuatus and rectus in a higher Ape correspond exactly to 

 those similarly named in the Macaques and Capuchin 

 Monkeys, even if it be admitted that the complex of the 

 two sulci in the Anthropoid represents the two elements of 

 the lower Apes. At the same time it often happens in 

 the case of the Anthropoid Apes, and even in Man, that 

 the exact forms of arcuate (inferior precentral) and straight. 

 (inferior frontal) sulci which occur in Macacns and Ct-lnm 

 may be developed. 



In the case under consideration, I am inclined to believe 

 that the sulcus arcuatus has become deeper and longer at 

 the expense of the sulcus rectus, which is a diminutive 

 element (triradiate on the left, arcuate on the right side) 

 in the position where the sulcus fronto*marginalifl of 

 Wernicke is found in the human brain. 



On the right hemisphere it is joined to the irregularly 

 linear orbital sulcus, the posterior part of which i.- al-<> 

 linked to the fronto-orbital by a transverse sulcus. 



Tin's soine\vliat lengthy digression lias l>en int rod need in 

 order to render intelligible the status of a suleus which 

 makes its appearance, external to the anterior extremity of 

 the sulcus rectus, to which it is obviously compensatory. 



