190 ON THE BRAIN OF THE SIAMANG. [Mar. 6, 



the left side it plainly stops short some little way above the 

 Sylvian fissure ; on the right side a faint groove continues it into 

 that fissure. Mesially neither fissure dips into the inter-cerebral 

 sulcus. That of the right side comes a little closer than does that 

 of the left ; and it is curious that this detail is repeated in my 

 brain of H. Twolock. The Assure of Rolando has no branches along 

 its course, and is entirely unrelated to other fissures. 



The hemisphere measures along its curvature 3| inches, and the 

 fissure of Rolando arises 2| inches from the anterior end. It is 

 thus considerably behind the middle of the cerebrum. In H. hoo- 

 lock, which has a longer brain, the corresponding figures are 3| 

 and 2|. 



The frontal lobes are thus larger in H. syndaetylug ; they have 

 a comparatively smooth appearance as in H. hoolocl: 



Of the furrows traversing this lobe I recognize the praecentralis 

 superior, the frontalis superior, and the frontalis inferior. 



The prcecentralig superior is deeply cut but not extensive. On 

 the left side it begins by being parallel to t':e, fissure of Rolando, 

 but ultimately bends much more forward. It is not connected 

 with the frontalis superior. On the right side the fissure is more 

 "normal" in direction, and is connected with the second fissure 

 referred to. It may be that the anterior half — the forwardly 

 directed portion — of the supposed pnecentralis is really the base 

 of the frontalis superior : but I think that it is not for the 

 following reasons : intermediate conditions are seen in two other 

 Gibbons' brains in my possession. In one (II. hudseus) the two 

 praxentralis fissures are quite parallel with the fissure of Rolando ; 

 in the other brain (//. hoolocl-) both fissures have so diverged 

 from the normal (?) that they are almost parallel to the inter- 

 cerebral sulcus. 



The frontalis superior is represented by detached tracts which 

 are deeply excavated. 



As is the case with the other two Gibbons' brains which 

 I have examined, the frontalis inferior is a very strongly marked 

 and long fissure. On the right side this fissure was forked 

 posteriorly, and it is quite likely that this region really represents 

 the praecentralis inferior, well developed in the other Gibbons, and 

 on both sides. 



Fntra-parietal fissure. — As can be seen from the drawingexhibited 

 (fig. 2, p. 189), this fissure is very much the same on both sides of the 

 brain ; that portion of the complex furrow termed by Dr Cunning- 

 ham postcentrals superior is not well developed and is detached 

 from the rest. It was also detached though very well developed 

 in II. leuciseus ; in //. hoolocl- it was perfectly confluent, the whole 

 fi-vsure being of the characteristic T-shape. I lay no stress upon 

 these differences, which are in all probability individual. They 

 only offer additional evidence of the unreliable nature of cerebral 

 tissures for systematic purposes. 



