NERVOUS SYSTEM. VERTEBRATA. 411 



in the pyriform lobe producing the vallecula Sylvii. The 

 part of the pyriform lobe in front of this vallocula ha> 

 become greatly reduced and would not be recogni/al>lr a- 

 such if it were not for the presence of the distinct olfactory 

 tract extending from the olfactory peduncle to the posterior 

 part of the lobe. The anterior rhinal fissure has vani-hrd. 

 The posterior rhinal fissure, however, forms a very distinct 

 incisura temporal is, which constitutes the lateral boundary 

 of the pyriform lobe. 



The Sylvian fissure appears to begin upon the base of the 

 brain as a lateral extension of this vallecula Sylvii of the 

 pyriform lobe. It then extends obliquely upward and 

 backward as a deep cleft upon the lateral aspect ; its dorsal 

 extremity almost reaches the parallel sulcus, from which it 

 is separated by a very narrow gyrus, which is often sub- 

 merged so that the Sylvian fissure appears to join the 

 parallel sulcus in a superficial view. 



If the lips of the Sylvian fissure be separated or cut 

 away (as has been done in the right hemisphere) a large 

 triangular depressed area the insula * will be found, over- 

 lapped chiefly by the dorsal (or fronto-parietal) operculum. 

 The dorsal limiting sulcus of the insula probably represents 

 the suprasylvian sulcus of other mammals (which in the 

 Primates forms the chief and most stable factor in the 

 complex called " Sylvian fissure "). When this fissure is 

 opened up in a Macaque, the anterior extremity of this dorsal 

 limiting sulcus will be found to extend forward into the deep 

 surface of the operculum just external to the junction of Syl- 

 vian vallecula and fissure. In the Gibbons and higher Apes 

 this limiting sulcus extends still further forward and becomes 

 visible. The anterior part of the fronto-parietal operculum 

 which borders on this upturned extremity of the limiting 

 sulcus, meets in the human brain another lip which arises 

 in front of the insula and forms the anterior limb (or limbs) 

 of the Sylvian fissure. In the brains of the Macaques 

 there is no trace of this anterior operculum (or opercula) ; 

 but in this brain there is a well-defined, simple, linear 



* It must, however, be remembered that in the Apes the anterior part of 

 the insula is not submerged but extends forward to the fronto-orbital sulcus. 



