Clark, Comparative Anatomy of the Insula. 85 



that the dorsal and not the lateral surface is homologous to the 

 lateral areas of most mammals and that the plane of the insula 

 is perpendicular to the lateral aspect. Presumably in this brain 

 as in the human the circuminsular fissure is the first developed 

 and in adults is usually complete. In younger specimens 

 however it is quite noticeable that this fissure is interrupted in 

 one or more places as on the lateral surface at the cephalic end 

 of Fig. 32 and at the dorso-caudal boundary of Fig. 33. With 

 the exception of the circuminsular but one fissure of this area 

 appears to be constant and is the only one extending upon the 

 lateral as well as the dorsal surface. This fissure is continuous 

 with the rhinal ; in it lies the largest branch of the medicere- 

 bral artery which passes dorso-mesad into the Sylvian fissure ; 

 it coincides throughout most of its course with the direction of 

 the Sylvian, divides more or less completely the area into a 

 preinsula and a postinsula and is the fissure which, in the dis- 

 cussion of the lateral surface, was called the transinsular fissure. 

 This branch of the medicerebral artery after traversing nearly 

 two thirds of the length of the transinsular from the lateral sur- 

 face mesad suddenly leaves it and passes up into the Sylvian 

 fissure, thus giving the appearance as if the nearly equilateral 

 triangular caudo-mesal portion of the caudal gyre of the pre- 

 insula had grown caudo-ventrad of the artery. Presumably 

 what has happened is that a displacement of the Sylvian fissure 

 has occurred due to a greater development of the temporal or 

 occipital lobes than the frontal and parietal ; that this growth 

 in an inclosed space has resulted in carrying cephalad the dor- 

 sal end of the Sylvian and with it the artery, thus removing 

 the artery from the mesal end of the transinular, where presum- 

 ably it originally was lodged. The vertical position of the Syl- 

 vian fissure would seem to indicate that such a displacement 

 had occurred. The relation of the Sylvian to the transinsular 

 fissure is very striking when compared with that in the raccoon, 

 where the Sylvian was mostly caudad of the transinsular. In 

 two instances only have fissures other than those above men- 

 tioned been seen on the lateral surface of the sheep brain and in 

 each case, as might be expected, they were in the preinsula. 



