Clark, Comparative Anatomy of the Insula. 87 



completely crossed the insula but was shallow at the ventral 

 margin. The area is more often incompletely divided by the 

 transinsular fissure which, while it is of considerable length and 

 depth, is almost always confined to the lateral surface. The 

 part most elevated in the five specimens examined lies in the 

 postinsula always directly caudad or caudo-mesad of the dorsal 

 end of the transinsular fissure. What has been assumed as the 

 plane of the insula is at an angle somewhat greater than ninety 

 degrees with the lateral surface. The caudal and caudo- lateral 

 surfaces of this elevated portion slope very abruptly from the 

 apex of this elevated portion to the temporal lobe. The slope 

 toward the mesal boundary is always more gradual and becomes 

 still more so as the cephalic boundary is approached. A cu- 

 rious condition exists at the caudal end of the preinsula where, 

 on the lateral surface, the appearance is as if a quite successful 

 effort had been made to tuck that part caudo-mesad under 

 the postinsula. This is more marked on the left side (see Fig. 

 26) than on the right (Fig. 27), and the crowded appearance of 

 this area seemed more marked in adult than in younger speci- 

 mens. The surrounding lobes appear to be well developed, 

 while the olfactory tract is enormous. The study of this brain 

 seems to confirm the impression that these remarkable features 

 are due to growth in an enclosed space. The insula shows the 

 effects of compression in a marked degree. 



The circuminsular fissure of the right side (Figs. 27-28) sharp- 

 ly defines the insular area except at the temporal lobe where the 

 fissure is interrupted and a junction of the insula with the tem- 

 poral lobe is formed at the lateral surface. This insula consists 

 of seven gyres only one of which belongs to the postinsula. 

 On the left side of the same specimen (Figs. 26 and 29) the 

 circuminsular fissure is also interrupted with the formation of a 

 similar junction as on the right side. Two interruptions of the 

 circuminsular are found at the cephalic margin on the left side. 

 The dorsal surface is divided into four gyres, three of which 

 belong to the preinsula. A zygal fissure^ (Fig. 29, zf) exists near 



1 See Wilder (Ol, 155). 



