LEMUR MONGOZ 29 



Surface Appearance of the Brain in Lemur Mongoz 



THE FISSURAL PATTERN 



Since it is the purpose of this comparative review to deal with the 

 structural evidence of an adaptive unfolding in the brain, it will be impossible 

 to present or discuss the morphological details with that completeness which 

 Jiiight be expected in an atlas on the subject. Especial emphasis is laid upon 

 structural features deemed to possess the greatest evolutional significance. 

 On the other hand, the effort has been made to present the necessary topo- 

 graphical correlations appearing in the several different levels of the brain. 

 Thus, certam prominent features on the external surface of the brain 

 appear to have such decisive evolutional significance as to require extended 

 consideration. Other features are, so to speak, structurally incidental and 

 are mentioned largely for purposes of identification and topography. 



The type of the cerebral hemisphere of Lemur mongoz (Eigs. 16 and 17) is 

 gyrencephalic. It presents a relatively simple fissural pattern. There is a 

 general tendency for the major fissure lines to arrange themselves about the 

 Sylvian sulcus, as is the rule in many of the gyrencephalic mammals of the 

 lower orders. On the other hand, the circumsylvian disposition of the sulci in 

 lemur shows a distinct tendency to depart from that concentric arcuate 

 arrangement so characteristic, for example, of the carnivores. The angulation 

 of the fissure of Sylvius in lemur with the base line of the brain is between s^i^° 

 and 60°. The definite circumferential arrangement of the other fissures which 

 surround the lissure of Sylvius disappears to the extent that there may be 

 found only a remnant of the postsylvian sulcus in the form of the sulcus 

 parallelus or superior temporal fissure. The sulcus lateralis also has lost 

 much of its circumsylvian disposition and now exists as the main portion 

 of the interparietal fissure. A well-defined sulcus in the area of transition 

 between the frontal and orbital surfaces constitutes the fronto-orbital sulcus 



