218 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



this date the human brain presents a marked resemblance in 

 the arrangement of its fissures to that of the dog-like ape 

 (Figs. 178 and 176). 



The fissures and sulci are caused by a rapid increase in number 

 and size of the cortical cells ; the increase of the area of the 

 cortex leads to a crumpling up of its surface. The increased rate 

 of growth appears to affect certain definite areas, hence the fairly 

 constant forms into which the surface of the brain is thrown. 



Affenspalte or Simian Fissure (Fig. 178). — In all ape brains 

 the anterior margin of the occipital lobe grows upwards and 

 forwards as an operculum, which covers the posterior margin 

 of the parietal lobe. The sulcus between the occipital operculum 

 and parietal lobe is the simian fissure or affenspalte. In the 

 human brain it is never developed owing to the great growth of 

 the posterior area of the parietal lobe ; the area which forms the 

 floor of the sulcus in apes is spread out on the surface of the 

 human brain. The ramus occipitalis of the intra-parietal fissure 

 lies in the floor of the simian fissure ; in the human brain the 

 ramus occipitalis is raised to the surface of the brain. 



Sensori-motor Areas of the Brain. — The fissure of Eolando, 

 which begins by two depressions — an upper and a lower — appears 

 in the fifth month ; it divides the sensori-motor areas into 

 anterior and posterior parts. Sherrington and Grunbaum found, 

 however, in their experiments on the brains of anthropoids, that 

 the posterior area gave no reaction when excited artificially, and 

 that the fissure of Eolando formed the posterior boundary 

 of the motor cortex. The anterior area is further sub- 

 divided by two fissures (Fig. 176), the inferior pre-central, an 

 L-shaped fissure which commences as soon or even before the 

 fissure of Eolando, and the superior pre-central, quite a late 

 development, and evidently an isolated part of the superior 

 frontal sulcus. The post-Eolandic area of the sensori-motor 

 cortex is limited behind by the inferior and superior parts 

 of the post-central fissure. The superior part is of late origin. 

 The inferior limb of the post-central fissure is an intrinsic part 

 of the intra-parietal fissure (Fig. 178). From the pyramidal 

 cells of the sensori-motor cortex, processes grow out and form the 

 middle part of the internal capsule. They reach the spinal cord 

 during the 4 th and 5 th months of foetal life and become 



