144 



THE CEREBRAL HEMISPHERES. 



which intervenes between the parietal and occipital lobes, the cleft tending obliquely 

 backwards, so that the occipital edge somewhat overlaps the parietal (occipital 

 operculum). This " Affenspalte ," is not represented in the adult human brain, but it 

 is apparently represented by a temporary fissure which is seen in the foetus during the 

 fifth and sixth months (external perpendicular fissure of Bischoff). 1 The parieto- 

 occipital fissure appears about the fourth or fifth month on the mesial surface. It is 

 about on a level with the lambda (see Vol. II., p. 83), or a little in front of the 

 level of that spot : more so in the child than in the adult (Cunningham). 



The calcariue fissure (Huxley). This is seen on the mesial surface of -the 

 hemisphere (fig. 102). It is a deep fissure, extending from near the posterior 



RuvL. 



.peer. oc.f. 



Pr.L 



Oc.L,. 



Fig.105. BRAIN OPCHIMPANZEE. 

 (From Gratiolet.) 



Fr. L. . frontal lobe ; Par. L. , 

 parietal lobe : Oc.L., occipital 

 lobe ; Temp. Sph. L., temporo- 

 sphenoklal lobe ; Sylv. /., fissure 

 of Sylvius ; f.Sy.a., f.Sy. p., its 

 anterior and posterior limbs ; . 

 f. Rol., fissure of Rolando ; 

 tr. fr. f., transverse frontal (pre- 

 central) fissure ; int. par. f., in- 

 tra parietal fissure ; par. oc. f., 

 parieto-occipital fissure. 



extremity of the brain. 



where it usually begins in 



a T-shaped fork, in a course curved at first upwards and then downwards, and 

 ending below the splenium of tfie corpus callosum. The forked posterior extremity 

 near the occipital lobe is sometimes cut off from the rest of the fissure, and appears 

 as an independent sulcus (sulcus extremus of Schwalbe). In many Quadrumana the 

 anterior extremity appears to be continued into the hippocampal fissure, but this is 

 usually superficial only, and occurs but rarely in the human brain. 



The anterior part of the calcarine fissure is by far the deepest part, and it is this part 

 which indents the posterior horn of the lateral ventricle and forms the calcar avis. A short 

 distance from its anterior extremity the calcarine receives the internal parieto-occipital 

 fissure, although in the depth of the latter fissure there is usually a small gyrus, passing from 

 the apex of the cuneus to the isthmus of the gyrus fornicatus (opposite 1. fig. 102) (cuneo- 

 limbic annectent gyrux). This sometimes comes to the surface and shuts off the continuity 

 of the two fissures, which otherwise form a Y-shaped figure, the stem of the Y being formed 

 by the anterior part of the calcarine, the limbs by the parieto-occipital and posterior part of 

 the calcarine respectively. This posterior part of the calcarine is developed independently of 

 the stem, which is a direct representative of one of the total fissures of the foetal hemisphere, 

 while the posterior part of the calcarine (posterior calcarine of Cunningham) is formed much 

 later by two depressions, which ultimately run together and into the true calcarine. The 

 original independence of these parts is indicated by the existence of two annectent gyri 

 (anterior and posterior cunco-linrjuar) concealed within the posterior part of the calcarine : 

 one at its junction with the stem of the Y, and the other near the hinder end. The places of 

 these annectent gyri are indicated by 2 and 3 in fig. 102. 



The parieto-occipital fissure is frequently interrupted in its depth by two annectent gyri. 

 One of these is the cuneo-limbic annectent gyrus already mentioned (fig. 102, 1), the other, 

 placed higher, may be termed the cnneo-qnadrate annectent (fig. 102, at 4). 



The parieto-occipital is preceded in the foetus by a fissure occupying about the same situa- 

 tion, but this usually disappears before the permanent fissure makes its appearance. 



The collateral fissure (Huxley). This is seen on the inferior surface, where 

 it lies below and parallel to the anterior part of the calcarine (figs. 102, 103, coll.}. 

 It extends forwards towards the tip of the temporal lobe, but does not reach the 



1 According to some authorities, the anterior occipital sulcus of the adult brain (see p. 152) repre- 

 sents the Affenspalte of Quadrumana. 



