PPvOSENCEPHALON OF MAMMALS. 



135 



121 



On the occipital surface are several but irregular fissures, 

 which, from their position, may be termed mid-, super-, ent-, and 

 post-occipital; they define, more plainly in Quadrumana than 

 in Man, the lambdoidal, fig. 119, p, suroccipital, q", midocci- 

 pital, q f , suboccipital, q"' ', and postoccipital, q*, folds. On the 

 tentorial surface they affect a longitudinal wavy course, and are 

 commonly three rn number; of these, the middle one is the 

 e tentorial' fissure, fig. 120, is, the inner one the ( entotentorial,' 

 ib. is', the outer one the ' ectotentorial,' is". On the surface 

 next the falx, or septum dividing the 

 hemispheres, fig. 121, the fissures have a 

 radiating tendency from the anterior angle 

 outward : the most constant and important 

 of these, in Man, has already received the 

 name of ' posthippocampal,' being a con- 

 tinuation of that deep fissure the corre- 

 sponding fold of which partly protrudes 

 into the posterior horn of the ventricle, 

 as the ' hippocampus minor ;' the rest I 

 called ( septal ' fissures, reserving the 

 term f falcial ' to those on the corre- 

 sponding surface of the anterior cerebral 

 lobe. The fissure above the ' posthippocampal ' is the ' septal ' 

 fissure, 19; that beneath the posthippocampal is the ( subseptal,' 

 i^' ; the fissure between the septal and entolambdoidal, is', fis- 

 sures is the superseptal, 19'; their outer ends are frequently lost in 

 a fissure following more or less extensively or interruptedly the 

 posterior contour of the posterior lobe; this is the postseptal fis- 

 sure, i'/"; it is peculiar to Man. The folds so defined on the sep- 

 tal surface are : the entolambdoidal, p' 9 superseptal, s' ', septal, s, 

 posthippocampal, a', subseptal, s" ', and postseptal, s'". 



The human brain, in its development, passes through stages in 

 some degree like those which are permanent in and characteristic 

 of the Quadrumana, in respect to its cerebral folds and fissures ; 

 but it early manifests its distinctive archencephalous proportions, 

 fig. 109, Foetus. About the twentieth week the fissures begin to ap- 

 pear upon the upper surface of the hemispheres, fig. 116, three 

 months' Foatus. After the ( hippocampal ' and 4 callosal ' have cleft 

 the inner surface, and the ( ectorhinal ' and ( sylvian ' the under sur- 

 face, the entolambdoidal ascends upon the mesial side of the upper 

 surface (fig. 116, 13); the postsylvian, 9, appears; then a faint 

 trace of the longitudinal fissure, fis;. 116, 14', indicative of the 



~ ' O ' J 



midfrontal and ectofrontal tracts. The ( coronal,' fig. 113, 12, is 



Inner or septal surface of posterior 

 lobe, human cerebrum. 



