MEDIAX SURFACE. 



531 



spot. Its anterior convolution is continuous at ths bottom of the 

 limiting sulcus with the adjacent posterior orbital ; its posterior con- 

 volution joins the teraporo-sphenoidal lobe. Externally it is separated 

 by a deep sulcus fi'om the contiguous convolutions of the operculum, 

 i. e., the extremity of the ascending parietal, ascending frontal, and 

 inferior frontal convolution. The island of Eeil covers the extra- 

 ventricular nucleus of the corpus striatum. It appears earlier than 

 any other division of the cerebrum, both in the foetus and in the animal 

 series. 



The Internal oh Mediae and Tentorial Surface (fig. 373), as 

 already said, does not present the same division into lobes as does 

 the external surface. 



Fissures. — The internal part of the parkto-occipital fissure is con- 

 tinuous, at the margin of the longitudinal fissure, with the external por- 

 tion. Thence it extends downwards and forv/ards, presenting a slight 

 curve with the convexity forwards, and blends with the calcarine fissure. 



The calcarine fissure commences at the posterior extremity of the 

 hemisphere, usually in a bifurcated manner, and extends forwards to 

 terminate beneath the posterior extremity of the corpus callosum. It 

 is joined by the parieto-occipital fissure. 



The calloso-marpnal fissure {c mf) commences beneath the anterior 



Fig. 374. 



Fig. 



i. — Right half op the Brain divided by a Vertical Antero-Posteriob 

 Section (from various sources and from nature). (Allen Thomson.) ^ 



po, parieto-occipital fissure ; cc, posterior extremity of calcarine fissure ; 1 , 1, marginal 

 convolution ; 2, 2, gyrus fornicatus ; 3, 3, 3, secondary convolutions passing between 

 this and the preceding ; 3a, quadrate lobule ; 36, cuneate lobule ; 4, the fifth ventricle, 

 and above it the divided corpus callosum ; 5, the third ventricle (see fig. 384) ; 5', 

 pituitary body ; 6, corpora quadrigemina and peneal gland ; + , the fourth ventricle ; 

 7, pons Varolii ; 8, medulla oblongata ; 9, cerebellum i, the olfactory bulb ; li, the 

 right optic nerve, the commissure cut through ; iii, right third nerve. 



M At 2 



