FORM AND DIVISIONS OF THE CEREBRUM. 531 



divides into two branches, anterior and posterior. It lodges the trunk and 

 primary divisions of the middle cerebral artery, and at its commencement 

 presents a spot pierced by numerous small arterial branches, and thence 

 named the locus perforatus anticus. 



The surface of the hemispheres is composed of grey matter, and is moulded 

 into numerous smooth and tortuous eminences, named convolutions, or gyri, 

 which are marked off from each other by deep furrows, called sulci, or 

 anfractuosities. 



Fig. 3G3. 



Fig. 363. LATERAL VIEW OF THE RIGHT CEREBRAL HEMISPHERE (from Sappey 

 after Foville). 



1, fissure of Rolando ; 2, anterior ascending parietal convolution ; 3, frontal convolu- 

 tions connected posteriorly with the anterior ascending parietal ; 4, union of two frontal 

 convolutions ; 5, posterior ascending parietal convolution ; 6, another parietal convolution 

 similarly Connected with those on the inner surface ; 7, 7, anterior part of the convolu- 

 tion of the fissure of Sylvius; 8, 8, horizontal part of the same convolution; 9, 9, 

 posterior part ; 10, 11, 12, anterior, middle, and posterior principal convolutions of the 

 island of Reil or central lobe ; 13, supraorbital convolution ; 14, part of the temporal 

 lobe ; 15, occipital lobe. 



CEREBRAL CONVOLUTIONS. The convolutions are covered closely through- 

 out by the vascular investing membrane, the pia mater, which sends pro- 

 cesses down to the bottom of the sulci between them, while the serous 

 covering, the arachnoid membrane, passes from one convolution to another, 

 over their summits and without dipping between them. The sulci are 

 generally from half an inch to an inch in depth ; but in this respect there 

 is much variety in different brains, and in different parts of the same brain ; 

 those upon the outer convex surface of the hemisphere being the deepest. 

 In general, the depth of a convolution exceeds its thickness ; and its thick- 

 ness, near the summit, is somewhat greater than through its base. 



Since the external grey or cortical substance is continuous over the whole 

 surface of the cerebral hemispheres, being found alike within the sulci and 

 upon the gyri, a far greater extent of grey matter is thus exposed to the 

 vascular surface of the pia mater with a given size of the brain, than could 

 have been the case had the hemispheres been plain and destitute of 

 convolutions. 



The general arrangement of the convolutions has been made the subject 



