EXTERNAL SURFACE. 523 



cleft the under surface rests in the anterior fossa of the base of the skull, 

 and behind it in the middle fossa, and further back still, on the ten- 

 torium cerebelli. 



The fireat longitudinal fissrire (fig. 372, 1 1), seen upon the upper sur- 

 face of the brain, extends from before backwards throughout its whole 

 length in the median plane, and thus separates the cerebrum, as already 

 stated, into a right and left hemisphere. On opening this fissure, it is 

 seen, both before and behind, to pass quite through to the base of tlie 

 cerelu'um ; but in the middle it is interrupted by a large transverse mass 

 of white substance, named the corpus caJIosum, which connects the two 

 hemispheres together. While the brain is in its natural situation, this 

 fissure is occupied by a vertical process of the dura mater — the falx 

 cerebri — which dips down between the two hemispheres, not quite 

 reaching to the corpus callosum. 



The Surface of the He]\iispheres is composed of grey matter, and 

 is moulded into numerous smooth and tortuous eminences, named con- 

 volutions or gyri, which are marked off ffom each other by deep ftuTows, 

 called sulci, or anfrcictuositics. 



Lobes of the cerebrum. — These are five in number, termed 

 respectively, frontal, imrietal, occipital, temporo-sjjhenoidal and central. 

 The three "former are in contact with the bones after which they are 

 named, though their limits do not correspond to those of the bones ; 

 the fourth occupies the middle or temporo-sphenoidal fossa in the base 

 of the skull, while the central lobe or island of Eeil lies within the 

 fissure of Sylvius. The divisions between these lobes are marked by 

 certain conspicuous fissures and by artificial hues connecting those 

 fissures. Their real limits will be better understood when the convolu- 

 tions and fissures have been described. 



Formerly it was customary to divide each hemisphere into three lol^es, an 

 anterior, in front of the fissure of Sylvius, a middle, behiad that fissui-e and rest- 

 ing ia the temporo-sphenoidal fossa, and a posterior lobe behind it. resting on 

 the tentorium cerebelli. The division into five lobes, now generally adopted, 

 was first made by Gratiolet. 



The Convolutions are covered closely throughout by the vascular 

 investing membrane, the pia mater, which sends processes down to 

 the bottom of the sulci between them, while the serous covering, the 

 arachnoid membrane, passing from one convolution to another, over 

 their summits and without dipping between them. In general, the 

 depth of a convolution exceeds its thickness ; and its thickness, 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 ex- 

 posed to the vascular surfiice 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 Fissures between the convolutions are generally from half an 

 inch to an inch in depth, but vaiy in this respect both in different 

 brains and in different parts of the same brain. In all brains 

 certain primary fissures can be recognised, on which the division into 

 lobes has been founded, and these it will be convenient to describe 

 first. 



