THE CEREBRAL HEMISPHERES. 137 



EXTERNAL CONFORMATION OF THE CEREBRAL HEMISPHERES. 



The cerebral hemispheres together form an ovoid mass, flattened on its under 

 side, and placed in the cranium with its smaller end forwards, its greatest width 

 being opposite to the parietal eminences. The hemispheres are separated in a large 

 part of their extent by a deep fissure, the great longitudinal fissure. On opening 

 this fissure by drawing asunder the two hemispheres, it is seen, both before and 

 behind, to pass quite through to the base of the cerebrum ; but for 10 centimeters 

 in the middle it is interrupted at a depth of about 30 mm. at the front to 40 mm. 

 at the back by a large transverse mass of white substance, named the corpus callosum, 

 which connects the two hemispheres together. While the brain is within the skull, 

 the longitudinal 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 in front but touching it behind. 



Each cerebral hemisphere has an outer, convex surface, in contact with the vault 

 of the cranium ; an inner or mesial, flat surface, which forms one side of the 

 longitudinal fissure ; and an irregular under surface, in which is a deep cleft, the 

 vallecula Sylvii. In front of this cleft the under surface (orbital division) rests in 

 the anterior fossa of the base of the skull, behind it in the middle fossa (temporal 

 division), and further back still, on the tentorium cerebelli (occipital division). The 

 surfaces pass into one another at the borders, which are also three in number, viz. : 

 two mesial (superior and inferior) and one lateral. The anterior extremity of each 

 hemisphere is known as the frontal pole, the posterior as the occipital pole. 



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 flssures or sulci of varying depth. 



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 arachnoid membrane passes from one convolution to another, 

 without dipping between them. In general, the height of a convolution exceeds its 

 width ; and its width at the surface is somewhat greater than at its base. The 

 convolutions present considerable variations of position, direction and complexity in 

 the brains of different individuals, and even in the two sides of the same brain, but 

 they are usually recognizable without much difficulty. It is far easier, however, to 

 trace the more important fissures and convolutions on the brain of a monkey, where 

 they are much less obscured by tortuosities and secondary sulci. 



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, it 

 is obvious that a far greater extent of grey matter is thus provided for with a given 

 size of the brain, than could have been the case had the hemispheres been plain and 

 destitute of convolutions ; indeed, it is calculated that the extent of " sunken " 

 surface is twice that of " exposed" surface (cf. pp. 176, 177). 



The sulci between the convolutions vary greatly in depth, being in some parts 

 quite shallow, in others as much as 25mm. (1 inch) or more deep. The average 

 depth is about 10mm. or 12mm. Certain well-marked sulci can easily be recognized, 

 and these serve as guides to the position of others which may at first be less readily 

 detected. 



It may be well in the first instance to describe the six chief or " interlobar " 

 fissures, so-called because they are used to mark the hemisphere off into lobes ; ihe 

 lobes themselves, with the convolutions and fissures which are included within 

 them, can then conveniently be taken in order. 



