644 



THE NERVOUS SYSTEM. 



an inner, flattened surface, which is vertical and directed toward the correspond- 

 ing surface of the opposite hemisphere (the two forming the sides of the longitu- 

 dinal fissure) ; and an under surface or base, of an irregular form, which rests in 

 front on the anterior and middle fossae of the base of the skull, and behind upon 

 the tentorium cerebelli. The hemispheres are composed of an outer stratum of 

 gray matter, called the cortical substance. It is thrown into a number of creases 

 or infoldings, which are termed fissures and sulci, and these separate the surface 

 into a number of irregular eminences, named convolutions or yyri. 



The infoldings or creases are of two kinds, fissures and sulci. The fissures are 

 of large size, and appear early in foetal life ; they are few in number, nearly con- 

 stant in their arrangement, and are produced by infoldings of the entire thickness 



INFERIOR 



FRONTAL 



CONV. 



Fissure of 

 Rolando. 



Parieto-occipital 



fissure. 

 FIG. 343. Upper surface of the. brain, the arachnoid having been removed. 



of the wall of the prosencephalon, producing corresponding elevations in the inte- 

 rior of the ventricle, and hence are termed complete fissures. They comprise (a) 

 the hippocampal, or dentate fissure ; (6) the anterior part of the calcarine fissure ; 

 (c) the collateral fissure. The sulci are more numerous ; they are superficial de- 

 pressions of the gray matter, which is folded inward and only indents the central 

 white substance. They produce no corresponding elevations in the interior of the 

 ventricle, and are therefore spoken of as incomplete fissures. They are fairly con- 

 stant in their arrangement, and have received names indicative of their position 

 and direction, but at the same time vary, within certain limits, in different indi- 

 viduals. They are similar, without being absolutely identical, on the two sides of 

 the brain. It therefore follows that the gyri or convolutions which lie between 

 these sulci are fairly constant in their general arrangement. 



