FISSURES. SL'LCI AND CONVOLUTIONS. 745 



the olfactory lobe from its sulcus, the dorsal ridge will be seen to Olfactory 

 become enlarged at its posterior end, forming the olfactory tubercle. 



POSITION OF THE PART. Now that the base of the cerebrum has Position f 

 Wn studied, the brain should be turned over for the examination of ^mine 

 the upper part. Something should then be placed beneath the fore upper part, 

 part, in order that it may be raised to the same level as the back ; 

 and a rolled-up cloth should loosely encircle the whole, to support 

 the hemispheres. 



THE FISSURES, SULCI AND CONVOLUTIONS OF THE CEREBRAL 

 HEMISPHERE. 



UPPER SURFACE OF THE CEREBRUM. Viewed from above, the Cerebrum is 

 cerebrum is ovoidal in form, and the upper surface is convex in abovefand 

 accordance with the shape of the skull. 



A median longitudinal fissure divides it incompletely into halves, divided into 

 At the front and back the hemispheres are quite separated by it ; but J^S^m* 

 at the middle and under parts they are united by connecting bodies, fissure, 

 the largest of which is the white corpus callosum. The falx cerebri 

 is lodged in the fissure. 



Each hemisphere is larger in front than behind, although the Hemisphere 

 greatest breadth is placed behind the middle. Its outer surface is 

 convex and applied to the skull, and the inner is flat and rests 

 against the falx cerebri. The surface of the hemisphere consists ofe 1 ^ 011 

 grey matter (cortex of the cerebrum), and is marked by tortuous marked by 

 eminences separated by grooves. The eminences are named conrolu- tionVand 

 tions or gyri; the grooves are either fissures or sulci. furrows. 



The grooves are of tv:o kinds. The greater number are superficial Furrows are 

 depressions which carry inwards the grey cortex and only indent the m 

 central white substance, and are called sulci ; a few, however, penetrate 

 more deeply, and are the result of folds involving the whole thickness and 

 of both grey and white substance of the hemisphere, so as to affect the comp e 

 form of the cavity (lateral ventricle) contained within or to give rise 

 to eminences projecting on its wall. These hollows are distinguished 

 as fissures. 



The convolutions and sulci, especially the smaller ones, vary in Convoiu- 

 different brains, and they are not exactly alike even in the two b" s vary> 

 hemispheres of the same cerebrum. Their general arrangement or 

 plan is, however, sufficiently constant, and there will seldom be plan is 

 much difficulty in recognising the several parts referred to in the uni orm ' 

 following description, 



INTERLOBAR SULCI AND LOBES OF THE HEMISPHERE. The outer Division of 

 surface of the hemisphere is divided into regions, or groups of con- sphere 1 " 

 volutions, known as the lobes of the cerebrum, by means of some of 

 the most constant fissures or sulci, aided by lines prolonged from 

 these. The interlobar sulci are three in number, viz. : 



The fissure of Sylvius (fig. 270, s) begins at the vallecula Sylvian 

 Sylvii, whence it extends transversely outwards across the under 

 surface of the hemisphere, separating the frontal and temporal lobes. 



