CHAPTER XIX. 

 THE CEREBRUM. 



The cerebrum is the largest portion of the encephalon, constitut- 

 ing about 85 per cent, of its total weight. In shape it is ovate, convex 

 on its outer surface, narrow in front and broad behind. It is divided 

 by a deep longitudinal cleft or fissure into halves, known as the 

 cerebral hemispheres. The hemispheres are completely separated 

 anteriorly and posteriorly by this fissure, but in their middle portions 

 are united by a broad white band, the corpus callosum. Each 

 hemisphere or hemi-cerebrum is convex on its outer aspect, and 

 corresponds in a general way with the cavity of the skull; the inner 

 or mesial surface is flat and forms the lateral boundary of the longi- 

 tudinal fissure. 



The surface of each hemi-cerebrum presents a series of alternate 

 indentations and elevations, known respectively as fissures or sulci, 

 and convolutions or gyres. A knowledge of the situation and extent 

 of the principal fissures and convolutions, as well as of their relation 

 one to another, is essential to a clear understanding of many phys- 

 iologic processes, clinical phenomena, and surgical procedures. 

 The general arrangement of the primary fissures and convolutions 

 is represented in Figs. 220 and 221. 



Fissures. 



1. The fissure of Sylvius, one of the most important of the primary 



fissures, is found on the side of the cerebrum. It begins at the 

 base and extends upward, outward, and backward to a point 

 corresponding to the eminence of the parietal bone, where it 

 usually terminates. Anteriorly a short branch is given off 

 which passes upward and forward into the frontal lobe. The 

 Sylvian fissure is the first to appear in the development 

 of the fetal brain, becoming visible at the third month. In the 

 adult it is deep and well marked and divides the hemi-cerebrum 

 into a frontal and a temporo-sphenoidal lobe. 



2. The fissure of Rolando, or central fissure, equally important, is 



found on the superior and lateral aspects of the cerebrum. It 

 runs from a point on the convexity of the hemisphere near the 

 median line transversely outward and downward toward the 

 fissure of Sylvius, but as a rule does not pass into it. It divides 

 the frontal from the parietal lobe. The inclination of the 



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