CHAPTER XXL 

 THE CEREBRUM. 



The cerebrum is the largest portion of the encephalon, constituting 

 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 of 

 nerve fibers, the corpus callosum. Each hemisphere or hemi-cerebrum is 

 convex on its outer aspect, and corresponds in a general way with each side 

 of the cavity of the skull; the inner or mesial surface is fiat and forms the 

 lateral boundary of the longitudinal fissure. 



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

 tions and elevations, known respectively as fissures or sulci, and convolutions 

 or gyri. 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 physiologic processes, clinical phenomena, 

 and surgical procedures. The general arrangement of the primary fissures 

 and convolutions is represented in Figs. 247 and 248. 



Fissures. 



1. The Sylvian fissure, 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 emi- 

 nence of the parietal bone, where it usually terminates in a more or less 

 vertically directed branch, the epi-sylvian branch. Anteriorly a short 

 branch is given off which passes upward and forward into the frontal 

 lobe and known as the pre-sylvian; a horizontal branch is known as the 

 sub-sylvian. The Sylvian fissure is the first to appear in the develop- 

 ment 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 Rolandic or central fissure, equally important, is found on the superior 



and lateral aspects of the cerebrum. It runs from a point on the con- 

 vexity 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 central fissure is such as to form with the longitudinal fissure an angle 

 of about 70 degrees. 



3. The intra-parietal fissure arises a short distance behind the central fissure. 



It then runs upward, backward, and downward to terminate near the 

 posterior extremity of the parietal lobe. It divides the parietal lobe 

 into'a superior and an inferior portion. 



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