THE CEREBRUM. 357 



middle or fundamental portion of the cerebellum; they are> 

 consequently, situated more internally, and are partially con- 

 cealed by the crus cerebelli, and have interposed between them 

 and the latter, the Corpus Rhomboideunvor Dentatum. - ^ 



SECT. V. OF THE CEREBRUM. 



The Cerebrum weighs about three pounds, and is seven times 

 as heavy as the cerebellum. It is ovoidal, and measures about 

 six inches in its antero-posterior diameter, five inches in its 

 greatest breadth, which is behind, and four or five in depth. It 

 is separated above by a deep fissure, (Fissura Longitudinalis,} 

 into two equal parts, called Hemispheres. At the bottom of 

 this fissure, by separating the contiguous surfaces of it, is to be 

 seen a broad lamina of medullary matter passing from side to 

 side, and called the Corpus Callosum, which connects the two 

 hemispheres together. The under surface of each hemisphere 

 is subdivided into three lobes; Anterior, Middle, and Posterior. 



The anterior lobes are placed upon the anterior fossae of the 

 base of the cranium ; the Middle, upon the middle fossae of the 

 same; and the Posterior Lobes rest upon the tentorium. The 

 two anterior lobes are completely separated by the Fissura 

 Longitudinalis, which extends between them to the base of the 

 cranium ; the same is the case with the posterior lobes ; the 

 middle lobes have interposed between them the annular protu- 

 berance and the crura cerebri. When the pia mater is removed, 

 the anterior lobe is seen to be marked off from the middle lobe 

 by a deep sulcus, the fissure of Sylvius, in the under surface of 

 the cerebrum, corresponding* in its position, with the posterior 

 margin of the Lesser Sphenoidal Wing* The boundary between 

 the middle and the posterior lobe is, by no means, well defined 

 on the basis of the brain, but it is agreed to consider as poste- 

 rior lobe, all that part of the hemisphere which rests upon the 

 tentorium. 



The periphery of the cerebrum is formed into convolutions, 

 (Gyri) which give it an unequal tortuous surface, resembling 

 the intestines of a small child. These convolutions are sepa- 

 rated by fissures (Sulci) of depths varying from an inch to two 



