CEREBRUM. 137 



internal, the latter external ; it covers the surfaces of the hemisphere which 

 are infolded, forming convolutions, for economy of space. 



Fissures. 



1. The Fissure of Sylvius is one of the most important ; it is the first to 

 appear in the development of the foetal brain, being visible at about the 

 third month ; in the adult it is quite deep and well marked, running from 

 the under surface of the brain upward, outward and backward, and forms 

 a boundary between the frojiial and temporo-sphenoidal lobes. 



2. The Fissure of Rolando is second in importance, and runs from a 

 point on the convexity near the median line transversely outward and down- 

 ward toward the fissure of Sylvius, but does not enter it. It separates the 

 frontal from the pajietal lobe. 



3. The Parietal fissure, arising a short distance behind the fissure of 

 ^ Rolando, upon the convexity of the hemisphere, runs downward and back- 

 ward to its posterior extremity. 



4. The Parieto-occipilal fissure separating the occipital from the parietal 

 lobes. Beginning upon the outer surface of the cerebrum, it is continued 

 on the mesial aspect downward and forward until it terminates in the calca- 

 rine fissure. 



\ 5. The Calloso-marginal fissure lying upon the mesial surface, where it 

 runs parallel with the corpus callosum. 



Secondary fissures of importance are found in different lobes of the 



^cerebrum, separating the various convolutions. In the anterior lobe 

 are found the pre-central, superior frontal and inferior frontal fissures ; 

 in the temporo-sphenoidal lobes are found the first and second temporo- 

 sphenoidal fissures ; in the occipital lobe, the calca rine and hippo-campal 

 fissures. 



Convolutions. Frontal lobe. 



The Ascending frontal convolution, situated in front of the fissure of 

 - Rolando, runs downward and forward ; it is continuous above with the 

 anterior frontal, and below with the inferior frontal convolution. 



The Superior frontal convolution is bounded internally by the longitu- 

 dinal fissure, and externally by the superior frontal fissure; it is connected 

 with the superior end of the frontal convolution, and runs downward and 

 forward to the anterior extremity of the frontal lobe, where it turns back- 

 ward, and rests upon the orbital plate of the frontal bone. 



The Middle frontal convolution, the largest of the three, runs from be- 

 hind forward, along the sides of the lobe, to its anterior part; it is bounded 

 above by the superior and below by the inferior frontal fissures. 

 J 



