THE BRAIN. 415 



from behind the posterior central convolution, and runs backward 

 through the parietal portion of the hemisphere, curving downward 

 toward its posterior extremity. Outside and below it are the arched 

 convolutions about the fissure of Sylvius ; inside and above it is a con- 

 volution running parallel with the great longitudinal fissure. 



Beside the fissures just named there are five others, which, though 

 less strongly marked, are constantly present and show considerable 

 regularity in their position and arrangement. The first runs parallel 

 with the fissure of Rolando, and a little in front of it; whence it is 

 called the "praecentral fissure." The second runs through nearly the 

 whole length of the frontal lobe, parallel in general direction with the 

 great longitudinal fissure. It divides the upper from the middle portion 

 of the frontal lobe, and is called the " superior frontal fissure." The 

 third is the " inferior frontal fissure," and surrounds the end of the short 

 anterior branch of the fissure of Sylvius. The two remaining fissures 

 of this grade are situated in the temporal lobe, below and behind the 

 fissure of Sylvius, with which they run in a general parallel direction. 



The numerous remaining fissures, which increase to a great extent 

 the convoluted aspect of the cerebral surface, are of secondary import- 

 ance and irregular in location. Some of them run longitudinally along 

 the middle of a convolution, dividing it into two narrower parallel 

 folds ; and some pass transversely between two fissures, across the inter- 

 vening convolution. But if the arachnoid and pia mater be removed, 

 these secondary fissures are found to be merely superficial indentations; 

 not penetrating, like the others, deeply into the brain. 



The principal convolutions on the convexity of the hemispheres are 

 as follows : 



The First Frontal Convolution runs from near the upper end of the 

 fissure of Rolando, forward along the edge of the great longitudinal 

 fissure to the anterior extremity of the frontal lobe, where it bends 

 downward and backward, terminating below in a straight convolution 

 next the median line, resting upon the upper surface of the orbital plate. 

 This convolution is divided and folded in many ways by secondary 

 transverse, oblique, and longitudinal fissures, but its general direction 

 is easily recognized. It is bounded externally by the superior frontal 

 fissure. 



The Second Frontal Convolution runs parallel with the foregoing 

 downward and forward over the anterior and lateral part of the frontal 

 lobe. This is the widest of the three frontal convolutions, and the 

 most abundantly variegated by secondary folds and fissures. It is sepa- 

 rated from the first frontal convolution by the superior frontal fissure, 

 and from the third by the inferior frontal fissure. 



The Third Frontal Convolution is situated at the lower and outer 

 part of the frontal lobe, and curves round the anterior branch of the 

 fissure of Sylvius. It communicates posteriorly with the lower end 

 of the anterior central convolution, and thus contributes to form the 

 operculum. 



