474: THE BKAIN. 



of secondary importance and more irregular in location, which increase 

 the convoluted aspect of the cerebral surface. Some of them run longi- 

 tudinally along the middle of a convolution, dividing it into two nar- 

 rower parallel folds ; and some of them pass transversely from one of 

 the main fissures to another, appearing to cut across the intervening 

 convolution. But in many instances, if the arachnoid and pia mater be 

 removed, it will be found that these secondary fissures are merely super- 

 ficial indentations on the surface, or furrows for the accommodation of 

 a vascular branch ; and that they do not, like the others, penetrate deeply 

 into the substance of the brain. 



The principal convolutions to be distinguished on the convexities of 

 the hemispheres are as follows : 



The First Frontal Convolution (1) runs from the upper end of the 

 anterior central convolution, just in front of the commencement 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, and resting upon the upper surface of the orbital 

 plate. This convolution is divided and folded in many ways by secon- 

 dary transverse, oblique, and longitudinal fissures, but its general direc- 

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

 frontal fissure. 



The Second Frontal Convolution (2) also takes its origin at the anterior 

 central convolution, from which it is more or less completely separated 

 by the praecentral fissure ; thence running 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 separated from the first frontal con- 

 volution by the superior frontal fissure, and from the third by the inferior 

 frontal fissure. 



The Third Frontal Convolution (3) is situated at the lower and outer 

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

 of the fissure of Sylvius. It communicates posteriorly with the lower 

 end of the anterior central convolution, and by this portion of its sub- 

 stance helps to cover in and conceal the island of Reil at the bottom of 

 the fissure of Sylvius. 



The Anterior Central Convolution (4) runs transversely outward and 

 forward along the front edge of the fissure of Rolando. It is usually 

 a single convolution, but is more or less folded laterally by transverse 

 indentations. It communicates with the first frontal convolution above 

 and with the third frontal convolution below. It also curves round the 

 lower end of the fissure of Rolando, to unite with the following convo- 

 lution, which may be said to be a continuation of it. 



The Posterior Central Convolution (5) also runs parallel with the fissure 

 of Rolando, but behind it. Above, it turns backward and is continuous 

 with the convolutions of the upper part of the parietal lobe. 



