572 TEXT-BOOK OF PHYSIOLOGY 



6. The par a- central fissure begins at the supero-mesial border of the hemis- 



phere. It then passes downward and forward for a variable distance 

 and then turns upward enveloping a lobule known as the para- 

 central lobule. 



7. The super-callosal fissure extends from a point just anterior to the para- 



central lobule downward and forward below the rostrum of the corpus 

 callosum. 



Secondary fissures of more or less importance are present in the different 

 lobes, subdividing the surface into convolutions: e.g., in the frontal lobe are 

 found the pre-central, the super -frontal, medi-frontal and sub-frontal fissures; 

 in the temporal lobe the super-temporal and medi-temporal fissures. 



Convolutions. The convolutions or gyri are the portions of the cere- 

 bral surface comprised between the fissures. The arrangement of the sur- 

 face is such that only the more superficial portions are visible. The depth 

 of the convolution, the portion bordering the fissure, is concealed from view. 

 Each lobe presents a series of such convolutions which differ considerably 

 in their relative physiologic importance. 



The Frontal Lobe. The frontal lobe presents on its convex surface 

 four convolutions, viz. : the anterior or pre-central convolution, and the 

 super-, medi-, and sub-frontal convolutions. 



1. The anterior or pre-central convolution or gyrus is situated just in front oi 



the Rolandic or central fissure, with which it corresponds in direction. 

 It is continuous above with the super-frontal and below with the sub- 

 frontal convolution. 



2. The super -frontal convolution or gyrus is bounded internally by the longi- 



tudinal fissure and externally by the super-frontal fissure. From the 

 upper end of the pre-central convolution, with which it is continuous; 

 it runs forward and downward to the anterior extremity of the frontal 

 lobe, where it turns backward and rests on the orbital plate of the 

 frontal bone. 



3. The medi-frontal convolution or gyrus is situated on the side of the lobe 



between the super-frontal fissure above and the medi-frontal fissure 

 below. Its general direction is downward and forward. 



4. The sub-frontal convolution or gyrus winds around the pre-sylvian brand 



of the fissure of Sylvius in the anterior and inferior portion of the fronta 

 lobe. It is continuous posteriorly with the lower end of the pre-centra 

 convolution. 



The Parietal Lobe. The parietal lobe presents three well-markec 

 convolutions, viz. : the posterior or post-central convolution, and the super 

 and sub-parietal. The latter is again subdivided into the marginal and the 

 angular convolution. 



1. The posterior or post-central convolution or gyrus is situated just behind tht 



Rolandic or central fissure, with which it corresponds in direction 

 Above, it is continuous with the super-parietal convolution; below, witl 

 the marginal and the pre-central convolutions. 



2. The super-parietal convolution or gyrus is bounded internally by the longi 



tudinal fissure and externally by the intra-parietal fissure. From th< 

 upper end of the post-central convolution, with which it is connected 

 it runs downward and backward as far as the parieto-occipital fissure 



3. The sub-parietal convolution or gyrus is connected anteriorly with th 



