OUTER CEREBRAL CONVOLUTIONS. 195 



front and behind by the lirnbs of the Sylvian fissure, and externally by a 

 deep groove separating it from the frontal and parietal lobes. It is of a 

 triangular form with the apex down. 



CONVOLUTIONS OF THE CEREBRUM. In different brains the convolu- 

 tions vary slightly in form, and even in the two hemispheres of the same 

 cerebrum they are not exactly alike : but there is always similarity 

 enough for the recognition of the chief eminences. Each lobe possesses 

 convolutions, but these run into each other by means of smaller gyri, 

 either on the surface of the brain or at the bottom of the sulci ; and the 

 student may experience some difficulty at first in defining the limits of 

 each. It is in the smaller gyri that the greatest variation will be found. 



A. CONVOLUTIONS OF THE OUTER SURFACE. About the middle of the 

 hemisphere are two straight vertical convolutions, one on each side of the 

 fissure of Rolando, R, which will serve as a starting point. In front of 

 those two the convolutions are longitudinal ; and behind they take an 

 oblique direction to the back of the brain. 



a. The frontal convolutions (fig. 60) form two sets, one on the outer, 

 the other on the under surface of the frontal lobe : those on the outer 

 aspect are four, viz., one vertical or posterior, and three longitudinal or 

 anterior, as follows : 



The ascending frontal ( 4 ) is the vertical, straight convolution, which 

 bounds in front the Rolando fissure. It reaches down from the median to 

 the sylvian fissure (posterior limb). Along the anterior border it is joined 

 by the three frontal convolutions ; and below it unites with the most ante- 

 rior convolution of the parietal lobe round the lower end of the fissure of 

 Rolando, R. 



The three longitudinal frontal convolutions are much subdivided and 

 blended, and are separated by two intervening sulci. They are named 

 superior ( 3 ), middle (*), and inferior (*) : they communicate behind by 

 secondary gyri with the ascending frontal ( 4 ), the highest having often two 

 processes ; and are directed forwards one outside another to the anterior 

 extremity of the hemisphere. 



The under or orbital surface of the frontal lobe, concave, is represented 

 in fig. 61. Near the inner margin is a sulcus, the olfactory, lodging the 

 olfactory nerve ; and internal to it is the lower end of the marginal convo- 

 lution ( 1? ). External to the sulcus lies a convolution, which is pointed 

 behind, but widened and subdivided' in front, and incloses smaller gyri 

 and sulci within its coil : this has been subdivided into three parts, an 

 inner (*), a posterior ( 3 ), and an external ( 6 ). 



b. The parietal convolutions (fig. 60), like the outer frontal, are four in 

 number ; an anterior, or ascending, which is vertical along the fissure of 

 Rolando, and three directed back from it. 



The ascending parietal ( 5 ), narrow and straight, limits behind the fissure 

 of Rolando, and reaches from the middle line to the hinder limb of the 

 Sylvian fissure, S. Above, it runs into the parietal lobule, 5 ' ; and below, 

 it joins the ascending frontal round the lower end of the fissure of Rolando. 

 Behind it is separated from the other gyri of the parietal lobe by a suclus, 

 IP. 1 



The parietal lobule ( 5/ ) appears to be an appendage to the upper end of 



1 The intraparietal sulcus (IP, fig. 60, Turner), is placed between the ascending 

 parietal and the supra-marginal convolution, A. Above, it is directed back near 

 the upper part of the hemisphere, separating the parietal lobule (5') and the 

 supra-marginal convolution, A. 



