CEREBRAL CONVOLUTIONS. 633 



volution, is one of the most distinct and symmetrical convolutions in the 

 whole brain. Commencing on the under surface of the brain, immediately 

 before the anterior perforated space, it ascends a short distance in front of 

 the anterior recurved extremity of the corpus callosum, and then runs back- 

 wards immediately above that body, as far as its posterior extremity : there 

 it turns downwards and forwards, embracing the cerebral peduncle, to reach 

 the entrance of the Sylvian fissure. This long convolution, therefore, describes 

 a sort of arch or ring, open or interrupted opposite the Sylvian fissure, and 

 embracing the corpus callosum above, and the cerebral peduncle below. It 

 thus, as was pointed out by Foville, forms a sort of rim or border to the 

 grey matter ; whence it is named by him convolution d'ourlet. The surface 

 of this convolution, especially towards its inferior termination, is covered by 

 a very thin cribriform layer of white substance, which, with the grey matter 

 beneath, gives the surface a mottled aspect. This has been called the reti- 

 culated white substance. 



The marginal convolution of the longitudinal fissure is a large convolution 

 which may be traced, more or less indented or interrupted however in its 

 course, along the line of junction between the convex and flat surfaces of 

 the hemisphere, forming the lip of the great longitudinal fissure. Ifc com- 

 mences on the under surface of the brain, in common with the gyrus 

 fornicatus, and, passing forwards, forms the inner border of the triangular 

 orbital surface of the anterior lobe. In this part it is cleft longitudinally 

 by a deep sulcus, into which the olfactory bulb is received, and which, it may 

 be mentioned, is developed at an earlier period than the convolution itself. 

 On the front and upper surface of the cerebrum, this convolution may 

 generally be traced for some distance along the margin of the longitudinal 

 fissure, but soon becomes marked by deep sulci ; and, thus interrupted, may 

 be followed round the posterior extremity, and along the under surface of 

 the hemisphere forwards as far as the point of the middle lobe, running 

 parallel for some space with the under portion of the gyrus fornicatus. Two 

 of the sulci which interrupt the marginal convolution are very constant, 

 viz., the vertical fissure with the fissure of the hippocampi, and the fissure 

 of Rolando. 



The fissure of the hippocampi has a deep origin in the inner margin of the 

 middle lobe of the brain between the fascia dentata and the gyrus fornicatus, 

 and, passing backwards, crosses that gyrus on the under surface of the brain, 

 behind the corpus callosum, and proceeds in a nearly horizontal course along 

 the inner face of the hemisphere. This fissure is important as forming the 

 reverse of the elevations of the hippocampi in the interior of the brain, and 

 as being (according to Reichert) produced at an early period in connection 

 with the general development of the hemispheres, and being comparable 

 therefore rather to the fissure of Sylvius than to a mere sulcus. The part 

 of the gyrus fornicatus beneath this fissure is distinguished as the gyrus 

 hippocampi. 



The fissure of Rolando, starting from behind the vertex, runs outwards and 

 forwards from the longitudinal fissure, so that the right and left grooves 

 form a V-shaped line open in front. It derives its importance from being 

 characteristic of the form of the brain of man and the quadrumana, and 

 separating two considerable convolutions, which extend from the superior 

 longitudinal fissure to the fissure of Sylvius. These convolutions, peculiar 

 to the greater number of simise and attaining their fullest development in 

 man, constitute the anterior and posterior transverse or anterior and 

 posterior ascending parietal convolutions. 



