MESIAL AXD TENTORIAL SURFACES OF HEMISPHERE G91 



Wlien the typical backward turn c^f the upper extremity of the fissure of Rolando is ]jresent, 

 it hooks backwards iiit(j tlie fbrwaid bend formed by the upper extremity of the calloso-marginal 

 fissure, and thus the frontal and jiarietal lobes are only connected by a narrow istlimus between 

 the two fissures. 



The gyrus fornicatus, or gyrus cinguli, Avhich forms part of tlie limbic lobe, 

 commences between the rostrum of the corj^us callosum and the extremity of the 

 marginal convolution, and, }»ursuing an arched course ]»arallel to the corpus 

 callosum, ends in a narrow convolution, the isthmus, which connects it to the 

 uncinate gyrus. It is placed between the calloso-marginal and callosal fissures, 

 and is thus clearly marked off from adjacent structures in the greater part of its 

 extent, but is often very imperfectly separated from the superior parietal lobule. 

 It may also be connected to the hinder end of the marginal convolution by a 

 small gyrus crossing the calloso-maruinal fissure. 



The praecuneus, or quadrate lobule, is a squarish mass of small gyri which is 

 continuous around the margin of the hemisphere with the superior parietal lobule. 

 It is l)ounded, in front by the upturned end of the calloso-marginal fissure; behind, 

 l)y the parieto-occi])ital fissure; and it may be marked off below, from the gyrus 

 fornicatus, by the sulcus subparietalis. 



The cuneus, or cuneate lobule, is a wedge-shaped mass occupying the interval 

 between the internal parieto-occipital and calcarine fissures. It is continuous 

 around the margin of the hemisphere with the superior occipital convolution. In 

 a properly hardened right hemisphere it shows a distinct bevelling near the margin. 

 This is produced by the impression of the superior longitudinal sinus. This 

 bevelling can be traced downwards to the extremity of the lobulus lingualis, and less 

 distinctly forwards to the precuneus. In rare instances it may be seen in the left 

 hemisphere instead of the right. 



Two sagittally directed convolutions are situated on the tentorial surface. They 

 are termed the superior and inferior occipito-temporal convolutions. The 

 superior of these is divided into an anterior part called the uncinate gyrus, and a 

 posterior part which is known as the lobulus lingualis. 



The uncinate convolution commences behind at the isthmus, by which it is 

 continuous with the gyrus fornicatus, and ends immediately behind the anterior 

 perforated space in a hook-like elevation, the uncus. It is separated from the 

 dentate convolution by the dentate fissure, it is marked off from the inferior 

 occi]^it<^)-tem])oral convolution by the collateral fissure and, in front of this, by the 

 incisura temporalis, it forms part of the limbic lobe. The outer part of the 

 anterior end is marked by a distinct gi'oove, which, as it is jiroduced by the free 

 edge of the tentorium cerelDelli, may be called the tentorial groove (fig. 411). 



The uncinate convolution is covered with a peculiar fine network of white fibres, the sub- 

 stantia reticularis alba. These fibres are derived from the stria? longitudinales laterales which 

 l>ass i'roni the splenium of the corpus callosum on to the istlimus, and from thence to the uncinate 

 gyrus. 



The dentate convolution or fascia dentata will be more conveniently described with the 

 anatomy of the lateral ventricle, as it is usually dissected in connection with that cavity. 



Tlie lobulus lingualis is the posterior part of the superior occipito-temporal 

 convolution. It is bounded by the calcarine and parieto-occipital fissures above, and 

 by the collateral fissure below. As the internal occipital border crosses this lobule 

 it divides it into two parts, an upper part which appears on the mesial surface, and 

 a larger loAver portion situated on the tentorial surface. 



The inferior occipito-temporal convolution extends from the apex of the 

 temporal lobe to the occi])ital })olc of the hemis})here. It is l)ounded above by 

 the collateral fissure, and below l)y the inferior temporal and inferior occipital 

 fissures. It is marked by the impressio petrosa at the junction of its anterior and 

 middle thirds. 



Three distinct impressions, which lie in a line with one another, are produced on the brain by 

 the petrous portion of the temporal bone. One of these (fig. 410, //) corresponds to the jwrtion 

 of 1)1 ine opposite the confluence of the .superior petrosal with the lateral sinus. Another, the 

 impressio petrosa, which may be large and conspicuous, is caused by the prominence of the 



