MESIAL AXD TEXTORIAL SURFACES OF THE HEMISPHERE G89 



The jiars ])arioto-falcit'onijis is usually termed pars temporo-parietalis ; a study of the devel- 

 opuieiit oi'tliis jiait of the island has shown, however, that it is connected with the falciform and 

 not with the temporal lobe. (Cunniniiham.) 



Dissection. — The student is recdmuiended to defer the study of the convolutions on the mesial 

 an<l tentorial surfaces of the hemisphere until the dissection of the corpus callosum is com]ileted, 

 and to procee<l as follows. He should luake a horizontal section through the uppermost part of 

 the cerebral hemispheres ; this will disjilay the central core of white luatter (centrum ovale 

 minus) surrounded by a convoluted border of grey matter. Within the white matter a number 

 of minute vessels will be seen which appear as bleeding points in afresh brain (the puncta vascu- 

 losa vel cruenta). He may then remove successive thin sections until he reaches the level of the 

 corpus callosum, the fibres of whicli, spreading out into the hemisphere, form an extensive white 

 layer called the centrum ovale majus. A far more instructive dissection, however, can be made 

 by cautiously tearing the brain substance in a direction from within outwards until the callosal 

 fissure is reached. By the latter method the following points can be made out in an ordinary 

 well-hardened brain : (1) The white fibres entering each convolution spreading out so as to end 

 lierpendicularly to the .surface. (2) The fibres from the body of the corpus callosum passing at 

 hvxt horizontally outwards into the hemisphere and then diverging, some sweeping upwards to the 

 l)arietal and frontal lobes, others bending downwards into the temporal lobe and following the long 

 axis of that lobe. At the point where they bend downwards they readily break,^ as they are here 

 intersected by fibres passing upwards from the inner cap.sule. (3) The fibres from the anterior 

 anil jiosterior extremities of the corpus callosum passing in a cuiTed manner into the frontal and 

 occipital lobes respectively, forming the forceps minor and the forceps major. (4) A set of 

 fibres, known as the cingulum, which ran within the gyrus fornicatus, forming a sagittally- 

 directed arch. The dissector should next study the corpus callosum and the convolutions on the 

 inner face of the hemisphere. The convolutions on the tentorial .surface may be learned from a 

 mounted specimen, or better still from a cast of a cerebral hemisphere, and the actual convolu- 

 tions can be examined when the hemispheres are cut away from the mesencephalon at a later 

 stage. 



Mesial and tentorial surfaces of the hemisphere. — The mesial surface of 

 the hemif^phere is marked off from both the convex siipero-external and the tentorial 

 surface by the margin of the great longitudinal fissure. This margin follows a 

 curved course as a well-defined border from the posterior extremity of the gyrus 

 rectus as far as the occipital pole of the cerebrum. From this point it passes for- 

 wards and pursues a slightly curved course to the splenium of the corpus callosum 

 immediately beneath which it ends. The latter part is termed by 8chwalbe the 

 internal occipital border ; it is only seen in carefully-hardened brains, and appears 

 as a rounded margin crossing the lobulus lingualis, and in front of this marking 

 the gyrus fornicatus immediately above its isthmus (fig. 411). The tentorial 

 surface is marked off from the supero-external surface by a well-defined border 

 which extends from the occipital pole to the extremity of the temporo-sphenoidal 

 k)be. forming a slight curve with the convexity upwards. The fissural system of 

 tlie mesial and tentorial surfaces is of considerable importance and interest, as all 

 the complete fissures of the cerebrum appear on these surfaces. These fissures are, 

 as already mentioned, the internal parieto-occipital. the calcarine, the dentate, and 

 the collateral. The calloso-marginal, a constant and important incomplete fissure, 

 is also to be .seen on the mesial surface. 



The calloso-marginal fissure commences below the genu of the corpus callosum 

 aufl pursues a curved course parallel to the corj)Us callosum until it reaches a point 

 a short distance behind the upper extremity of the fissure of Rolando. It then 

 bends upwards and extends on to the convex surface for a short distance, where it 

 indents the upper end of the ascending parietal convolution. Tavo branches which 

 pass from this fissure deserve special mention as. although not constantly present, 

 they form useful lines of demarcation.. One of these, the sulcus paracentralis, 

 ]>asses upwards between the paracentral and the marginal convolutions. Another, 

 the post-limbic or sulcus subparietalis, passes backwards from the bend of the 

 main fissure and se}>aratrs tlio in-ieeuneus from the gyrus fornicatus. 



The parieto-occipital fissure commences at the isthmus of the gyrus fornicatus, 

 and passes at first backwards and upwards, and then almost vertically upwards to 

 reach the margin of the hemisphere, where it bends outwards and ends as the 

 external ])arieto-occipital fissure. 



The calcarine fissure commences by a bifid extremity near the occipital pole, 

 and passes horizontally forwards with a sliglitly curved course to become confluent 

 \vith the parieto-occipital fissure. 

 44 



