THE WHITE MATTER OF THE CEEEBRAL HEMISPHERES. 647 



direction, nearer the anterior end of the hemisphere than the posterior end, is 

 the stem of the lateral fissure. This deep cleft divides the inferior surface into an 

 anterior or orbital area, which rests on the orbital part of the frontal bone and is 

 consequently concave from side to side, and a more extensive posterior or tentorial 

 area, which lies on the floor of the lateral part of the middle cranial fossa and upon 

 the superior surface of the tentorium cerebelli. This surface is arched from before 

 backwards, and looks medially as well as downwards. In its posterior two-thirds it 

 lies above the cerebellum, from which it is separated by the tentorium cerebelli. 



The borders which intervene between these surfaces are the supero-medial, the 

 superciliary, the infero-lateral, the medial occipital and medial orbital. The supero- 

 medial border, convex from before backwards, intervenes between the convex lateral 

 surface and the flat medial surface of the hemisphere. The superciliary border is 

 highly arched and separates the orbital surface from the lateral surface. The 

 infero-lateral border marks off the tentorial surface from the lateral surface. The 

 medial occipital border can be seen only in cases where the brain has been hardened 

 in situ and faithfully retains the natural form. It extends from the posterior end 

 of the hemisphere towards the posterior extremity of the corpus callosum, and inter- 

 venes between the medial and tentorial surfaces. It is the border which lies along 

 the straight blood sinus, and it therefore occupies the angle which is formed by the 

 attachment of the posterior part of the falx cerebri to the superior surface of the 

 tentorium cerebelli. The medial orbital border separates the medial surface from 

 the orbital surface. 



The most projecting part of the anterior end of the cerebral hemisphere is called 



limiting sulcus 

 area x ; area y 



limiting slllcus axiai sulcus 

 area x I area y ! area y area 



operculated sulcus 



area z area y area x 



FIG. 575. DIAGRAMS TO EXPLAIN THREE TYPES OP CEREBRAL FURROWS. 



the frontal pole, whilst the most projecting part of the posterior end is termed the 

 occipital pole. On the inferior surface of the hemisphere the prominent point of. 

 cerebral substance which extends forwards below the lateral fissure receives the 

 name of the temporal pole. In a well-hardened brain a broad groove is usually 

 present on the medial and inferior aspect of the occipital pole of the. right hemisphere. 

 This corresponds to the commencement of the right transverse venous sinus. A less 

 distinct groove on the occipital pole of the left hemisphere frequently indicates the 

 commencement of the left transverse sinus. On the tentorial surface, a short 

 distance behind the temporal pole, a well-marked depression, impressio petrosa, is 

 always visible. This corresponds to the elevation on the anterior surface of the 

 petrous portion of the temporal bone over the superior semicircular canal. 



THE WHITE MATTEK OF THE CEREBRAL HEMISPHERES. 



According to the connexions which they establish the fibres forming the white 

 medullary matter of the hemispheres may be classified into three distinct groups, 

 iz., (1) commissural fibres; (2) association fibres; and (3) projection fibres. 



Commissural Fibres. These are fibres which link together portions of the 

 pay cortex of opposite cerebral hemispheres. They are arranged in three groups 

 >rming three definite structures, viz., the corpus callosum, the anterior commissure, 

 and the hippocampal commissure. 



The corpus callosum has in a great measure been already studied (p. 628). As 

 it enters each hemisphere, its fibres spread out in an extensive radiation (the radia- 



