830 THE NERVOUS SYSTEM 



the fibres of Gudden's commissure (inferior cerebral commissure). This com- 

 missure consists of fibres which connect the medial geniculate bodies of the two 

 sides with each other, and which, instead of crossing the mid-line through the mesen- 

 cephalon, course in the optic tracts and cross by way of the posterior portion of 

 the optic chiasma. It consists of fibres which both arise and terminate in each of 

 the bodies, and, therefore, of fibres coursing in both directions. 



THE CEREBRAL HEMISPHERES 



The cerebral hemispheres in man form by far the largest part of the central 

 nervous system. Together, when viewed from above, they present an ovoid surface, 

 markedly convex upwards, which corresponds to the inner surface of the vault of the 

 cranium. The greatest transverse diameter of this surface lies posteriorly in the 

 vicinity of the parietal eminences of the cranium. The outline of the dorsal aspect 

 varies according to the form of the cranium, being more spheroidal in the brachy- 

 cephalic and more ellipsoidal in the dolichocephalic forms. The hemispheres are 

 separated from each other dorsally by a deep median slit, the longitudinal fissure, 

 into which fits a duplication of the inner layer of the dura mater known as the falx 

 cerebri. The posterior or occipital extremities of the hemispheres overlap the cere- 

 bellum, and thus entirely conceal the mesencephalon and thalamencephalon. They 

 are separated from the dorsal surface of the cerebellum and the corpora quadrigemina 

 by the deep transverse fissure. This is occupied by the tentorium cerebelli, which is 

 similar to and continuous with the falx cerebri and is connected with the tela chori- 

 oidea of the third ventricle below. 



Each of the hemispheres is usually described as having three poles or projecting 

 extremities, and three surfaces bounded by intervening borders. The most anterior 

 projection is the frontal pole. This is near the mid-line, and with its fellow of the 

 other hemisphere, forms the frontal end of the ovoid contour of the cerebrum. The 

 occipital pole is the most projecting portion of the posterior and inferior end, and is 

 more rounded and thus not so near the mid-line as the frontal pole. The ventro- 

 lateral portion of the hemisphere is separated anteriorly. by the deep lateral fissure 

 (fissure of Sylvius) into a distinct division, the temporal lobe, and the anterior portion 

 of this lobe projects prominently forwards and is known as the temporal pole. 



The surfaces of the hemisphere are (1) the dorso-lateral or convex surface; (2) 

 the medial surface; and (3) the basal surface. The convex surface comprises the 

 entire rounded aspect of the hemisphere visible previous to manipulation or dissec- 

 tion, and is the surface subjacent to the vault of the cranium. The mesial surface is 

 perpendicular, flat, and parallel with that of the other hemisphere, the two bounding 

 the longitudinal fissure and for the most part in contact with the falx cerebri. 

 The superomesial border intervenes between the convex and medial surfaces, and 

 is thus convex and extends from the frontal to the occipital pole. 



The more complex basal surface fits into the anterior and middle cranial fossse, 

 and posteriorly rests upon the tentorium cerebelli. Thus it is subdivided into (a) 

 an orbital area, which is slightly concave, since it is adapted to the orbital plate of the 

 frontal bone, and is separated from the convex surface by the necessarily arched 

 superciliary border and from the mesial surface by the internal orbital border, the 

 latter being straight and extending from the frontal pole mesial to the olfactory bulb 

 and tract; (b) a tentorial area, or surface, which is arched in conformity with the dorsal 

 surface of the cerebellum. This is separated from the convex surface by the infero- 

 lateral border, which runs from the occipital to the temporal pole; and from the 

 mesial surface by the internal occipital border, which is a more or less rounded 

 ridge extending from the occipital pole obliquely upwards in the angle formed by the 

 junction of the perpendicular falx cerebri and the horizontal tentorium cerebelli. 

 This border is best seen in brains which have been hardened with the membranes 

 in situ. The remainder of the basal surface includes the optic portion of the hypo- 

 thalamus already considered, and the small depressed and punctate area, the anterior 

 perforated substance, which is penetrated by the antero-lateral group of the central 

 branches of the anterior and middle cerebral arteries and into which the striae of the 

 olfactory trigone disappear. In addition to the orbital area the basal surface of the 

 hemisphere shows signs of the impress of the petrous portion of the temporal bone and 

 of the great wing of the sphenoid. 



