850 THE NERVOUS SYSTEM 



radiation (radiatio occipito-thalamica, fig. 699). Whether any fibres of the optic radiation arise 

 in the superor quadrigeminate body is doubtful. It also is in large part composed of fibres 

 arising from the cells of the visual area, which pass from the cortex to the pulvinar, superior 

 quadrigeminate bodies, and possibly some to the medulla oblongata and spinal cord. 



The mesial root of the optic tract contains few true visual fibres. It runs into the medial 

 geniculate bod}', and neither it nor this body are appreciably affected after extirpation of both 

 eyes. It may be considered as largely representing the fibres of Gudden's commissure (infe- 

 rior cerebral commissure). This commissure consists of fibres which connect the medial genicu- 

 late bodies of the two sides with each other, and which, instead of crossing the mid-line through 

 the mesencephalon, 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. It is also claimed that the fibres of Gudden's 

 commissure connect the medial geniculate body of each side with the inferior colliculus of the 

 opposite side. 



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 upward, which corresponds to the inner surface of the 

 vault of the cranium. The greater transverse diameter of this surface lies 

 posteriorly in the vicinity of the parietal eminences of the cranium. The outline 

 of the superior aspect varies according to the form of the cranium, being more 

 spheroidal in the brachycephalic and more ellipsoidal in the dolichocephalic 

 forms. The hemispheres are separated from each other superiorly 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 extrem- 

 ities of the hemispheres overlap the cerebellum, and thus entirely conceal the 

 mesencephalon and thalamencephalon. They are separated from the superior 

 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 con- 

 tinuous with the falx cerebri and is connected with the tela chorioidea 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 pointed than the frontal pole. The infero-lateral por- 

 tion of the hemisphere is separated anteriorly by the deep lateral fissure (fissure 

 of Sjdvius) into a distinct division, the temporal lobe, and the anterior portion 

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



The surfaces of the hemisphere are — (1) the 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 

 dissection, 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 

 fossae, and posteriorly rests upon the tentorium cerebeUi. Thus it is subdivided 

 into — (a) an orbital area, which is slightly concave, since it is adapted to the 

 orbital phite of the frontal bone, and is separated from the convex surface by the 

 necessarily arcliod superciliary border and from the mesial surface by the medial 

 orbital border, the latUT being straight and extending from the frontal pole 

 mesial to the olfactory bulb and tract; (6) 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 medial occipital border, 

 which is a more or less rounded ridge extending from the occipital pole obliquely 

 upward 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 hardcmed with the meml)ranes in situ. The remainder of the basal surface 

 includes the optic j)ortion of the hypothalamus already considered, and the small 



