THE TELENCEPHALON. 1133 



matter, the caudate and the lenticular 7iucleiis, represents the internal nucleus of the 

 end-brain. Certain commissural structures, as the corpus callosum, the anterior com- 

 missure and the/brw/.v are to be regarded as secondary and as serving to connect 

 the halves of the great brain. The immediate free or outer surface of the pallium is 

 everywhere formed by a thin peripheral layer of cortical gray matter, which, as an 

 unbroken sheet, clothes the various ridges and intervening furrows — the convolutions 

 and fissures — which model the exterior of the cerebrum and provide the necessary 

 extent of surface. Beneath the cortical gray substance lies the white ^natter, which 

 constitutes the bulk of the hemisphere and consists of the tracts of nerve-fibres pass- 

 ing to and from the cortex, as well as of those connecting the various regions of the 

 cortex with one another. Embedded within the core of white matter and lying 

 much nearer the basal than the superior surface of the hemisphere (Fig. loog), the 

 corpus striatum is closely related to the ventricular cavity by means of the caudate 

 nucleus on the one hand, and to the cortical gray matter by the lenticular nucleus 

 on the other. In view of the rudimentary condition of the rhinencephalon and 

 the over-shadowing development of the pallium in man, it is usual and convenient 

 to regard most of the parts derived from the telencephalon as belonging to the 

 hemispheres, the latter term being used in a less restricted sense than warranted 

 by a precise interpretation of its developmental significance. 



The Cerebral Hemispheres. 



Viewed from above, the human brain presents an ovoid form, the parrower end 

 being directed forward and the broader backward, the greatest width corresponding 

 with the parietal eminences (Fig. 984). The convex surface formed by the 

 hemispheres is divided by a deep median sagittal cleft, the longitudinal fissure 

 (fissura longitudinalis cerebri), that, for a distance less than one-third of its length 

 anteriorly and more than one-third posteriorly, completely separates the hemi- 

 spheres. In its middle third or more, the fissure is interrupted at a depth of about 

 3.5 cm. by the arched upper surface of the corpus callosum, the chief connection 

 between the hemispheres.- The upper and back part of the longitudinal fissure, 

 throughout its length, is occupied by the sickle-shaped mesial fold of dura mater, 

 the falx cerebri, which incompletely subdivides the space occupied by the 

 cerebrum into two compartments. Under the name, transverse fissure (fissura 

 transversa cerebri), is sometimes described the deep cleft which separates the 

 postero-inferior surface of the hemisphere from the cerebellum, the corpora quad- 

 rigemina and the pineal body. This cleft, so evident after the brain has been 

 removed from the skull, when the parts are in situ is filled behind by the tentorium 

 cerebelli and in front by a fold of pia. 



The hemispheres are advantageously studied after being separated from each 

 other by sagittal section, and from the brain-stem by cutting across the mid-brain. 

 When examined after such isolation, especially when hardened before removal from 

 the skull, each hemisphere presents a dorso-lateral, a mesial and an inferior surface. 

 The dorso-lateral surface (Fig. 980) is convex both from before backward and 

 from above downward and closely conforms to the opposed inner surface of the 

 cranial vault. The mesial surface (Fig. 987) is flat and vertical and bounds the 

 longitudinal fissure. It is in contact with the sagittal fold of dura, the falx cerebri, 

 except in front and below where the partition is narrow; here the mesial surfaces 

 of the hemispheres, covered of course by the pia and arachnoid, lie in apposition. 

 The inferior surface (Fig. 989) is irregular, its approximate anterior third 

 resting in the anterior cerebral fossa of the cranial floor, the middle third in the 

 lateral part of the middle fossa, whilst the posterior third is supported by the 

 upper aspect of the tentorium, which separates it from the subjacent cerebellum. 

 At the juncture of its anterior and middle thirds, the inferior surface of the 

 hemisphere is crossed transversely, from within outward, by the stem of the Sylvian 

 fissure and thus subdivided into an anterior and a posterior tract. The former 

 and smaller, known as the orbital area, rests upon the orbital plate of the frontal 

 bone and is modelled by this convex bony shelf into a corresponding slight con- 

 cavity from side to side. The tract behind the deep Sylvian cleft is at first convex 



