THE CEREBRAL HEMISPHERES 947 



roofed in by the corpus callosum, while the floor consists of the fused formers 

 and the rostrum. Each hemiseptum bounds a part of the anterior cornu and body 

 of the lateral ventricle in its mesal wall, and in a lateral view is of triangular out- 

 line. The hemisepta represent the thin, undeveloped parts of the mesal walls 

 of the cerebral vesicles, which were enclosed within the rapidly developing arch of 

 the corpus callosum. The cavum septi pellucidi is therefore a closed-off part 

 of the original intercerebral cleft and not a part of the neural cavity, as its older 

 name, "fifth ventricle," seems to imply. 



The Anterior Commissure. The anterior commissure, or precommissure, is a bun- 

 dle of white fibres, of oval outline in a sagittal section, which crosses the midline as 

 a localized reinforcement of the lamina terminalis, slightly bulging into the frontal 

 part (aula) of the third ventricle and clothed by its ependyma. It is a comparatively 

 insignificant intercerebral commissure in the human brain, having become dimin- 

 ished as the corpus callosum increased in mammalian development. It courses 

 from side to side frontad of the anterior pillars of the fornix, ventrad of the head 

 of the caudate nucleus, and passes, in part, through the frontal end of the lenticular 

 nucleus (Fig. 701). Its fibres radiate chiefly to the cortex of the temporal lobe and 

 to certain parts of the rhinencephalon. 



The bundle is slightly twisted in each lateral, buried part. Two divisions are 

 distinguishable: (1) The pars anterior or frontal part (in the median plane) con- 

 tains two groups of fibres belonging to the olfactory apparatus (a) fibres arising 

 from the mitral cells in the olfactory bulb of one side to the same layer in the 

 opposite bulb; (6) fibres which associate the uncus of one side with that of the 

 other. (2) The pars posterior contains the fibres passing between the cortices 

 of the two temporal lobes. 



Gray Masses in the Cerebral Hemisphere. Aside from the cortex, the cerebral 

 hemisphere contains certain gray ganglionic masses in its interior, more or less 

 embedded in the white centrum, and called, because of their proximity to the base 

 of the cerebrum, the basal ganglia. These comprise the caudate, the lenticular, 

 and the amygdaline nucleT It is usual to include the claustrum among the basal 

 .ganglia, but morphologicall this structure belongs rather to the insular cortex of 

 the island of """" 



Conventionally^the caudate nucleus and lenticular nucleus together are described 

 as the corpus striatum (xtriatum), a ganglionic mass which in earlier vertebrate 

 1 trains bore intimate relations with the olfactory apparatus, but later, with the 

 rise in functional dignity and growth of the neopallium, underwent specialization 

 and differentiation concomitant with the reduction of the rhinencephalon. The 

 intrusion of great projection fibre masses, thrusting the cortical gray outward, 

 has not been everywhere uniform, and we still find, in the human brain, a common 

 ground in which the neopallial cortical gray, the corpus striatum, rhinencephalon, 

 and amygdaline nucleus meet the site of fusion being in the gray substance 

 of the anterior perforated substance. To the cortical mantle they are regarded 

 as bearing the relation of subordinate (subcortical) centres. In the human brain 

 .the corpus striatum so called because of its striated appearance in sections 

 Js composed of two masses, the caudate and lenticular nuclei, directly continuous 

 with each other at their frontal ends (Fig. 703). The connecting gray bridge 

 becomes broken up into numerous small bands of gray substance as the fibre 

 masses of the internal capsule insinuate themselves between the two nuclei 

 (Fig. 700). 



The caudate nucleus (caudatum; nucleus caudatus) (Figs. 702, 703) presents 

 a ventricular and a capsular surface; the ventricular surface, covered by ependyma, 

 forms part of the floor of the body and anterior cornu, while in the middle cornu 

 it is a constituent of its roof, owing to its arched contour in correspondence with 

 the sweeping curve of the ventricle itself. It is of a pyriform shape with a very 



