OLFACTORY LOBE 



II. Central Portion 



THE TELENCEPHALON. n 55 



as its hind-end and then, as the dentate gyrus, extends forward along the inner sur- 

 face of the hippocampus to the uncus. The septum lucidum (page 1 159), a sickle- 

 shaped partition which lies between the lateral ventricles, the corpus callosum and 

 the fornix, is also a constituent of the olfactory path, as are also, probably, the 

 tsenia semicircularis (page 1162) and the nucleus amygdalae (page 1172). 



In the foregoing description of the rhinencephalon only such parts have been included as 

 seem warranted on morphological grounds (Turner, Elliot Smith and Cunningham ). It should 

 be pointed out, however, that the German and French anatomists include also the limbic lobe, 

 the division and constitution of the rhinencephalon accordingly being as follows : 



RHINENCEPHALON. 

 I. Peripheral Portion 



A. Anterior part : 



1. Bulbus olfactorius 



2. Tractus olfactorius 



3. Tuberculum olfactorium 



4. Area parolfactoria 



B, Posterior part : 



5. Substantia perforata anterior 



6. Gyrus subcallosus 



1. Gyrus callosus 



2. Gyrus hippocampi 



3. Gyrus uncinatus 



4. Hippocampus 



5. Gyrus dentatus 



6. Gyrus supracallosus 



ARCHITECTURE OF THE CEREBRAL HEMISPHERES. 



On drawing apart the walls of the great longitudinal fissure, it will be seen that, 

 while in front and behind this cleft completely separates the hemispheres, the latter 

 are connected in the intervening part of their length by a robust commissure, the 

 corpus callosum, which floors the fissure along the middle part of its course. On 

 making sections of the hemisphere above the level of this bridge, either in the frontal 

 or transverse plane, the hemibrain is found to be composed of the thin reddish brown 

 sheet of cortical gray matter (substantia corticalis), which everywhere constitutes an 

 unbroken stratum, and the enclosed large tract of white matter, the centrum ovale. 

 Beneath the corpus callosum lies the lateral ventricle, the cavity enclosed within the 

 hemisphere, in whose lateral wall and floor appears the mesial division of the corpus 

 striatum, the caudate nucleus, whilst further outward is lodged the lateral division of 

 the nuclear mass of the end-brain, the lenticular nucleus. Attached to the under 

 surface of the posterior half of the corpus callosum is the arched layer of fibres known 

 as theforni.v, and below the latter, covering to a large extent the upper surface of 

 the thalamus which forms a part of the floor of the lateral ventricle, lies the thin 

 highly vascular sheet, the velum interpositum. These and the other structures more 

 or less closely related to the lateral ventricle claim fuller description, which may now 

 be undertaken. 



The Corpus Callosum. This structure is the great commissure which con- 

 nects the hemispheres and, in addition, affords passage to fibres that arise from the 

 thalamus and, probably, other nuclei outside the hemisphere and proceed to the 

 cerebral cortex. It lies considerably nearer the anterior than the posterior end of 

 the hemisphere and occupies approximately one half of the latter' s length. Seen in 

 mesial sagittal section (Fig. 996), the corpus callosum appears as a robust arched 

 structure, white in color and composed of nerve-fibres transversely cut, whose ends 

 are considerably thicker than the intermediate portion, the body (truncus corporis 

 callosi). Its upper surface is convex, partly free and partly covered by the overlying 

 hemisphere, and its lower one is concave and, where not attached to the fornix and 

 the septum lucidum, clothed by the ependyma lining the ventricle. Its length is 

 about 7 cm. (2^ in.) and its greatest thickness, at its posterior extremity, is about 

 8 mm. It is widest behind, where it measures about 20 mm., and somewhat 

 narrower in front. The thickened front end, the genu, bends backward and is 



