THE CORPUS CALLOSUM 



851 



depressed and punctate area, the anterior perforated substance, which is pene- 

 trated by the antero-lateral group of the central branches of the anterior and 

 middle cerebral arteries and into which the striiB 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. 



The corpus callosum. — In their early development as lateral dilations of the 

 anterior primary brain-vesicles, the hemispheres are connected with each other 

 only at the anterior end of the thalamencephalon, where they are both continuous 

 with the lamina terminalis. As development proceeds and the hemispheres 

 extend upward, backward, forward, and laterally to completely conceal the base, 

 and as the pallium, or cortex, thickens and its folds begin to appear, the two hemi- 

 spheres become united across the mid-line above the thalamencephalon and the 

 third ventricle by the inter-growth of the great cerebral commissure, the corpus 

 callosum. After removal of the falx cerebri from the longitudinal fissure, the 



Fig. 671. — Mesial and Tentorial Surfaces of Right Cerebral Hemisphere, Viewed from 

 THE Left. (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.) 



Sulcus of corpus callosum 

 Body of fornix i 

 Body of corpus callosum ! ; Thalamus 



Interventricular foramen 

 Genu of corpus callosum 



Crus of fornix 



Cut surface of cerebral peduncle 



Splenium of corpus callosum 



Columns of fornix 

 Anterior commissure/ 

 Optic chiasma 



Isthmus of gyrus fornicatus 

 \ ' Chorioid fissure 

 .^ > Fimbria 



'> Hippocampal fissure 

 1 1 Impressure for petrous bone 

 Mammillo-thalamic fasciculus Dentate fascia 



Columns of formx' 

 Corpus mammillare 



dorsal surface of the corpus callosum may be exposed by drawing apart the 

 contiguous mesial surfaces of the hemispheres. It consists of a dense mass of 

 pure white substance coursing transversely, and arises as out-growths from the 

 cortical cells of both hemispheres. Thus it is the great pathway which associates 

 the cortex of the two sides of the telencephalon. Only the smaller medial 

 portion of the body lies free in the floor of the longitudinal fissure, by far the greater 

 part being concealed in the substance of the hemispheres, where its fibres radiate 

 to and from different localities of the pallium, forming the radiation of the corpus 

 callosum. Its surface shows numerous transverse markings, the transverse strim, 

 which indicate the course of its component bundles of fibres. In addition there 

 may be seen two delicate, variable longitudinal bands running over its surface 

 on each side of the mid-line. The medial longitudinal stria {stria Lancisii) 

 runs close to the median plane, around the anterior end from the gyrus subcallosus 

 (fig. 672), and over the posterior end downward and lateral ward to disappear in 

 the hippocampal gyrus of the base of the telencephalon. The lateral longitudinal 

 stria is more delicate than the mesial stria, courses lateral to the medial stria, and 

 can be seen only within the sulcus of the corpus callosum (fig. 672). Both 

 strise are composed largety of axones having to do with the olfactory apparatus. 



When severed along the median plane, it may be seen that the anterior margin 

 of the corpus callosum is turned abruptly downward, forming the genu, and that 

 this turn continues, so that the tapering edge of the body points posteriorly and 



