142 ANATOMY OF VERTEBRATES. 



under the microscope, before it quits the ventricular wall to 

 descend, -with the radiating fibres, into the crus. From this 

 stage in the mammalian series the great transverse commissure 

 grows with the growth and complexity of the hemisphere. It 

 consists mainly of white or fibrous neurine, but some grey matter 

 ( f nucleus lcnticularis')is superadded to the inferior fibres external 

 to the radiated cone, and between this and the ' island of Keil ' 

 there is also a thin layer of grey neurine (' nucleus tamire- 

 formis '). 



Always maintaining its closest connection with the part of the 

 fornix called e lyra,' or hippocampal commissure, whence its 

 development began, the increasing body of transverse fibres 

 extends forward and upward, with a bend or ( genu,' fig. 123, C, O, 

 corresponding in extent with elevation and expansion of the front 

 lobes of the cerebrum. In Man its narrow anterior beginnino; is 



O o 



connected by the ( lamina cinerea ' with the optic commissure, 

 receives a small part of the grey substance of the thalamus, and 

 sends off two bands, called ' peduncles of the corpus callosum,' 

 which, diverging, pass backward across the ( perforated space ' to 

 the lower part of the sylvian fold. The corpus callosum, expand- 

 ing as it rises, bends backward, and presents on its upper surface 

 a medial longitudinal groove, called ( raphe,' bounded laterally by 

 the white f strire longitudinales : ' it terminates behind in a slightly 

 down-bent, thickened, free border or ' pad.' Some way in advance 

 of this the attachment of the under surface of the corpus callosum 

 to the fornix begins, and, as the hemispheres increase in the pla- 

 cental series, so does the extent of the filmy inner walls of the 

 lateral ventricles (' septum lucidum,' Anthro.,fig. 123, b) between 

 the body of the fornix and the great superadded transverse com- 

 missure, the fibres of which extend over the roof of those ventricles. 

 The most intelligible illustrations of the comparative anatomy of 

 this interesting part of the cerebral structure is obtained by dis- 

 secting and exposing the lateral ventricle from the outer side, as 

 in the views of the brains of the Opossum, Kangaroo, and Ass, 

 showing the relative proportions of the hippocampus, and of the 

 part of the inner wall distinct therefrom, called ' septum lucidum,' 

 in LXX', pi. vii. In fig. 5, the vascular fold of pia mater called 

 ( choroid plexus ' is shown passing beneath the fore part of the 

 f taenia hippocampi ' through the canal of communication between 

 the lateral ventricles, in both marsupial and placental brains. 

 The supraventricular neurine, being folded upon its stem, the 

 cavity is a reflection of the external surface, and is lined by a 

 continuation of the pia mater, although the fissure by which it 



