100 ANATOMY OF VERTEBRATES. 



fibres of other t commissural ' tracts either ( longitudinal.' con- 



O * 



necting parts of the same hemisphere, or ' transverse,' and bringing 

 a greater proportion of the two hemispheres into mutual com- 

 munication. But there are steps in this differentiation. 



Eacli hemisphere of the cerebrum begins as a vesicle of neurine, 

 the cavity of which receives the growth from the ' crura ' forming 

 the ' corpus striatum.' This, in Birds, mainly fills the ( ven- 

 tricle ' or remnant of the primitive cavity of the sac. But, in 

 Mammals, the w r all of the vesicle is augmented bv folds, of which 



CJ * 



the first and most constant is pushed from the mesial or inner 

 side of the ventricle into its cavity, giving rise to the convexity, 

 figs. 70, 71, h, fig. 75, n, representing the part called ( hippo- 

 campus ' in anthropotomy, The ( fissure upon which the hippo- 

 campus is folded ' 1 is numbered 4 in the ' Table of Cerebral 

 Fissures,' p. 136, as in fig. 69, et seq. 



In Lyencephala it extends from the fore part of the inner sur- 

 face of the hemisphere backward and downward in a curve with 

 the concavity toward the centre or ' nucleus cerebri,' fig. 69, b. 

 It is not, however, a mere doubling of the wall of the hemispheral 



vesicle ; longitudinal fibres are de- 

 veloped therein for commissural office ; 

 they cause a definite production of the 

 lower part of the fold within the ven- 

 tricular cavity called hippocampal band 

 (t&nia hippocampi}, or, because in Man 

 it is plaited, ' corpus frmbriatum : ' its 

 im,er surface of hemisphere, vertical m f er ior hinder termination is in the 



section of brain, Ormthorhynchus. 



' pes hippocampi ; ' its upper or anterior 



one becomes the ( posterior pillar ' of the fornix. ( Fornix ' is the 

 anthropotomical term for the anteriorly continued and transversely 

 connected longitudinal fibres of the hippocamp : the ' posterior 

 pillars,' fig. 69, a, one from each hemisphere, converge as 

 they advance, are united by a commissure of their own, ib. o, 

 beyond which some fibres pass forward and radiate upon the 

 inner surface of the fore part of the hemisphere ; while others 

 bend down, as the f anterior pillars ' of the fornix, pass between 

 the anterior commissure, ib. c, and the nucleus cerebri, b, and 

 terminate in the mammillary body already mentioned. 



Delicate fibres, running on the inner surface of the hemisphere 

 at right angles to the line of the hippocampal fissure, are con- 

 tinued into the ventricle, where they cover the longitudinal fibres 



1 So defined in i.xx'. p. 90 (1837) 



