428 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



broken up into simple folds or gyri, by the forma- 

 tion of intervening fissures ; the arrangement of 

 these is better studied in a small than in a large 

 animal, for with increase in size the primitive pattern 

 is obscured by the increase of the convolutions 

 (Flower). These gyri may be distinguished as the 

 superior, middle, and inferior external gyri (Fig. 1 84 ; 

 8, m, i) ; below is the temporal lobe, separated by the 

 hippocampal sulcus (A). To these other grooves may 

 be added on, such as the supraorbital (Fig. 184; o), 

 and the complexity of the surface of the brain be 

 increased by the development of annectent gyri 

 between the primary folds of a simpler brain. 



It is not, however, to the surface that the complexi- 

 ties of the brains of the higher Mammals are limited, 

 the inner as well as the outer face of the cerebral 

 hemispheres becomes convoluted. The corpus callosum, 

 which is at first a thin straight band of connecting 

 fibres, becomes thicker, especially in front and behind, 

 and so curved on itself that anteriorly it forms the 

 " genu " of human anatomy. Behind and below this 

 corpus callosum is the " fornix," and these two 

 structures are peculiar to mammalian brains ; the 

 former is developed from what is, morphologically, the 

 inner portion of the surface of each cerebral hemisphere, 

 and there is, therefore, a space left which is bounded 

 on either side by a thin wall (septum lucidum) ; this 

 space is known as the fifth ventricle, but the name 

 is an unfortunate one, inasmuch as this fifth ventricle 

 is not developed, as are the others, from the original 

 cavity of the cerebrospinal axis, but is merely a space 

 between two overgrown walls. The fornix is similarly 

 derived from the hinder part of the walls of the cere- 

 bral hemispheres. 



The thickening in the floor of the cerebral hemi- 

 sphere of either side (corpus striafuiii) is much 

 more prominent in the Mammalia than in other 



