BRAIN 



159 



connecting the left and right halves of the brain ; Cr, C. crura cerebri ; Di. 

 diencephalon ; for. JM. foramen of Monro ; i, iter, or aqueduct or Sylvius ; inf. 

 infundibulum ; liled. obi. medulla oblongata ; Olf. I. olfactory lobe ; opt. ch. 

 optic chiasma ; Opt.^I. optic lobe ; opt. 7: optic \'entricle ; //«. stalk of pineal 

 body , pit. pituitary body ; .?/, at. spinal cord ; -'■•. third ventricle ; v^. fourth 

 ventricle; I — X, cerebral nerves; iS/>.2.Sp.y first and second spina! nerves; 

 (A — C, after Gaupp ; D, from Wiedersheim, after Osborn). 



plexus, of a thickening of pia mater, containing numerous 



blood-vessels. It helps to roof over a narrow slit-like 



cavity, the third ventricle 



(Figs. 49 D and 50, w^), the 



sides of which are formed by 



thickenings of nervous matter, 



the optic thalami {Di). On 



the ventral surface of the brain 



the diencephalon is continued 



into a funnel-like extension, 



the infundibitliii?i{Y\g. 49, inf), 



to which is attached a rounded 



structure, the pituitary body 



{pit). On the dorsal surface, 



just behind the choroid plexus, 



is the delicate stalk {piji) of 



ihepinealbody — the vestige of 



a sensory apparatus (Fig. 148 



C), part of which in some 



lizards, for example, has the 



structure of an eye, and which 



was probably functional in the 



ancestors of the frog. We 



shall meet with other examples 



of such vestigial organs in the 



course of our studies. 



In front of the 'tween-brain comes a pair of long, oval 

 bodies, wider behind and narrower in front. These are the 

 cerebral hemispheres {Cer. H). Each contains a cavity, the 



Sfj. cci 



Fk;. 50. — Diagrammatic horizontal 

 section of frog's brain. 

 c. c. central canal ; Cer. //. cerebral 

 hemisphere ; Di. diencephalon ; 

 for. M. foramen of Monro ; i. iter ; 

 I^at. V. lateral ventricle ; Med. 

 ohl. medulla oblongata ; Nv. /, 

 olfactory nerve ; Olf. L. olfactorj- 

 lobe; Olf.v. olfactory ventricle; 

 Opt. I. optic lobe : Opt. V. optic 

 \entricle ; Sp. cd. spinal cord ; 

 -'. J, third ventricle ; v. 4, fourth 

 ventricle. (After Ecker and Wie- 

 dersheim.) 



