BRAIN 



159 



Clf.v 



or commissures connecting the left and right halves of the brain ; Cr. C. crura 

 cerebri ; Di. diencephalon ; for. M, foramen of Monro ; z. iter, or aqueduct of 

 Sylvius ; inf. infundibulum ; Med. obi. medulla oblongata ; Olf. I. olfactory 

 lobe ; opt. ch. optic chiasma ; Opt. I. optic lobe ; opt. v. optic ventricle ; pin. stalk 

 of pineal body , pit. pituitary body ; Sp. cd. spinal cord ; zfi. third ventricle ; 

 7' 4 . fourth ventricle ; I X, cerebral nerves ; i Sp. sSp., first and second spinal 

 nerves ; (A C, after Gaupp ; D, from Wiedersheim's Comp. Anatomy, 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 (z/ 3 ), 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 infundibulum (Fig. 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 (pin) of 



the pineal body- the vestige of 



a sensory apparatus, part of 



which in some lizards, for 



example, has the structure of 



-an eye situated beneath the 



skin on the top of the head, 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. If). Each contains a cavity, the 



FIG. 50. Diagrammatic horizontal 



section of frog's brain. 

 c. c. central canal ; Cer. H. cerebral 

 hemisphere ; Di. diencephalon ; 

 for. M. foramen of Monro j z'. iter ; 

 Lat. v. lateral ventricle ; Med. 

 obi. medulla oblongata ; Nv. /, 

 olfactory nerve ; Olf. L. olfactory 

 lobe; Olf.v. olfactory ventricle; 

 Opt. I. optic lobe ; Opt. v. optic 

 ventricle ; Sp. cd. spinal cord ; 

 v. 3, third ventricle ; v. 4, fourth 

 ventricle. (After Ecker and Wie- 

 dersheim.) 



