XIII 



PHYLUM CHORDATA 



285 



of hearing not hitherto met with. Bounded externally by the tym- 

 panic membrane and internally by the outer wall of the auditory 



as, 



cptch if if. II I I ]a 



CerM fcr.M 



'pit V 



ch plx.S 



Afed. obi 



Fig. 985. — Brain of Rana. A, from above ; B, from below ; C, from the side ; D, in longitudinals 

 vertical section. Ch, cerebellum ; Cer. H, cerebral hemispheres ; c/i. plx'^, anterior and 

 cli. plx^, posterior choroid plexus (removed in A) ; com, commissures, the two in front the 

 anterior and hippocampal, the two above the supenor or habemilar and the posterior ; Cr. C, crura 

 cerebri ; Di, diencephalon ; for. M, foramen of Monro ; i, iter, or aqueduct of Sylvius ; in/, 

 infundibulum ; Med. obi, medulla oblongata ; Olf. t, olfactory lobe ; opt. ch, optic chiasma ; 

 Opt. I, optic lobe ; opt. v, iptic ventricle ; jnw, stalk of pineal body ; jjii, pituitary body ; 

 Sp. cd, spinal cord ; v^, third ventricle ; vi, fourth ventricle ; / — X, cerebral nerves ; ISp. Stip. 

 ' spinal nerves. (From Parker's Practical Zoology. A — C, after Gaupp ; D, from .Wieders- 

 heim's Comparative Anatomy ; after Osborn.) 



