I" TYMPANIC CAVITY 45 



Two apparently insignificant structures connected with 

 the skull must be described because of their connection with 

 the organ of hearing. Behind the suspensorium is a recess, 

 roofed over by the squamosal, and, in the entire frog, 

 converted by muscle and other tissues into a chamber, the 

 tympanic cavity (Fig. 10, tymp. cav), bounded externally by 

 the tympanic membrane, and communicating with the mouth 



memiXab 



ezca.t 

 o.st ^fen.oi/ 



Fig. 10. — Transverse section (diagrammatic) through the tiead of a Frog at the 

 level of the tympanic cavity. The various parts of the sl^ull shown in section 

 are black, the muscle, &c., grey, and the skin and mucous membrane white. 

 an. tytnp. tympanic ring ; b. hy. body of hyoid ; biic. cav. buccal cavity ; 

 ch.plx. choroid plexus : col. columella ; eits. t. Eustachian tube ; fen. ov. fenestra 

 ovalis : tned. obi. medulla oblongata ; mciitb. lab. membranous labyrinth ; umd. 

 mandible ; Nv. VIII. auditory nerve ; o. st. omosternum ; pt^. pterygoid ; 

 qn.ju. quadrato-jugal ; stp. stapes ; tyvip. cav. tympanic cavity ; iytnp. vicmb. 

 tympanic membrane. (From Parker and Haswell's Zoology.) 



by the Eustachian tube. Supporting the tympanic mem- 

 brane, as the frame of a tambourine supports the parch- 

 ment, is a cartilaginous ring, the tympanic ring (shown in 

 section in Fig. 10, an. tynp). Stretching across the t)'m- 

 panic cavity from the outer wall of the auditory capsule to 

 the tympanic membrane is a small, hammer-shaped rod, the 

 columella {col), having a bony handle and a cartilaginous 



