Ill 



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 



o.sl 



FIG. 10. Transverse section (diagrammatic) through the head of a Frog at the 

 level of the tympanic cavity. The various parts of the skull shown in section 

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

 an. tymp. tympanic ring ; b. hy. body of hyoid ; hue. cav. cavity of pharynx ; 

 ch.plx. choroicf plexus ; col. columella ; eus. t. Eustachian tube ; fen. ov. fenestra 

 ovalis ; med. obi. medulla oblongata ; memb. lab. membranous labyrinth ; mnd. 

 mandible ; Nv. VIII. auditory nerve ; o. st. omosternum ; ptg. pterygoid ; 

 gu.j'u. ^quadrato-jugal ; stp. stapes ; tymp. cav. tympanic cavity ; tymp. inemb. 

 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. tymp]. Stretching across the tym- 

 panic cavity from the outer wall of the auditory capsule to 

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

 columella (cot), having a bony handle and a cartilaginous 



