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. lo, tymp. cav), bounded externally by 

 the tympanic membrane, and communicating with the mouth 



mtmbZab 



a^nXrh 



ezta.t 



o^t 



^fenxftf 



Fig. io. — 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 andmucous membrane white. _ 

 an. tyinp. tympanic ring ; h. ky. body of hyoid ; bjic. cav. buccal cavity ; 

 ck. plx. choroid plexus ; col. columella ; eus. f. Eustachian tube ; fen. ov. fenestra 

 ovalis ; med. obi. medulla oblongata ; menib. lab. membranous labyrinth ; 7nnd. 

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

 qu.ju. quadrato-jugal ; stp. stapes ; tymp. cuv. tympanic cavity ; tymp. memb. 

 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. lo, 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 



