262 ANATOMY AND PHYSIOLOGY FOR NURSES 



entrance of the auditory canal. It presents several ridges 

 and furrows, arising from the folds of the cartilages that 

 form it. 



The external auditory meatus is a canal, partly cartilag- 

 inous and partly bony, about i inch in length, whose walls 

 extend inward from the pinna to the membrana tympani, 

 or drum of the ear. It is narrower in the middle than at 

 the extremities. It is lined by an extremely thin pouch 

 of cuticle, which, when withdrawn after maceration, pre- 

 serves the form of the canal. 

 Stiff, short hairs that stretch 

 across the tube are often found 

 in the interior of the channel, 

 and prevent the ingress of in- 

 sects. Beneath the cuticle are 

 a number of small follicles that 

 secrete the wax (cerumen) of the 

 ear. 



The membrana tympani is 

 a thin, semitransparent mem- 

 brane of an oval shape. It is 

 about I inch in dimeter, and is 

 inserted into the groove around 

 the circumference of the meatus, 

 This membrane is placed ob- 

 it is concave toward the 



FIG. 137 The external 

 ear: a, Helix; b, fossa of 

 anthelix ; c, tragus ; d, lobule ; 

 e, antitragus ; /, concha ; g t 

 anthelix; h, fossa of helix 

 (Randall). 



near its termination. 



liquely across the tube. 



meatus and convex toward the tympanum. 



The tympanum is an irregular bony cavity, situated 

 within the temporal bone. It is bounded externally by 

 the membrana tympani; internally, by the inner wall; 

 and in its circumference, by the petrous portion of the 

 temporal bone and mastoid cells. The tympanum is 

 traversed by a chain made of three small bones the 

 malleus, incus, and stapes. (See Fig. 139.) 



The mastoid cells are very numerous, and occupy the 



