THE EAU. 261 



tube is closed by the well-known tympanic membrane 

 or drum {tympanum) ; a thin membrane of oval form (c), 

 placed in a vertical position, but slightly inclined. This 

 membrane separates the outer ear-canal from the so-called 

 cavity of the drum (cavum. tympani). This is a small 

 cavity enclosed in the petrous part of the temporal bone, 

 which is filled with air and connected by a special tube with 

 the moutli-cavity. This tube is somewhat longer, but much 

 narrower than the outer ear-canal ; it leads inward and 

 forward in an oblique direction from the inside wall of the 

 tympanum and opens behind the inner nostrils (or Choana) 

 into the upper part of the cavity of the throat (pharynx). 

 This canal is called the Eustachian tube (tuha Eustachii). 

 It equalizes the pressure of the air in the tympanic 

 cavity, and the outer atmospheric air which enters by 

 the ear canal. Both the Eustachian tube and the tympanic 

 cavity are lined by a thin, mucous membrane, which 

 is a direct continuation of the mucous membrane of the 

 throat. Within the tympanic cavity are the three bonelets 

 of the ear, which, from their characteristic shape, are called 

 the hammer, the anvil, and the stirrup (Fig. 244 /, g, h). 

 The hammer (/) lies furthest outward, just within the 

 tympanic membrane; the anvil (g) is wedged in between 

 the two others, above the hammer, and further in than the 

 hammer ; and, lastly, the stirrup (h) lies next to the anvil 

 toward the inside, and touches with its base the outer waD 

 of the internal ear, or the auditory sac. All these parts of 

 the middle and external ear belong to the sound-conducting 

 apparatus. Their principal office is to convey the waves ol 

 sound from without throu!ih the thick side-wall of the head, 

 to the internal ear. In Fishes these parts are entirely unre- 



