132 HUMAN PHYSIOLOGY 



of the membrane, and its paralysis diminishes the acuteness 

 of hearing. 



The Eustachian tube has a double function. It allows the 

 escape of mucus from the middle ear, and it allows the en- 

 trance of air, so that the pressure is kept equal on both sides 

 of the membrana tympani. Its lower part is generally closed, 

 but opens in the act of swallowing. It is surrounded by an 

 arch of cartilage to one side of which fibres of the tensor 

 palati are attached, so that when this muscle acts in swallow- 

 ing, the arch of cartilage is drawn down and flattened, and 

 the tube opened up (Fig. 69). 



When the Eustachian tube gets occluded, as a result of 

 catarrh of the pharynx, the oxygen in the middle ear is ab- 

 sorbed by the tissues, and the pressure falls. As a result, the 

 membrane is driven inwards by the atmospheric pressure, and 

 does not readily vibrate, and hearing is impaired. 



C. Internal Ear. The internal ear is a somewhat complex 

 cavity in the petrous part of the temporal bone, the osseous 

 labyrinth. It consists of a central space, 

 the vestibule ( V), into which the fenestra 

 ovalis opens. From the anterior part of 

 this, a canal makes two and a half turns 

 round a central pillar, and then, turning 

 sharply on itself, makes the same number 

 of turns down again, and ends at the 

 fenestra rotunda. This is the osseous 



FIG. 69.- Transverse cochlea (Fig. 68, Cock.) The ascending 

 Section through Car- p. 



tiiaginous lower part and descending canals are separated from 



of Eustachian Tube O ne another, partly by a bony plate, 



L" ?<% by a membranous partition-the 



and the way in which basilar membrane. At the base, the 



it is pulled down and h ony i aine n a j s broad, but at the apex 



the tube opened in . , . . n 



swallowing (shaded). lts place is chiefly taken by the mem- 

 brane, which measures at the apex more 

 than ten times its width at the base. 



From the posterior and superior aspect of the vestibule 

 three semicircular canals (Fig. 70) open, each with a 

 swelling at one end. One runs in the horizontal plane, 

 and has the swelling or ampulla anteriorly (Fig. 70, h.c.). 



