364 



ADVANCED PHYSIOLOGY 



the air in the middle ear at the same pressure as that outside, 

 so that the ear drum may be kept flat. The Eustachian tube 

 is, therefore, of especial use in enabling one to adapt himself 

 to different altitudes. At the sea-level, atmospheric pressure 

 is much greater than on the tops of mountains; if the Eu- 

 stachian tube did not thus regulate internal pressure in the ear, 

 the drum would sometimes be pressed inward or again out- 

 ward, perhaps to the breaking point. The Eustachian tube 

 is also useful as a drainage way for the middle ear; its lining is 

 ciliated, and mucus which is formed in small quantities in the 

 tympanum is thus carried to the pharynx. 



The Eustachian tube is not generally open; it is rather in 

 the condition of a thin rubber tube, with its sides collapsed. 

 Its closing prevents the voice, which is produced in the voice 

 box just below the opening of the tube into the pharynx, from 

 passing up the tube and creating a loud disturbance during 

 ordinary conversation. Moreover, if the tube were con- 

 stantly open, air would be continually passing in and out of 

 the middle ear as one breathes. This would keep the thin 

 membrane of the drum and the partition between the middle 

 and inner ear cavities under constantly changing pressures^ 

 and irritate the hearing organ seriously. 



Mastoid Cavities. Whether or not the mastoid cavities 

 of any value is not clear; but they are sometimes the cause oi 

 serious trouble. When there is inflammation in this region 

 and the cavities become filled with pus, producing a disease 

 called mastoiditis, the most skilled physician or surgeon 

 should be given charge of the case. The distance from the 

 mastoid region to the brain is so short that inflammation can 

 easily spread through the thin, bony walls of the brain cavity, 

 and if the brain becomes involved the trouble may be fatal. 



The Ear Bones. There are three tiny bones stretching in 

 a zigzag course across the middle ear cavity, from the ear 

 drum to the foramen ovale; Fig. 182. The first is the malleus 

 (hammer), and is fastened to the drum at one end. From 



