EAE. 389 



closes it. This very delicate membrane separates the canal, 

 or middle-ear, from the internal ear. 



914. The cavity of the internal ear is behind the membrane 

 of the tympanum. This cavity is filled with air. It has no 

 outlet to the external ear. But there is a passage or tube, 

 called the eustachian tube, which leads from the back part of 

 the mouth to the cavity of the internal ear. The air has free 

 access from the mouth to the inner ear through this tube. 



The air may be forced from the mouth through this tube 

 into the internal ear, by closing the lips and the nostrils, and 

 pressing the air from the lungs through the windpipe. The 

 air is then felt pressing into the ear with a sound, and some- 

 times with a loud sound. The acts of gaping and swallowing 

 have a somewhat similar effect ; the latter creates a distinct 

 murmuring, and the former a sort of explosive sound in the 

 inner ear. 



This tube holds the same relation to the drum of the ear 

 as the hole in the side of a martial drum does to that musical 

 instrument. When the vibration of the air strikes upon the 

 membrane of the tympanum, the air within receives the im- 

 pression, and partly escapes through the eustachian tube, and 

 thus the impression is modified. 



915. There are within the drum of the ear three small bones, 

 which are so arranged as to connect the membranous cover- 

 ing with the labyrinth, &c., where the auditory nerve is 

 spread, and to convey the impressions, which are made by the 

 undulations of the atmosphere on the outside of the membrane, 

 to the nervous filaments within. 



916. There are also three semicircular canals, (Fig. LXIX. 

 e,) and the cochlea, (Fig. LXIX./*,) which have their use in 

 the function of hearing, but precisely what use, it is not easy 

 to explain. These are placed within the parts of the bone at 

 the side and the base of the head. 



917. The auditory nerve passes from the brain through a 

 hole at the bottom and side of the skull, and is spread about 

 in the labyrinth of the ear. 



918. All the several parts of the ear are adapted to receive 



33* 



