ANATOMY OF THE fiAR 167 



communicate them to the brain. No part of the human 

 structure, however, is so complicated, and so difficult to 

 be comprehended by one who has never personally ex- 

 amined it, as the ear. For this reason I shall not per- 

 plex your mind with much of its minute anatomy, since 

 the clearest descriptions in the world, even if assisted by 

 plates, would fail to give any thing like an adequate con- 

 ception of it. The external ear is composed of an elas- 

 tic cartilaginous substance, which collects the sonorous 

 rays, and may be compared to the large extremity of a 

 trumpet. Thence the sounds are transmitted through 

 the auditory canal which winds along in the substance 

 of the bone. At its termination, there is stretched di- 

 rectly across it a thin tense membrane, called the mem- 

 brane of the tympanum, or drum of the ear. On , the 

 other side of this membrane, is a cavity called the tym- 

 panum, in which is contained a chain of four very small 

 bones, connected with the drum of the ear and the op- 

 posite sides of the cavity. An opening into the pha- 

 rynx, establishes a free communication with the air. 

 Various canals and cavities, closed by tense membranes, 

 lead from the tympanum, containing fluids, in which are 

 expanded the soft and delicate filaments of the auditory 

 nerve. This is enough of the anatomy of the ear, for 

 you to understand how the vibrations are communicated 

 to the nerve. 



The membrane of the tympanum on which sounds 

 first strike after passing through the external auditory 

 opening, may be compared to the head of a drum, and 

 the cavity of the tympanum being supplied with air, 

 serves to complete the resemblance to that instrument. 



Emily. The use of the Eustachian tube, then, is to 

 supply the cavity of the tympanum with air but why 

 is this necessary ? 



Dr. B. The necessity of such a supply of air will 

 appear very obvious, if you consider thrt if a drum be 

 exhausted of its air, no sound would be produced. 

 Whether the membrane of the tympanum be under the 



