CHAPTER VIII. 



THE EAR. 



The organ of hearing consists of three divisions — the external, the 

 middle, and the internal ear. The first of these comprises the osseous 

 external auditory process, and the trumpet-like organ which collects 

 the waves of sound and transmits them along that process to the 

 middle ear. It is described at page 163. 



The middle and the internal ear are cavities excavated in the sub- 

 stance of the petrous temporal bone. From their situation and the 

 minuteness and intricacy of their parts, their dissection is extremely 

 difficult. The student is therefore recommended to study the anatomy 

 of these parts on the models and special dissections to which he is 

 likely to have access, and by the aid of the fuller description given in 

 systematic text-books. At the same time, an outline description will 

 be here given, which the student may illustrate to himself by procuring 

 two or three petrous temporal bones and dissecting them after they 

 have been decalcified in a hydrochloric or chromic acid solution. 



THE MIDDLE EAR. 



The Middle Ear — called also the Tympanum, or drum of the ear — is 

 a cavity in the petrous temporal bone. It contains air, and across it 

 there stretches a chain of minute bones, which transmit the sound 

 waves from the outer to the inner ear. The inner wall of the chamber 

 is formed by that portion of bone in which the divisions of the internal 

 ear are excavated, and it shows the following objects : The 'promontory 

 — a projection, or bulging, which corresponds to the first turn of the 

 cochlea. Above the promontory, the fenestra ovalis — an opening which 

 is closed by the base of the stapes (the innermost of the auditory 

 ossicles). Below the promontory, another opening — the fenestra rotunda^ 

 which is closed by a thin membrane. A pin passed through the fenestra 

 ovalis would enter the vestibular division of the internal ear j if passed 

 through the fenestra rotunda it would penetrate the scala tympani of 

 the cochlea. The outer wall of the chamber is formed mainly by the 

 membrana tympani. This is a thin, translucent membrane which forms 

 the septum between the tympanum and the outer ear. The rim of the 

 membrane is fixed in a groove of the bone. The membrane is slightly 



