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 Avhich collects the 

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

 ear. It is described at page 159. 



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

 stance of the petrous tempoi-al 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 hydrochloi-ic or chromic acid solution. 



THE MIDDLE EAR. 



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

 is a cavity of 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 fonned by that portion of bone in which the divisions of the 

 internal ear are excavated, and it shows the following objects : — The 

 2)rom<mtory — a projection, or bulging, which con-esponds 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 innennost 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 ; 

 if passed through the fenestra rotunda, it would peneti'ate the scala 

 tympani of the cochlea. The outer wall of the chamber is formed 

 mainly by the memhrana tympani. This is a thin, translucent membrane 

 which forms the septum between the tympanum and the outer ear. 



