SMALL BONES OF EAR. INTERNAL EAR. 185 



also communicates with the mastoid cells, numerous sinuses 

 which are found in the mastoid process of the temporal bone, 

 containing air, and designed to multiply the vibratory surfaces; 

 and lastly, it unites by a sort of funnel with the Eustachian 

 tube, a canal about one inch and a third in length, which 

 opens into the upper portion of the pharynx, and admits the 

 air into the middle ear. 



The ossicles or small bones of the ear. These are four in 

 number; they are articulated together, and form a bony 

 chain which runs from the membrana tympani to the fenestra 

 ovalis, following a broken line. They have been named the 

 hammer (malleus), the anvil (incus), the lenticular\)QK<t, and the 

 stirrup (stapes), from their form or their functions. Special 

 muscles act upon the malleus and the stapes, which are at 

 the two extremities of the chain ; the incus and the lenticular 

 bone serving as media for the propagation of the vibrations. 

 The motion impressed upon one of these extremities is com- 

 municated to the other by a sort of see-saw movement of the 

 little bones, the mechanism of which is pretty nearly repre- 

 sented by that of a bell. One extremity of the hammer, the 

 handle, is fitted into the membrane of the tympanum, and 

 when the muscle of the hammer contracts, the membrane 

 tightens, a phenomenon which will be discussed further on. 

 The muscle of the stirrup attaches the flat part of this bone 

 to the fenestra ovalis, and according to M. Longet prevents 

 it from being forced in a contrary direction under the influ- 

 ence of the muscle of the hammer, of which it is the an- 

 tagonist. 



Labyrinth or internal ear. The internal ear is that por- 

 tion of the organ of hearing which perceives the impression 

 of sound, and transmits it directly to the brain. It is hollowed 

 out in the petrous bone, and is divided naturally into three 

 distinct compartments, named the vestibule, the semicircular 

 canals, and the cochlea or snail-shell. These divisions form 

 together one of the most complex and delicate pieces of me- 

 chanism in the human body. 



The labyrinth is composed of a bony cavity which incloses 

 a membranous cavity in a portion of its space, and from this 

 circumstance arises the distinction made by anatomists 



