646 A TEXTBOOK OF PHYSIOLOGY 



On the inner wall of the middle ear there are two apertures: an 

 upper oval one, known as the fenestra ovalis, or the oval window; 

 a lower, smaller, round one the fenestra rotunda, or round window. 

 Each of these is closed by a membrane. Into that of the oval 

 membrane is inserted the terminal process of the chain of ossicles. 



Leading away from the front of the tympanum is a channel divided 

 into two by a ledge of bone. The upper compartment contains a 

 muscle the tensor tympani while the lower passage connects with 

 the pharynx, and is known as the Eustachian tube. 



The action of two muscles of the middle ear requires consideration 

 namely, the tensor tympani and the stapedius. The tensor tympani, 

 supplied by the fifth nerve, arises, as we have seen, in the upper 

 compartment of the channel leading from the front of the middle 

 ear, and is inserted by its tendon, which crosses the tympanum, into 

 the inner part of the handle of the malleus. Its action is to maintain 

 by its tone a constant tension on the tympanic membrane. When it 

 contracts, it renders the membrane more taut an action which is 

 believed to limit the movement of the membrane, and thus dampen 

 the effect of loud notes. 



The stapedius lies in a space behind the tympanum, and its tendo: 

 passes through a perforation in the bone behind the oval window, 

 to be inserted into the neck of the stapes. Its action is doubtful. 

 It may be that it moderates the force of the stapes against the fenestra 

 ovalis, or it is possible that, by pulling on the stapes, it acts through 

 the chain of ossicles upon the tympanic membrane, and induces 

 relaxation of it an action antagonistic to that of the tensor tympani. 

 It is supplied by the facial nerve, and when this nerve is paralyzed 

 loud sounds are heard with painful intensity. The muscles may help 

 to tune the tympanic membrane for the reception of sound. 



The Eustachian tube serves to keep the pressure equal 011 the 

 two sides of the tympanic membrane. It is normally irepf closed 

 except during the act of swallowing. This has a double usefulness. 

 If the tube were always open, then, in the first place, since all parts of 

 the head are affected by the waves of sound, the tympanic membrane 

 would tend to become pushed upon on both sides at once, and the 

 effect of the vibrations damped. Secondly, were it open, there 

 would be a great reverberation of our own voice in our ears. 



The equality of pressure on both sides of the tympanic membrane 

 is of great importance for normal hearing. Deafness from " colds 

 in the head " is due to the occlusion of the passage by the congestion 

 of its mucous membrane. The Eustachian tube allows the escape of 

 mucus from the middle ear. During exposure to increasing or rare- 

 fying atmospheric pressure hearing becomes defective, unless the tube 

 be kept open by swallowing or forced expiratory movements with the 

 nose and mouth shut. It is important for the airman to keep the 

 pressure equal on both sides of the tympanic membrane in this manner 

 both for the sake of hearing and for the sake of correct balance. 



The Internal Ear consists of a thick-walled cavity in the temporal 

 bone, known as the osseous labyrinth. It is filled with a lymph^like 

 fluid, known as the perilymph. Lying in this perilymph is a smaller 





