254 HUMAN PHYSIOLOGY. 



membrane in accordance with the pitch of the sound wave. The 

 tension of this muscle playing over the processus cochleariformis and 

 attached at also a right angle to the handle of the malleus will, 

 when the muscle contracts, pull the handle inward, increase the con- 

 vexity of the membrane, and at the same time increase its tension ; 

 with the relaxation of this muscle, the handle of the malleus passes 

 outward and the tension is diminished. The contractions of the 

 tensor muscle are reflex in character and excited by nerve impulses 

 reaching it through the small petrosal nerve and otic ganglion. The 

 number of nerve stimuli passing to the muscle and determining the 

 degree of contraction will depend upon the pitch of the sound wave 

 and the subsequent excitation of the auditory nerve. The tensor 

 tympani muscle may be regarded as an accommodative apparatus 

 by which the tympanic membrane is so adjusted as to enable it to 

 receive vibrations of varying degrees of pitch. 



Function of the Ossicles. The function of the chain of bones 

 is to transmit the sound wave across the tympanic cavity to the in- 

 ternal ear. The first of these bones, the malleus, being attached to 

 the tympanic membrane, will take up the vibrations much more readily 

 than if no membrane intervened. Owing to the character of the 

 articulations, when the handle of the malleus is drawn inward, the 

 position of the bones is so changed that they form practically a solid 

 rod, and are therefore much better adapted for the transmission of 

 molecular vibrations than if the articulations remained loose. As 

 the stapes bone is somewhat shorter than the malleus, its vibrations 

 are slighter than those of the tympanic membrane, and by this 

 arrangement the amplitude of the vibrations is diminished, but their 

 force increased. 



The function of the stapedius muscle is, according to Henle, 

 to fix the stapes bone so as to prevent too great a movement from 

 being communicated to it from the incus and transmitted to the peri- 

 lymph. It may be looked upon, therefore, as a protective muscle. 



The function of the Eustachian tube is to maintain a free com- 

 munication between the cavity of the middle ear and the nasopharynx. 

 The pressure of air within and without the ear is thus equalized, and 

 the vibrations of the tympanic membrane are permitted to attain 

 their maximum, one of the conditions essential for the reception of 

 sound waves. The impairment in the acuteness of hearing which is 



