892 



THE AUDITORY OSSICLES AND THEIR MUSCLES. 



Although the vibrations of the tympanic membrane are transmitted 

 through the malleus to the incus, there is, however, a loss of about one- 

 fourth of their original amplitude. 



As the excursions of the ossicles caused by the sound-vibrations are 

 extremely small, the articulations do not change position with every 

 vibration. Such a change occurs probably only when larger movements 

 are produced by the muscles, as will now be explained. 



The muscles of the auditory ossicles affect the position of the latter and 

 also the tension of the tympanic membrane, as well as the pressure in 

 the endolymph. The tensor tympani muscle is situated in an osseous 

 groove above the Eustachian tube; its tendon is deflected over a bony 

 process of this prolonged groove in a direction outward almost at right 



angles, and is inserted on 

 the malleus just below its 

 axis of rotation (Fig. 321, 

 M). When the muscle 

 contracts (in the direction 

 of the arrow t, Fig. 320) 

 the handle of the malleus 

 ( n ) pulls the tympanic 

 membrane (M) inward and 

 makes it tense. The incus 

 and stapes are moved at 

 the same time, and the 

 stapes (S) is pressed more 

 deeply into the fenestra 

 ovalis, as has been already 

 fully described. When the 

 muscle relaxes, the original 

 position is again assumed 

 as a result of the elasticity 

 of the rotated axial liga- 

 ment and the tense tym- 

 panic membrane. The 

 motor nerve of the muscle 

 comes from the trigeminus 

 and passes through the 

 otic ganglion. C. Ludwig 

 and Politzer observed the 

 motion described follow 

 irritation of the fifth nerve 

 in the cranial cavity. 



Ihe stretching of the tympanic membrane effected by the tensor 

 has a double purpose: (i) The tense membrane offers greater resistance 

 to sympathetic vibration when the sounds are loud, as tense membranes 

 are always the more difficult to set in sympathetic vibration the more 

 they are stretched. In this connection the tensor acts as a protection 

 for the ear, by preventing the transmission of excessively strong impulses 

 through the tympanic membrane to the nerve-endings. (2) The tension 

 of the tympanic membrane must vary according to the degree of con- 

 traction. In this way the membrane'has a different fundamental tone 

 according to the tension, and is therefore enabled alwavs to vibrate more 



FIG. 320. Tympanic Membrane and Auditory Ossicles (left), en- 

 larged: A.G. External auditory canal; M, tympanic membrane 

 with which the handle of the malleus (n) and its short process 

 (p) are in contact; h. head of the malleus; a, incus; K, its short 

 process with its fixation-ligament; 1, its long process; s, 

 ossicle of Sylvius; S, stapes. A x, A x, the axis of rotation of the 

 ossicles (it is drawn in perspective and must be conceived as 

 stuck through the surface of the paper); t, direction of action 

 of the tensor tympani muscle. The other arrows indicate 

 the movement of the ossicles when the tensor tympani contracts. 



