73 6 TEXT-BOOK OF PHYSIOLOGY 



are transmitted to it. This it does by vibrating in unison with them. The 

 vibrations which the membrane exhibits correspond in amplitude, in fre- 

 quency, and in form to those of the atmosphere. That this membrane 

 actually reproduces all vibrations within the range of audibility has been 

 experimentally demonstrated. The membrane, not being fixed as far as its 

 tension is concerned, does not possess a fixed fundamental note, like a station- 

 ary fixed membrane, and is, therefore, just as well adapted for the reception of 

 one set of vibrations as another. This is made possible by variations in its 

 tension in accordance with the pitch or frequency of the atmospheric vibra- 

 tions. In the absence of vibration the membrane is in a condition of re- 

 laxation; with the advent of sound-waves possessing a gradual increase of 

 pitch, as in the ascent of the music scale, the tension of the membrane in- 

 creases until its maximum is reached at the upper limit of the range of 

 audibility. By this change in tension certain tones become perceptible and 

 distinct, while others become imperceptible and indistinct. 



The Function of the Tensor Tympani Muscle. The function of this 

 muscle is, as its name indicates, to change and to fix the tension of the tym- 

 panic membrane, so that it can most readily vibrate in unison with vibrations 

 of varying degrees of rapidity. The tendon of this muscle playing around 

 the processus cochleariformis is attached almost at a right angle to the handle 

 of the malleus. Hence as the muscle contracts it exerts its traction from the 

 process and draws the handle of the malleus inward, thus increasing the 

 convexity of the tympanic membrane and at the same time its tension. 

 With the relaxation of the muscle the handle of the malleus passes outward, 

 and the convexity and tension diminish. 



In the ascent of the music scale, each note corresponding to an increase 

 in vibration frequency requires for its perception an increase in tension and 

 an increase in the force of the contraction of the tensor muscle. In the 

 descent of the music scale the reverse conditions obtain. The contraction of 

 the muscle is of the nature of a single twitch, and of just sufficient force and 

 duration to tense the membrane for a given rate of vibration. 



The contraction of the muscle is excited reflexly. The afferent path is 

 through fibers of the trigeminal nerve distributed to the tympanic mem- 

 brane; the efferent path is through fibers in the small root of the trigeminal. 

 The stimulus is sudden pressure on the tympanic membrane. The more 

 frequently and forcibly the stimulus is applied, the greater is the muscle 

 response. The tensor tympani muscle may therefore be regarded as an 

 accommodative apparatus by which the tympanic membrane is adjusted 

 for the reception of vibrations of varying degrees of frequency. 



The Function of the Chain of Bones. The function of the chain of 

 bones is to transmit the effects of the atmospheric vibrations to the fluid of the 

 labyrinth. The manner in which this is accomplished becomes evident 

 from the relation which the bones of this chain bear to one another and to the 

 tympanic membrane on the one hand and to the fluid of the labyrinth on 

 the other. 



^ When pressure is made on the outer surface of the tympanic membrane 

 it is at once pushed inward, carrying with it the handle of the malleus, the 

 head at the same time rotating outward around an axis corresponding to its 

 ligamentous attachments. As the handle moves inward a small ledge of bone 

 just below the malleo-incudal joint locks with, and hence pushes inward, 



