646 TEXT-BOOK OF PHYSIOLOGY. 



*a condition of relaxation; 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 increases 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 tympanic 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 tym- 

 panic membrane ; 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 accom- 

 plished 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 mem- 

 brane 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, the long process of the 



