THE SENSE OF HEARING. 685 



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, 

 the long process of the incus. Since this process is united at almost a 

 right angle to the stapes bone, the latter is forced toward and into the 

 foramen ovale, thus producing a pressure on the perilymph. With the 

 cessation of the pressure the elastic forces of the membrane and of the liga- 

 ments return the handle of the malleus to its former position; by the unlock- 

 ing of the malleo-incudal joint the entire chain also returns to its former posi- 

 tion without exerting undue traction on the basal attachment of the stapes. 



As the long process of the incus is shorter than the handle of the malleus, 

 and as the movement between them takes place around an axis from before 

 backward, it follows that the excursion of the incus and stapes will be less 

 than that of the malleus, while the force will be greater. Hence as the 

 vibrations are transferred from the tympanic membrane of large area to the 

 base of the stapes of small area (20 to 1.5), they lose in amplitude but in- 

 crease is force. Their pressure on the perilymph is therefore 13 . 3 times 

 greater than on the membrana tympani. In addition to its function as a 

 transmitter of vibrations, the chain of bones serves as a point of attachment 

 for muscles which regulate the tension of the tympanic membrane and 

 the pressure on the labyrinth. 



The Function of the Stapedius Muscle. The function of thestapedius 

 muscle is a subject of much discussion. According to Henle, its function 

 is so to adjust the stapes bone that it will be prevented from exerting an undue 

 pressure on the perilymph during the inward excursions of the incus process. 

 According to Toynbee, its function is to press the posterior part of the stapes 



