SENSE OF HEARING. 227 



been lost, it is doubtful if it is at all necessary for hearing. Nevertheless an 

 individual with dull hearing may have the perception of sound increased by 

 placing the pinna at an angle of 45 degrees to the side of the head. The ex- 

 ternal auditory canal transmits the sonorous vibrations to the tympanic mem- 

 brane. Owing to the obliquity of this canal it has been supposed that the 

 waves, concentrated at the concha, undergo a series of reflections on their 

 way to the tympanic membrane, and, owing to the position of this mem- 

 brane, strike it almost perpendicularly. 



Function of the Tympanic Membrane. The function of the tympanic 

 membrane appears to be the reception of sound vibrations by being thrown by 

 them into reciprocal vibrations which correspond in intensity and amplitude. 

 That this membrane actually reproduces all vibrations within the range of 

 audibility has been experimentally demonstrated. The membrane not being 

 fixed, so far as its tension is concerned, does not possess a fixed fundamental 

 note, like a stationary fixed membrane, and is, therefore, just as well adapted 

 for the reception of one set of vibrations as for another. This is made pos- 

 sible by variations in its tension in accordance with the pitch of the sounds. 

 In the absence of all sound the membrane is in 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 tympanic membrane is gradually 

 inceased until its maximum tension 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 indistinct and inaudible. 



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

 is, as its name indicates, to increase the tension of the 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, in- 

 crease the convexity of the membrane, and at the same time increase its ten- 

 sion; with the relaxation of this muscle, the handle of the malleus passes out- 

 ward 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 accomodative 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 trans- 

 mit the sound wave across the tympanic cavity to the internal ear. The first 



