g04 ACTION OF THE STAPEDIUS. 



of the incus and stapes (S) which must be pressed more deeply into the fenestra 

 ovalis as already described. When the muscle relaxes, then owing to the elasticity 

 of the rotated axial ligament and the tense membrana tympani itself, the position 

 of equilibrium is again restored. The motor nerve of this muscle arises from the 

 trigeminus, and passes through the otic ganglion (p. 597). C. Ludwig and Politzer 

 observed that stimulation of the fifth nerve within the cranium [dog] caused the 

 above mentioned movement 



Use of the tension. The tension of the membrana tympani caused by the 

 tensor tympani has a double function (Joh. Miiller) 1. The tense membrane offers 

 very great resistance to sympathetic vibrations when the sound-waves are very in- 

 tense, as it is a physical fact that stretched membranes are more difficult to throw 

 into sympathetic vibrations the tenser they are. Thus, the tension so far protects 

 the auditory organ, as it prevents too intense vibrations applied to the membrana 

 tympani from reaching the terminations of the nerves. 2. The tension of the 

 membrana tympani must vary according to the degree of contraction of the tensor. 

 Thus, the membrana for the time being has a different fundamental tone, and is 

 thereby capable of vibrating to the correspondingly higher tone, it, as it were, being 

 in a certain sense accommodated for. 



Comparison with Iris. The membrana tympani has been compared with the iris. Both 

 membranes prevent by contraction narrowing of the pupil and tension of the membrana 

 tympani the too intense action of the specific stimulus from causing too great stimulation, and 

 both adapt the sensory apparatus for the action of moderate or weak stimuli. This movement 

 in both membranes is brought about rejlexly, in the ear through the N. acusticus, which causes 

 ;t reflex stimulation of the motor fibres for the tensor tympani. 



Effect of Tension. That increased tension of the membrana tympani renders it less sensitive 

 to sound-waves is easily proved, thus : Close the mouth and nose, and make either a forced 

 expiration, so that the air is forced into the Eustachian tube, which bulges out the membrana 

 tympani, or inspire forcibly, whereby the air in the tympanum is diminished, so that the 

 membrana bulges inwards. In both cases, hearing is interfered with, as long as the increased 

 tension lasts. If a funnel with a small lateral opening, and whose wide end is covered by a 

 membrane, be placed in the external meatus, hearing becomes less distinct when the membrane 

 is stretched {Joh. Miiller). If air be blown into the external auditory meatus, both tensores 

 tympani contract, and in consequence of this the hearing of the other ear is temporarily 

 affected [OelU . 



Normally, the tensor tympani is excited rejlexly. The muscle is not directly and by itself 

 subject to the control of the will. According to L. Fick, the following phenomenon is due to 

 an "associated movement " of the tensor : When he pressed his jaws firmly against each other, 

 he heard in his ear a piping, singing tone, while a capillary tube, which was fixed air-tight into 

 the meatus, had a drop of water which was in it rapidly drawn inwards. During this experi- 

 ment, a person with normal hearing hears all musical tones as if they were louder, while all the 

 highest non-musical tones are enfeebled (Lucae). When yawning, v. Helmholtz and Politzer 

 found that hearing was enfeebled for certain tones. 



Contraction of the tensor. Hensen showed that the contraction of the tensor 

 tympani during hearing is not a continued contraction, but what might be termed 

 a " twitch." A twitch takes place at the beginning of the act of hearing, which 

 favours the perception of the sound, as the membrana tympani thus set in motion 

 vibrates more readily to higher tones than when it is at re3t. On 

 exposing the tympanum in cats and dogs, it was found that this 

 contraction or twitch occurs only at the beginning of the sound, 

 and that it soon ceases, although the sound may continue. 



Action of the Stapedius. The muscle arises within the emi- 



nentia pyramidalis, and is inserted into the head of the stapes and 



Sylvian ossicle (fig. 583); when it draws upon the head of the 



stapes, as indicated in fig. 577, by the small curved arrow, it must 



Right stapedius place the bone obliquely, whereby the posterior end of the plate of 



muscle. the stapes is pressed somewhat deeper inwards into the fenestra 



ovalis, while the anterior is, as it were, displaced somewhat outwards. The stapes 



is thereby more fixed, as the fibrous mass [annular ligament] which surrounds the 



