THE AUDITORY OSSICLES AND THEIR MUSCLES. 893 



strongly in sympathy with the especial tone for which it is, as it were, 

 adjusted. By this means the perception of feeble tones is facili- 

 tated. 



In this respect the tympanic membrane has been well compared with the iris. 

 Both membranes, by contracting narrowing of the pupil and stretching of the 

 tympanic membrane prevent the excessive action of the specific stimulus from 

 causing excessive irritation, and both adapt the sensory apparatus to the action 

 of moderate or weak stimuli. The movement in both membranes is the result 

 of a reflex action: for the ear through the auditory nerve, which constitutes the 

 path for reflex stimulation of the motor fibers of the tensor. 



That increased tension of the tympanic membrane makes it less sensitive 

 to sound-vibrations can be readily shown by closing the mouth and nostrils, and 

 either making a forcible expiration, so that air is forced through the Eustachian 

 tube into the tympanic cavity, and the tympanic membrane is bulged outward, 

 or by making a strong inspiration, so that the tympanic membrane is drawn 

 inward as a result of rarefaction of the air in the tympanic cavity. In both cases 

 hearing is interfered with as long as the increased tension persists, as may be dis- 

 tinctly observed on listening for a note to die out. 



If air is blown into the external auditory canal of a normal individual, by 

 means of a rubber bag, both tensors of the tympanum contract, and in consequence 

 the ear not blown into becomes momentarily hard of hearing. Johannes Miiller 

 made the same action clear by means of the following experiment: If a funnel, 

 with a small lateral opening, be placed in 

 the auditory canal, and the wide end be 

 covered with a tense membrane, hearing is 

 less acute as soon as the membrane is made 

 more tense by means of a traction-apparatus. 

 In other words, the membrane of the funnel 

 represents a second tympanic membrane, 

 which is placed in front of the ear. 



The normal mode of stimulation of 

 the tensor tympani is, as has been said, 

 reflex. The muscle is not directly and 

 solely under the influence of the will. 

 L. Fick explains the following phenomenon 

 as an associated movement of the tensor : 

 When he pressed his jaws firmly together, 

 he heard in his ear a high peeping-singing . 

 tone, and in a capillary tube, placed air- 

 tight in the auditory canal, he saw a drop move FIG. 321. Tensor Tympani Muscle; the 

 quickly inward. During this experiment an Eustachian Tube (Left), 



individual with normal acuteness of hearing 

 perceives a reinforcement of all musical tones, 



but a weakening of all high, nonmusical tones. In yawning, with great stretch- 

 ing of the muscles of the face and jaws, v. Helmholtz and Politzer found an im- 

 pairment of hearing for certain tones, which Landois also was able distinctly to 

 perceive in himself, and which he was more inclined to ascribe to an increased 

 activity of the stapedius. 



Hensen found that the tensor tympani muscle takes part in the act of hearing 

 by sudden movements, and not by tonic contraction. At the commencement 

 of the act a contraction occurs that facilitates the perception because the mem- 

 brane, when set in motion by the muscle, vibrates more readily in sympathy 

 with the higher tones than when at rest. On exposing the tympanum in dogs 

 and cats, he showed that the contraction takes place only at the commencement 

 of the sound, and that it then quickly ceases, although the sound may continue. 



The stapedius muscle, which is situated within the pyramidal 

 eminence, and is inserted from behind forward on the head of the stapes 

 and the sesamoid bone of Sylvius, has the following action: by pulling 

 on the head of the stapes (indicated in Fig. 314 by the small curved 

 arrow) it places the bone in an oblique position, so that the posterior 

 extremity of the base of the stapes is pressed more deeply into the fenes- 

 tra ovalis, and the anterior extremity is displaced outward. The stapes is, 



