d»« THE SPECIAL SENSES. 



reception of sound vibrations and are used, therefore, in attentive 

 listening. They form, in fact, a mechanism of accommodation simi- 

 lar in its general functions to the ciliary muscle of the eye. Hensen* 

 has shown that both muscles contract reflexly to sounds, and that 

 the contractions of the tensor tympani are stronger, the higher the 

 pitch of the sound. This contraction seems to take place at the 

 beginning of the sound, but is not maintained for a long period. 

 The reaction is apparently a reflex movement the sensory path of 

 which lies in the acoustic nerve and the reflex center in the medulla 

 oblongata. That a similar reflex adjustment takes place in man is 

 indicated by the following experiment described by Hensen, If 

 while listening to a tuning-fork (400 to 1000 v. d.) a metronome is set 

 going at a rate of 40 to 60 beats per minute, the tone of the tuning- 

 fork becomes obviously strengthened. The stimulus of the noise 

 caused by the metronome may be supposed to excite the reflex con- 

 tractions of the muscles of the ear and thus increase its responsive- 

 ness to the vibrations of the tuning-fork. According to this view, 

 therefore, the ear muscles are kept constantly in play by sounds or 

 sudden variations in the intensity of sounds, and perhaps the obvious 

 effort experienced in listening intently to a sound is also due to a con- 

 traction of these muscles. 



The Eustachian Tube. — Through the Eustachian tube a com- 

 munication is established between the tympanic cavity and the 

 pharynx, and through this latter with the exterior. The obvious 

 advantage of this arrangement is that it keeps the air within the 

 tympanum under the same pressure as the outside air, — that is, the 

 pressure on the two sides of the tympanic membrane is kept the 

 same. The pharyngeal opening of the tube is normally closed, but it 

 may be opened by raising or lowering the pressure in the pharynx. 

 This happens, for instance, in the act of swallowing, and we perform 

 this act, therefore, whenever our sensations from the tympanic mem- 

 brane warn us of an inequality in pressure upon the two sides. When, 

 for instance, one enters a caisson in which the external pressure is 

 increased over the normal atmospheric pressure the tympanic 

 membrane would be driven inward by the excess of external pres- 

 sure were it not for the existence of the Eustachian tube. Under 

 these conditions swallowing movements will open the pharyngeal end 

 of the tube and thus bring the tympanic cavity under a barometric 

 pressure equal to that on the outside. In nasal catarrh the tube 

 may be occluded so as to prevent this equalization, and under such 

 conditions, as is well known, the delicacy of hearing is much im- 

 paired, until by raising the pressure in the pharynx or by other 

 means the tube is opened. 



The Projection of the Auditory Sensations. — Auditory sen- 

 sations are projected to the exterior and, indeed, to the supposed 

 * Hensen, "Archiv f. d. gesammte Physiologie," 87, 355, 1901. 



