\^II EUSTACHIAN TUBE 389 



source of a sound heard through a snow storm. Again, 

 in ventriloquism our judgment is upset, not only as 

 regards the nature nf the source of sound, but also of its 

 distance and direction, by carefully planned simulation 

 and suggestion. 



(xiii) The Functions of the Tympanic Muscles and 

 Eustachian Tube. — It has already been explained that 

 the stapedius and tensor tijmpani muscles are competent 

 to tighten the membrane of the fenestra ovalis and 

 that of the tympanum, and it is probable that they come 

 into action when the sonorous impulses are too violent, 

 and would produce too extensive vibrations of these mem- 

 branes. They may therefore be of use in moderating the 

 eft'ect of intense sound, in much the same way that, as we 

 shall find, the contraction of the circular fibres of the iris 

 tends to moderate the effect of intense light in the eye ; 

 they may, however, have other purposes. 



The function of the Eustachian tube is, probably, to 

 keep the air in the tympanum, or on the inner side of the 

 tympanic membrane, of about the same tension as that on 

 the outer side, which could not always be the case if the 

 tympanum were a closed cavity. The unpleasant sensa- 

 tion often experienced, as of a " tightness " in the ear, 

 when diving under water, is due to the compression of 

 the air in the tympanic cavity under the increased external 

 pressure. It may be largely removed by merely performing 

 the movements of swallowing. By the.se movements the 

 end of the Eustachian tube which opens into the pharynx 

 is opened and the pressure on the two sides of the 

 tympanum is equalised. 



10. Sensations governing co-ordination and equili- 

 brium. — The sense of sight is associated with the eye and 

 with no other sense organ, that of hearing with the ear 

 and so forth ; in each case we are fully conscious of our 

 use of these faculties. But there is another sense of 

 which we are as a rule unconscious, this is the sense of 

 co-ordination ; when this sense fails us however we become 



