SENSE OF HEARING. 421 



amount, as we saw in regard to the stomach and the digestive 

 tract. In our study of deglutition we made use of this 

 special and intermittent working of the Eustachian tube for 

 the purpose of demonstrating the strict occlusion of the 

 naso-phjaryngeal orifice ; and then observed that the hearing is 

 obstructed (on account of the rarefaction of the air in the 

 tympanic cavity), if one or more deglutitions are performed 

 with the nostrils closed ; and showed the necessity of degluti- 

 tion performed with the nostrils open in order to restore the 

 sense of hearing to its natural state (see p. 226). 



.The tympanic cavity is crossed by a nerve (the chorda 

 tympani) which leads to the salivary glands, and whose 

 function consists of exciting the secretion of saliva ; so cer- 

 tain sounds, especially sharp ones, cause an abundant secre- 

 tion of saliva, no doubt by means of their action on the 

 chorda tympani through the medium of the membrane 

 against which its nerve fibres are applied ; at all events, we 

 cannot avoid associating this anatomical fact of the passage 

 of the nerve of the salivary secretion in the cavity of the 

 tympanum, with the physiological fact which we have just 

 been studying, namely, the essential connection between the 

 secretion of the saliva and deglutition with the opening of 

 the Eustachian tube, and, consequently, with the keeping up 

 of the normal pressure in the cavity of the tympanum. This 

 relation between the middle chamber of the ear and the 

 pharynx is made plain by the study of embryology : in the 

 foetus these parts are confounded together in the first pha- 

 ryngeal cleft, and the Eustachian tube is the remnant of this 

 fetal communication (see p. 405, Physiology of the Chorda 

 Tympani). 



C. Internal ear. 



The vibrations reach the fluid of the labyrinth either 

 through the columella cochlea} (chain of small bones), this 

 being the usual means of communication, or by the bones of 

 the head, especially the walls of the external and the middle 

 ear ; the latter is the case with persons who, although they 

 have lost the chain of small bones, are yet not completely 

 deaf. The fluid of the labyrinth then communicates these 

 vibrations to the various terminal organs of the auditory 

 nerve which are situated in the vestibular sacks (utricula 

 and saccula), in the semicircular canals (ampulla3 and 

 crest), and in the cochlea (spiral lamina with the organ 

 of Corti). Nothing, however, is yet positively known as to 



