THE SENSE OF HEARING. 



763 



in the middle ear. Deglutitions periodically open the Eustachian tube 

 and form an auxiliary function to that of hearing. These acts follow 

 each other at short intervals and are repeated often during the day, 

 even during sleep. 



In a deep mine where the atmosphere is considerably more dense 

 than that on the surface, the uninitiated is instructed to swallow 

 every few minutes. By so doing he maintains an equable pressure 

 upon both sides of the membrana tympani. 



The secretory nerve of the submaxillary gland is the chorda 

 tympani, which passes through the middle ear and may be considered 

 as a proof of the functional unity which belongs to the salivary secre- 

 tion and hearing. 



Fig. 316. I. The Mechanics of the Auditory Ossicles. (After HELM- 

 HOLTZ.) II. Section of the Middle Ear. (MuNK, after HENSEN.) 



I. a, Malleus, h, Incus, am, Long process of incus, s, Stapes. The arrows 

 show the direction of motion. 



II. G, External auditory canal. M.t., Membrana tympani. C, Tympanum. 

 H, Malleus. L.8., Superior ligament. S, Stapes. 



Movements of the Ossicles. To the tympanic membrane is at- 

 tached the handle of the malleus, whilst projecting above the edge 

 of the membrane, into the tympanic cavity, is the head of the bone. 

 Helmholtz states that the malleus-incus articulation, in its action, 

 may be compared with the points of the Breguet watch-keys, which 

 have rows of interlocking teeth which offer scarcely any resistance 

 to revolution in one direction, but allow no revolution in the other. 

 Hence, when the handle of the malleus moves inward toward the 

 tympanic cavity, the incus and its long process, which is parallel with 



