562 



COMPARATIVE PHYSIOLOGY; 



muscle, but a supporting ligament for the maUeus. The stape- 

 dius, however, has the important function of regulating the 

 movements of the stapes, so that it shall not be too violently 

 driven against the membrane covering the fenestra ovalis. 



The two muscles, stapedius and tensor, suggest the accom- 

 modative mechanism of the iris. The motor nerve of the sta- 

 pedius is derived from the facial; of the tenSor, from the tri- 

 geminus through the otic ganglion. 



The Eustachian Tube. — Manifestly, if the middle ear were 

 closed permanently, its air would gradually be absorbed. The 

 drum-head would be thrust in by atmospheric pressure, and 

 become useless for its vibrating function. The Eustachian 

 tube, by communicating with the throat, keeps the external and 

 internal pressure of the middle ear balanced. Whether this 

 canal is permanently open, or only during swallowing, is as yet 

 undetermined. 





II' 



' ife.. 





Fig. 405 — D agram intended to llustrate the procesees of hearing (after Landois). ■ A G, 

 external auditory meatus, T^ tympanic membrane: K, malTeus; o, incns; F, mid- 

 dle ear; o, fenestra ovalis; r, fenestra rotunda; pi, Bcala tympani; vt, scala vesti- 

 buli; F, vestibule; 5', saccule; fT, utricle; .ff, semicircular canals; 7W, Eustachian 

 tube. Long arrow indicates line of traction of* tensor tympani; short curved one 

 that of Stapedius. 



One may satisfy liimseK that the middle ear and pharynx 

 communicate, by closing the nostrils and then distending the 

 upper air-passages by a forced expiratory effort, when a sense 

 of distention within the ears is experienced, owing to the rise 

 of atmospheric pressure in the tympanum. 



