356 HEARING. LECT. XIX 



strong light. Under the influence of a prolonged and vio- 

 lent noise, the stirrup and the hammer, by contracting 

 their muscles, render tense the membrane upon which they 

 are fixed, and thus give rise to a temporary, and, under 

 the circumstances, a useful deafness. 



What is the function of the entire chain of ossicles ? 

 Why are there four connected bones together rather than 

 a single one ? Why n&t suppress the foramen rotundum, 

 or the foramen ovale, or even both of them, by applying 

 the vestibule upon the membrane of the tympanum ? It 

 is impossible, with OU-E present acoustical knowledge, to 

 answer these questions satisfactorily. The chain of ossi- 

 cles, besides having the faculty of varying the tension of 

 the membranes upon which its extremities are fixed, also 

 fulfils in the ear the same office as the bridge in stringed 

 instruments, and the cylinder of a drum. If we sprinkle 

 sand on one of the membranes of a drum we observe 

 that the sand vibrates, and forms itself into divisions when 

 we make the other membrane sound ; and this propagation 

 varies according to the arrangements of the cylinder. The 

 chain of ossicle is a kind of music-bridge suspended in the 

 tympanum, which not only propagates the vibrations from 

 one wall to the other, but receives also impulses of air upon 

 the various parts of its surface. The membrane of the fenes- 

 tra ovalis, which communicates with the vestibule, where the 

 nerve is, has its elasticity and tension better protected by- 

 being placed in an air cavity contained in the interior of 

 the ear, than it would be if it were in direct contact with 

 the atmosphere. It is, undoubtedly, for the purpose of 

 giving to the ear a more varied and extended mode of ac- 

 tion, that it is provided with two openings, supplied with 

 stretched membranes in the internal part of the ear, and in 

 proximity with the acoustic nerve ; one of the openings 

 being free, and the other in contact with the membrane of 



