120 PHYSIOLOGY. 



mental note, hence, if it be unduly tense from any cause, the indivi- 

 dual will be deaf to low sounds. In its natural condition it is rather 

 lax than tense ; by this means it can reciprocate a greater variety of 

 sounds. The integrity of this membrane does not seem to be essen- 

 tial to hearing, since it may be perforated, or destroyed, without the 

 loss of the sense. 



The chain of hones is intended to conduct the sonorous undulations 

 across the tympanum to the internal ear. The tensor tympani 

 muscle which is inserted into the handle of the Tnalleus has a pro- 

 tective agency over the organ of hearing, analogous to that of the 

 iris. When this muscle contracts it draws in the handle of the 

 malleus and renders the membrana tympani tense, and thus takes 

 away from it the power of reciprocating low sounds. It is also 

 capable of being excited to reflex action by loud sounds, and thus 

 putting the membrane into such a state of tension as not to recipro- 

 cate them. It may, therefore, be fairly compared to the iris, which 

 contracts ever the more powerfully the stronger the light that im- 

 pinges upon it. 



The tympanum isolates the chain of bones and allows free vibra- 

 tion to the membrane at either end of it. Moreover the air which it 

 contains reverberates, and the walls and neighbouring spaces and 

 cells reflect sonorous pulses, which thus fall strengthened upon the 

 walls of the labyrinth, and particularly upon the membrane of fene- 

 stra ovalis and fenestra rotunda. The principal use of the Eustachian 

 tube seems to be, to maintain the equilibrium between the air of the 

 tympanum and the external air, so as to prevent undue tension of 

 the membrana tympani. It also serves to conduct away the secre- 

 tions of the middle ear, which it discharges into the cavity of the 

 throat by means of cilise vibrating upon its surface. 



In regard to the uses of the different parts of the labyrinth nothing 

 certain is known. The semicircular canals are supposed by their 

 direction to give information as to the direction of sounds. The 

 cochlea^ in like manner, is thought to enable us to judge of the pitch 

 of notes, an idea which seems to derive confirmation from the corre- 

 sponding development of this portion in animals, and the variety in 

 the pitch of the sounds, which it is important they should hear dis- 

 tinctly. The vestibule^ from its uniform presence, may be considered 

 as the essential part of the organ of hearing. 



The chain of bones, it was said, connects the external and internal 

 ears. The base of the stapes is attached to the membrane filling up 

 the fenestra ovalis. Immediately beneath this is a circular opening, 

 called the foramen rotundum, which also is filled up with a mem- 

 brane, and is capable of receiving reciprocal vibrations from the air 

 of the middle ear, and transmitting them to the internal ear. 



The vestibule and semicircular canals have lying within them, a 

 membranous lahyrinih^ having the same general shape as the cavi- 



