80 PKACT1CAL UAKP ClLLTL r .KK. 



to open with a click at every swallow and which allows the passage of air. 

 If closed by disease deafness is likely to follow as in catarrh and after scar- 

 let fever. 



Here again it will be observed we have nothing but a transmitter, 

 though one whose presence is of great importance to the perfection of the 

 sense of hearing in the higher animals. Yet it is not the essential part of 

 the organ, nor does its absence destroy the sense or prevent the action of 

 the other parts if present. 



It will be observed that the air wave in reaching and striking the drum- 

 head is changed into movement of the membrane, just as the movement 

 of a drum stick is lost or converted into movement of the stretched parch- 

 ment of the drum. This movement is passed on to the first bone of 

 the series which is a bent lever. This transmits it to the second, thence it 

 passes to the third, which from its shape is called the stirrup-bone. This 

 lies against a membrane that closes an opening in the bone of the inner 

 ear, and which consequently moves with every movement of the bone. 

 Here for the present we leave this sound-wave. 



The third and most intricate and yet by far the most important part of 

 the human organ of hearing is the inner ear. This from its complexity is 

 well named the labyrinth. The annexed figures will aid the reader in fol- 

 lowing the description, but the whole structure cannot be well understood 

 without actual dissection. Some account of it is, however, necessary for 

 the comprehension of what is to follow. 



The labyrinth or inner ear is a chamber excavated in the ear-bone 

 the hardest in the body filled with a watery liquid, adjoining the middle 

 ear and having a hole in the bony wall that separates the two, which, how- 

 ever, is not open but closed with a delicate membrane. Against this lies 

 the stirrup-bone already mentioned. The form of this chamber is exceed- 

 ingly complicated, but speaking broadly, it consists of three parts. These 

 are, first a central portion separated from the middle ear by the bony wall 

 containing the opening already mentioned ; second and freely communi- 

 cating with this is a posterior portion in form like the shell of a snail and 

 severed from the middle ear only by a second delicate membrane stretched 

 across a second opening in the ear-bone. Third, there are three semicir- 

 cular tubes communicating freely at both ends with the central chamber. 

 These three canals are roughly speaking at right angles with each other 

 and like the whole of the inner ear are full of liquid. All this mechanism, 

 both the spiral portion and the canals is securely imbeded in cavities in the 

 bone, which makes their study in the higher animals both tedious and dif- 

 ficult. 



We have now reached the central portion of the organ the part by 

 which the motion of the small bones is converted into nerve motion the 

 part without which no hearing is possible.* Inside the labyrinth lie the 

 fine hair-like tips of the seventh nerve the nerve of hearing floating as 



*From the facts here stated it may be seen how many parts of the ear may be destroyed 

 while the hearing yet remains in greater or less perfection. Indeed it appears as though 

 the sense could never be entirely abolished so long as the liquid of the inner ear and the 



