CHAPTER V. 



THE OEGANS OF HEA.HINO. 



That many of the lower animals have special organs 

 for the production of sound, and possess the sense of 

 hearing, has been shown in the preceding chapter. 



I now proceed to consider the mechanism by which 

 sounds are perceived. In our own ear we have, first 

 of all, the external ear, much less important in man 

 than in many other animals, as in the horse, for 

 instance, where it may be seen moving continually, and 

 almost automatically assuming the position most favour- 

 able for conveying the waves of sound down the outer 

 passage (Fig. 46, D) to the tympanum, or drum. This is 

 a membrane stretched between the outer air on the one 

 hand, and the drum on the other, which also contains 

 air, transmitted through the mouth by means of the 

 Eustachian tube (Fig. 46, E). The drum is separated 

 from the brain by a hard, bony partition in which are 

 two orifices, one oval and the other round. Across the 

 drum stretches a chain of little bones (Fig. 47) ; first 

 the " hammer," secondly the " anvil," and lastly the 

 " stirrup." The flat plate of the stirrup, again, lies 

 against the oval orifice, or fenestra ovalis, as it is techni- 

 cally called, of the drum. Thus the sounds are intensi- 



