The Senses. 317 



The tympanic membrane is placed obliquely across the 

 canal of the external meatus, and owing to its attachment 

 to the malleus on the opposite side, it does not present a 

 plane, but a concave surface towards the external ear. It 

 is not tightly stretched across the space ; by its attachment 

 to the malleus excessive vibration is prevented, for this 

 connection acts the part of a damping apparatus. 



The bones of the ear transmit the vibrations from the 

 tympanic membrane to the internal ear. The small bones 

 have a certain amount of movement on each other, and 

 there are special muscles for bringing it about ; by means 

 of these movements the tension of the membrane may be 

 altered. As before mentioned, this chain of bones lies in 

 the middle ear, which is also an air cavity deriving its 

 supply through the Eustachian tube. It is necessary for 

 the perfect production of sound that the same atmospheric 

 pressure should exist on both sides of the tympanic mem- 

 brane, and this is provided for by the Eustachian tube. 



Whatever part those remarkable sacs (confined solely to 

 solipeds) are intended for — viz., the guttural pouches — it is 

 probable, from their anatomical connection, that they take 

 some part in the sense of hearing, perhaps that of supplying 

 the needful amount of air to the middle ear. The actual 

 use of the guttural pouches is involved in obscurity, but we 

 may provisionally consider them as part of the middle ear. 

 In man, acuteness of hearing is produced by listening with 

 an open mouth ; the fact that the horse cannot breathe 

 through the mouth may explain the presence of these large 

 air-sacs beneath the skull ; in other words, they are pro- 

 bably associated with acuteness of hearing. 



The tension of the tympanic membrane, produced by the 

 muscles acting on the small bones of the ear, assists in pro- 

 tecting the internal ear from violent noises, and the same 

 movement which slackens or tightens the drum-head in- 

 creases or decreases the pressure on the perilymph of the 

 internal ear. 



The stapes being fixed against the membranous labyrinth 

 through a foramen in the osseous structure, every alteration 



