92 THE HORSE. 



and ends in a large pouch or bag. The cartilage, p. protects the mv'Uth of 

 this bag, and prevents the food from entering it ; and lilvewise enables it 

 occasionally to unclose for purposes connected with the faculty of hearing 



'I'he impression, then, has been conveyed by the rnecnanisrn of \iie bones, 

 from the membrane of the drum, h, to the membrane on which ilie stirrup 

 rests, f ; and which closes the fenestra ovalis, or oval window, or opening 

 into the labyrinth of the ear. This mechanism, however, deeply seated as 

 it is in the head, and guarded by the stony hardness of the temporal bone, 

 is liable to injury, and we are next led to admire many provisions for 

 preserving the sense of hearing, even when much mischief has been done to 

 the machine. The membrane may be punctured or ruptured. It is occa- 

 sionally so by accident or violence, and lately purposely done in the human 

 subject, to remedy deafness produced by obstruction of the Eustachian 

 tube. Tlie vibrations of tlie external air would proceed down the passage 

 a, and be communicated, although imperfectly, to the little bones at the 

 bottom, c, d, e, and carried on the oval window, f, and hearing would 

 remain. Supposing that the three first of the little bones were diseased or 

 removed, the vibration of the external air would be communicated to the 

 air in the drum, and by that to the stirrup, f, and the animal would not be 

 entirely deaf: or even if the whole of the little bones were destroyed, yet 

 the membrane of the oval window remaining, some vibration might bo 

 communicated to it, and some sound perceived. 



Passing the oval window, y, we arrive at the true seat of hearing. A 

 strangely irregular cavity, h, presents itself, filled with an aqueous fluid, 

 while the substance or pulp of the porlio mollis or soft portion of the seventh 

 pair of nerves, the auditory nerve, expands on the membrane which lines 

 the walls of this cavity. Why is this cavity filled with a liquid? First, 

 that the membrane which covers the passage into it, might always be pre- 

 served in a proper state to receive and communicate vibrations. If the 

 labyrinth had contamed a fluid possessed of much expansibility, in the 

 considerable changes of temperature to which the frame. is subject, this 

 membrane might be stretched beyond the power of vibrating, and almost 

 to bursting by tlie increased bulk of that fluid. Air is highly expansible. 

 That is of no consequence in the drum of the ear, I, because, as it expanded, 

 it would rush out of the Eustachian tube ; but in the labyrinth it would be 

 highly injurious, because that is a closed cavity. These interior chambers 

 then are filled with water instead of air, because it is not one-hundredth 

 part so expansible as air. If, however, the labyrinth be completely filled 

 with this aqueous fluid, how can any undulation or vibration take place ? 

 Undulation supposes a change of figure, an enlargement in some direction ; 

 but there can be no enlargement in a bony cavity completely filled. This 

 was not forgotten in the wonderful construction of the ear, and, therefore, 

 at the base of the shell, m, and between the stirrup and the shell, is an 

 opening, covered likewise with membrane, called the round window, or 

 communication between the drum and tiie labyrinth. When any force, 

 then, is impressed on the membrane under the stirrup, this membrane 

 yields to the impression, and sufl^ers the vibration to be propagated through 

 the whole of the labyrinth. When the vibration ceases, and the fluid is at 

 rest, the membrane over this opening returns to its natural situation, and is 

 ready to yield to the next impression. 



T.'iere is another important reason why these cavities are filled with 

 aqueous fluid. The principal object of the mechanism of the little bones, 

 we have seen to be, perfectly to convey, and even to increase the effect of, 

 the vibration first connnunicated to the membrane of the drum. The vibra- 



