?2 THE HORSE. 



and ends in a large pouch or bai^. The cartilage, p, protects the mouth of 

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

 occasionally to unclose for purposes connected with the faculty of hearing. 



The impression, then, has been conveyed by the mechanism of the bones, 

 from the membrane of the drum, 6, to the membrane on which the stirrup 

 rests, /; 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 pre- 

 serving 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. The vibrations of the external air would proceed down the passage «, 

 and be communicated, although imperfectly, to the little bones at the bot- 

 tom, c, c/, e, and carried on to the oval window, /, 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, / and the animal would not be entirely 

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

 brane of the oval window remaining, some vibration might be communi- 

 cated to it, and some sound perceived. 



Passing the oval window,/, 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 portio 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 liq\iid ? First, that 

 the membrane which covers the passage into it, might always be preserved 

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

 had contained 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 the 

 increased bulk of that fluid. Air is highly expansible. That is of no 

 consequence in the drum of the ear, /, 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 expan- 

 sible as air. If, however, the labyrinth be completely filled with this aqueous 

 fluid, how can any undulation or vibration take place? Undulation sup- 

 poses a change of figure, an enlargement in some direction ; but there can 

 be no enlargement in a bony cavity completely filled. This was not for- 

 gotten 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 be- 

 tween the drum and the labyrinth. When any force, then, is impressed on 

 the membrane under the stirrup, this membrane yields to the impression, 

 and suffers the vibration to be propagated through the whole of the laby- 

 rinth. 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 niBxt impression. 



There 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 communicated to the membrane of the drum. The vibra- 



