THE SENSE OF HEARING. 



151 



their passage, strike against and rebound from its walls, so that none can reach 

 the tympanum in a quite direct and unbroken line ; further, we may conjecture 

 that this reflection of the air-waves from the walls gives tliera additional force, 

 on the same principle that the rebound from a vaulted surface is known to 

 strengthen sounds; but we are not in a position to determine the course of each 

 wave in this tube with so much precision as physics might desire. The hairs 

 at the entrance are an important protection against the intrusion of foreign 

 bodies, especially particles of dust, which, at least to a certain extent, is thereby 

 prevented. Respecting the true use of the ear-wax there is still a question; 

 the covering of the Avails with a glutinous substance would be judged a priori 

 to be detrimental to the conducting capacity of the tube, as too great a secre- 

 tion and accumulation of it are in fact found to be. A recital of the conjectures 

 which have been formed on this subject may well be spared. 



We come now to a very important part of the apparatus for conveying sound, 

 the tymjmnum, and its connection with the reservoir of the nerves by means of 

 the ossicles of the tympanum, a chain of small movable bones. In order to give 

 the reader a correct idea of this mechanism, it is indispensable to convey as 

 clear a conception as possible of the anatomical arrangement, form, position, and 

 adjustment of its several parts ; an undertaking which would be easy had we 

 before us, for the purposes of demonstration, either preparations from nature or 

 artificial models, but which presents no inconsiderable difficulty when it is to be 

 accomplished by means of description and mere delineations. 



The tympanum is a delicate membrane, which is stretched like the head of a 

 drum over tlie inner extremity of the auditory canal, and forms a wall of separa- 

 tion between that passage and a small interior cavity, known as the cavitij of 

 the tympanum. This membrane is not placed perpendicularly, but obliquely, 

 in such manner that its exterior surface next the canal looks obliquely down- 

 ward, and at the same time somewhat backward, to the floor of the canal, while 

 the interior surface, turned towards the tympanic cavity, looks upward and some- 

 what forward. The auditory canal is thus obliquely cut off at its inner extremity, 

 its floor or lower wall being longer than its roof or upper wall, the hinder wall 

 somewhat longer than the front one, the cavity of the tympanum lies more above 

 than behind that membrane. The annexed 

 figure represents the outer ear 0, the au- 

 ditory canal G, and tympanum T, of the 

 right side, seen directly in front. In order 

 to place the membrane of the tympanum 

 in open view, the ossicles connected with 

 it are left out, and for the same reason the 

 outline of the cavity is not given. But 

 the membrane of the tympanum forms not 

 a plane surface. It is arched in the di- 

 rection of the cavity, so that its outer sur- 

 face is concave, its inner convex, while 

 between the two layers of which it is com- 

 posed the handle of the malleus, a small 

 bone, to be presently described, descends 

 from the upper ed^e as far down as the 

 middle, where it is farmly attached, and, by its position, gives to the whole mem- 

 brane the curvature which has been just mentioned. If we imagine a stick 

 firmly affixed to the centre of a drum-head, while the other end projects over 

 the rim, and that a force is applied to this outer and movable end, so that the 

 skin of the drum is at once stretched and pressed inward, we shall obtain some 

 idea of the arrangement of the malleus and the tympanum. 



Let us examine now the action of the sound-producing undulations, the be- 

 fore-described waves of condensation and rarefaction on the membrane in ques- 



