THE SENSE OF HEARING. 



155 



action may take effect, the reservoir is provided with a second opening h, which 

 is closed with an ehistic membi'one, {membrana tympani secondaria.) In pro- 

 portion as the stirrup-plate is pressed deeper into its opening, the yielding fluid 

 drives the membrane 7i before it, bending and stretching it outwards. We shall 

 see hereafter that, in nature, this reservoir of fluid consists of a sac-shaped space 

 in which is the opening for the stirrup or stapes, and of a long spirally convolved 

 canal ; the other opening h lies at the extreme end of this canal, so that the 

 movement of the yielding fluid, propagated from particle to particle, must tra- 

 verse the whole canal before it arrives at the elastic membrane. This extensive 

 circuit is of the utmost importance, because the greatest part of the excitable 

 nerves have their extremities within this canal, by all of which extremities there- 

 fore the liquid waves must pass before they reach the membrane. If the con- 

 densed wave, whose effect we have been considering, is succeeded by a rarified 

 wave, all the movements will of course be reversed. The tympanum recoils and 

 oscillates outwards, thus turning the rod a b on the axis d so that the lower por- 

 tion a c moves outwards with the tympanum while its upper portion c b moves 

 inwards ; the portion c a draws a f after it, and this in turn draws the stirrup 

 g in a corresponding degree out of the opening of the reservoir L ; the fluid pur- 

 sues the withdrawing stirrup-plate and the membrane h becomes level or may 

 even be drawn inwards. If condensed and rarified air-waves now follow one 

 another in rapid succession, as in the case of our vibrating string, the little lever 

 a h turns with equal velocity on its axis and moves the foot plate of the stirrup 

 in its opening up and down, causing in the reservoir advancing and retreating 

 waves, equal in number to the vibrations of the sound-producing object. 



After this general representation of the scheme and action of the mechanism, , 

 it will not be difficult to comprehend that of the rather complex apparatus of 

 the ossicles of the tympanum in its actual state. This apparatus consists of 

 three members connected with one another — three small, peculiarly shaped 

 bones — the malleus, the incus, and the stapes. The malleus and incus (hammer 

 and anvil) together correspond to the system of levers a b, d e, and a f oi our 

 scheme, the stapes to our stirrup, which is the meaning of its Latin name. We 

 will first consider the form of the several bones. The 

 malleus consists, as is shown by the annexed figure, of 

 a handle a b and of a spherical head c at the upper end. 

 Beneath this head is found on one side a grooved de- 

 pression running obliquely around the neck from above 

 downwards, into which a corresponding surfoce of the 

 incus is fitted ; on the other side proceeds from about 

 the middle of the bone a long and slender outgrowth or 

 projection e f, running to a point, corresponding to the 

 portion d c of the transverse axis d e in the previous 

 figure. The incus much resembles a bicuspid t lOth with 

 widely separated roots. It consists of a short, thick 

 body a, whose upper surface b is of an oval shape and 

 is so channelled out as to be concave in a longitudinal and con- 

 vex in a transverse direction, fitting acciu-ately into the depression 

 at the neck of the malleus. From the body a proceed, nearly at 

 a right angle with one another, two processes, one short and thick, 

 c, the other, d, long and somewhat bent towards its extremity, on 

 \e which it bears a small lens-shaped bone e. With this terminal 

 bone of the incus articulates the head of the stapes, 

 whose form corresponds to its name ; it consists of a 

 head, two divergent branches unequally bowed, a b, and 

 a flattened oval-shaped plate c which forms the base or 

 foot of the stirrup. The position, ligature, and connec- 

 tion of these bones with each other, the tympanum and 



Fig. 5. 



