80 THE HORSE. 



Between the membrane at which we have arrived, 5, and a smaller one 

 almost opposite, pleading- to the still interior part of the ear, and on which 

 the nerve of hearing is expanded, are four little bones, c, d, e,/, united to 

 these membranes, and to each other. Their office is to convey, more per- 

 fectly than it could be done through the air of the cavity, the vibrations 

 which have reached the membrana tympani. 



The first of these Httle bones {ossiculi) is called the malleus, c, from its 

 supposed resemblance to a hammer. The longer arm of the hammer is 

 attached to the edge of the membrane, and reaches to its centre, and is so 

 strongly united to it, as to draw it down into a kind of funnel-shape. It 

 is the bracing of the parchment of the soldier's drum ; and it must be 

 sufficiently evident, that every vibration given to the membrane must 

 be communicated to the hammer-bone. 



From the hammer, the vibration is communicated to the incus, d, so 

 named from its imagined likeness to a blacksmith's anvil, although it is a 

 great deal more like a molar tooth or grinder, with two fangs, and one of 

 them much lengthened and curved. The hammer, however, is so formed and 

 placed, that the impression or vibration is not merely conveyed, but consi- 

 derably increased. Between the extremity of the handle of the hammer, and 

 its head resting on the anvil, is a sharp process, received into a hollow 

 in the bony wall of the drum, and which is evidently the fulcrum, or centre 

 of motion, on which the hammer turns ; and this is much nearer the head 

 of the bone, than the extremity of the handle. It is then a lever, and it 

 acts upon the principle of the lever. The point of the handle is the place 

 where the vibration is received, or the power applied ; the little process is 

 the fulcrum or prop, or turning point ; and the head of the bone is the ex- 

 tremity of the other arm, where the weight is to be hung, or the eiFect pro- 

 duced. Now, in proportion (as we shall have again and again to demon- 

 strate, when we speak of the construction of the limbs) as the distance of 

 the power from the fulcrum exceeds that of the weight, so will be the 

 mechanical advantage gained, or so will the effect be increased. Here the 

 extremity of the hammer is twice as far from the centre as the head ; and, 

 therefore, the effect will be doubled, and the vibration received by the 

 extremity of the handle, will be conveyed with double intensity to the 

 anvil. 



The bodies of these bones are elastic; and the heads of all bones are 

 covered by a substance, cartilage, elastic in the highest degree : therefore, 

 the impression or vibration communicated from the hammer to the anvil, 

 will not be deadened, but rather increased by the collision of these elastic 

 bodies. 



The anvil d is another lever, and not only would the vibration be com- 

 municated undiminished through its substance, but, one of the projections 

 or fangs being received into an opening in the wall of the drum, and the 

 distance of the point at which the impression was received, or the power 

 resides, being greater from the centre than that where the impression is to be 

 conveyed or given up to the next bone, or, in other words, where the effect 

 is to be produced, mechanical advantage is here, likewise, gained, and the 

 effect on the next bone, e, may fairly be reckoned at three times the inten- 

 sity of the original vibration. 



The round bone, e, a very minute one, is the next in order. It is the 

 smallest bone in the body ; and its use seems to be, to form a more 

 complete and moveable joint between the anvil and the stirrup, and to cause 

 the impulse or vibration to be communicated to the stirrup-bone in a per- 

 pendicular direction. 



