50 THE HORSB. 



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

 almost opposite, f, leading to the still interior part of the ear, and on which 

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

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

 perfectly than it could be done through the air of the cavity, the » ibrations 

 which have reached the membrana tympani. 



The first of these little 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 

 considerably 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 extremity of the other arm, where the weight is to be hung, 

 or the effect produced. Now, in proportion (as we shall have again and 

 again to demonstrate, 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 intensity 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 slirrup-bone in a 

 perpendicular direction. 



