SENSITIVE SYSTEM. 397 



with, the membranes closing the external meatus and the oval 

 fenestra are four little bones {ossicu/a), by whose intervention 

 vibrations of sound agitating the membrana tympani are trans- 

 mitted to the membrane opposite — that closing the fenestra 

 ovalis : for these bones are connected to each other by ligaments, 

 and are so disposed in relation to one another as to offer the 

 greatest mechanical advantages in this chain of communication 

 for the purpose of augmenting the intensity of the vibration. 

 The names of the ossicula are, malleus, incus, os orhkulare, and 

 stapes. 



The malleus (so called on account of its similitude to a 

 hammer or mallet) is the one most outwardly situated. Its long 

 process, manubrium or handle, is attached to the central part of 

 the membrana tympani, which is drawn inward by it into the 

 shape of a funnel. The head of the malleus stands out from its 

 body in an oblique direction, very similar to that of the os femo- 

 ris ; the handle forming an angle with the head and neck, and 

 tapering to its termination against the membrane. From the up- 

 per part of the handle, immediately below the neck, issues an 

 acute tapering process ; and this is received into a depression in 

 the bone (in which it rests) in the side of the cavity. This pro- 

 cess forms the centre of motion of the malleus, and as such it is 

 worthy of remark here, that it originates near its head and at a 

 distance from the extremity of the handle; a point of consider- 

 able import in the physiology of hearing. 



The incus, though it has received its name from its fancied 

 resemblance to a blacksmith's anvil, bears a much nearer ap- 

 proach in figure to a molar tooth. Answering to the tooth, too, 

 it has a depression upon its surface, adapted to receive the head 

 of the malleus. Like the malleus, it possesses two processes — a 

 superior and short one turned backwards to be let into a depres- 

 sion in the wall of the tympanum ; a longer one projected down- 

 wards into the cavity, whose extremity is curved a little, and 

 whose point is attached to the os orbiculare. 



The OS orjjiculare is not only the smallest of the ossicula^ 

 but it is the smallest bone in the whole body. Soemmering dis- 

 believes in the existence of such a bone ; but it may, with a 

 little care, be found and demonstrated, although it hardly ex- 

 ceeds in magnitude a grain of sand. It forms the medium of 

 junction and communication between the incus and stapes : 

 through it, the joint existing between the two obtains additional 

 freedom and facility of motion. 



The stapes (or stirrup-bone) seems to be the most aptly 

 named of them all ; since it really possesses much of the form 

 and character of the common iron stirrup. Its base (which is 



