96. Within the tympanum is arranged a chain of remarkable "little bones," or _ossicles_. They are chiefly three in number, and from their peculiar shapes bear the following names: _malleus_, or the mallet; _incus_, or the anvil; and _stapes_, or the stirrup. A fourth, the smallest bone in the body, in early life intervenes between the incus and stapes, but at a later period it becomes a part of the incus. It is called the _orbicular_ bone. Small as are these ossicles--and they, together, weigh only a few grains--they have their little muscles, cartilages, and blood-vessels, as perfectly arranged as the larger bones of the body. One end of the chain of ossicles, the mallet, is attached to the membrane of the tympanum, or outer drum-head, while the other end, the stirrup, is firmly joined by its foot-piece to a membrane in the opposite side of the cavity. The chain, accordingly, hangs suspended across the drum between the two membranes; and when the outer one vibrates under the influence of the sound-wave, the chain swings inward and transmits the vibration to the entrance of the inner ear. [Sidenote: 97. The Eustachian tube? Describe it, and state its use.] 97. The musical instrument, the drum, is not complete if the air within be perfectly confined: we therefore find in all instruments of this kind a small opening in the side, through which air may pass freely. By this means the pressure of the air upon the vellum which forms the head of the drum is made equal upon all sides, and the resonance of the drum remains unaffected by the varying density of the atmosphere. It will, therefore, emit its proper sound, whether it be struck in the rarified air of the mountain-top, or in the condensed air of a mine. The tympanum, {221} or drum of the ear, in like manner has an opening by means of which it communicates freely with the external air. This opening is a narrow canal, about an inch and a half long, called the _Eustachian tube_, after the name of its discoverer, Eustachius. [Illustration: FIG. 54.--SECTION OF THE RIGHT EAR. A, The Concha. B, Auditory Canal. C, Membrane of the Drum, (the lower half.) D, A small muscle. E, Incus, or Anvil. M, Malleus, or Mallet. I, Eustachian Tube. G, Semicircular Canals. H, Cochlea, or snail's shell.] [Sidenote: 98. What can you state of the action of the Eustachian tube?] 98. The course of this passage is indicated in Fig. 54, I, directed downward and inward: its other extremity opens into the upper part of the throat. The passage itself is ordinarily closed, but whenever the act of swallowing or gaping takes place, the orifice in the throat is stretched open, and the air of the cavity of the tympanum may then be renewed. Air may at will be made to enter through this tube, by closing the mouth and nose, and then trying {222} to force air through the latter. When this is done, a distinct crackle or clicking sound is perceived, due to the movement of the membranes, and of the little bones of the ear.