93. As in the case of the nostrils, a number of small, stiff hairs garnish the margin of the auditory canal, and guard it, to some extent, against the entrance of insects and other foreign objects. The skin, too, covering its outer half, is furnished with a belt of little glands which secrete a yellow, viscid, and bitter substance, called "ear-wax," which is especially obnoxious to small insects. As the outer layer of this wax-like material loses its useful properties, it becomes dry, and falls out of the ear in the form of minute, thin scales, a fresh supply being furnished from the little glands beneath. In its form, the auditory canal resembles the tube of an ear-trumpet, and serves to convey the waves of sound to the middle portion of the ear. [Sidenote: 94. What is the middle ear? Why called tympanum?] 94. THE MIDDLE EAR, OR TYMPANUM.--The middle ear is a small cavity, or chamber, of irregular shape, about one-fourth of an inch across from side to side, and half an inch long (see Fig. 53, A, _d_). From the peculiar arrangement of its various parts it has very properly been called the _tympanum_, or the "drum of the ear." The middle ear, like the external canal, contains air. [Sidenote: 95. What is the membrana tympani? Describe it.] 95. The circular membrane, already mentioned as closing the auditory canal, is the partition which separates the middle from the external ear, and is called the _membrana tympani_ (_c_), and may be considered as the outer head of the drum of the ear. It is sometimes itself spoken of as the "drum," but this is incorrect; since a drum is not a membrane, but is the hollow space across which the membrane is stretched. This membranous drum-head is very tense and elastic, and so thin as to be almost transparent; {220} its margin is fastened into a circular groove in the adjacent bone. Each wave of sound that impinges against this delicate membrane causes it to vibrate, and it, in turn, excites movements in the parts beyond. [Sidenote: 96. What are the ossicles? Their number and names? Their arrangement?]