EAR, THE, ANATOMY AND PIIYSIOLOGY OF. The apparatus of hearing, as it exists in man and the niammalia, is composed of three parts—the external ear, the middle ear or tympanum, and the internal ear or labyrinth.
The external ear consists of two portions, the auricle or pinna (the part popularly recognized as the ear), and the auditory canal or external meatus.
The auricle, on its outer or more exposed surface, presents various eminences and depressions, resulting from the form of its cartilaginous frame-work. These have received special anatomical names, to which it is unnecessary to advert further than to mention that the deep capacious central space to which several grooves converge, is i termed the concha, and that the lowest and pendulous portion of the ear is termed the lobe. The cartilage forming the basin of the external ear consists of one principal piece, in which there are several fissures, which are filled up by fibrous membrane. Several muscles are deScribed as passing from one part of the auricle to another, but they are so little developed in man that they do not require notice7 there are additionally three attollens aurem (or superior auras), the attraltens aurem (or anterior aurzs), and the retrahens aurem (or posterior auri.$), which pass from adjacent parts of the scalp to the E., and which, though more developed than thd previous group, are of little or no real importance in man (at least in his civilized state), but are of considerable use in many mammals. Their actions are sufficiently indicated by their names.
The auditory canal passes from the concha inwards, and a little forwards, for rather more than an inch. It is narrower at the middle than at either extremity; and on this account there is often considerable difficulty in extracting foreign bodies that have been inserted into it. The membrane of the tympanum which terminates it is placed obliquely, in consequence of the lower surface of the meatus being longer than the upper. The canal is parly cartilaginous and partly osseous; the osseous portion con sisting in the foetus of a ring of bone, across which the membrane is stretched, and in many animals remaining persistently as a separate bone. The orifice of the
meatus is concealed by a pointed process, which projects from the facial direction over it like a valve, and which is called the tragus, probably from being sometimes covered with bristly hair like that of a goat (tragos); and it is further defended by an abundance of ceruminous glands, which furnish an adhesive, yellow, and bitter secretion (see CERIIMEN), entangles small insects, particles of dust, and other small foreign bodies, and prevents their further passage into the meatus.
The middle ear, or rarity of the tympanunz• is a space filled with air which is received from the pharynx (q.v.) through the Eustachian tube, and traversed by a chain of very small movable bones, which connect the membrane of the tympanum with the internal ear. It lies, as its name implies, between the external meatus and the labyrinth or internal E., and opens posteriorly into the cells contained in the mastoid portion of the temporal bone, which are also filled with air, and anteriorly into the Eustachian tube. The cavity is of an irregular shape, and is lined by a very delicate ciliated epithelium, which is a prolongation of that of the pharynx through the Eustachian tube.
Its external wall is mainly formed by the membrane of the tympanum, which is nearly oval, and placed in a direction slanting inwards, so as to form an angle of about 45° with the floor of the auditory canal. The handle of the malleus (or hammer), the first of the chain of ossicles, is firmly attached to the inner side of this membrane in a vertical direction as far downwards as the center, and by drawing it inwards, renders its external surface concave.
Its internal wall has two openings communicating with the internal E., each of which is closed by a delicate membrane. These openings are termed, from their respective shapes, the fenestra avails, and the fenestra rotunda; the former leads to the vestibule, and is connected by its membrane with the base of the stapes (or stirrup bone), the last of the chain of ossicles; while the latter opens into the cochlea.