Fig. 138.—The Pinna, or Auricle.

In studying the structure of the ear, and how it is fitted to respond to sonorous vibrations, we may divide it into three parts: the sound-conducting part, known as the external ear, the middle ear, and the deeply placed nerve portion, the inner ear.

343. The External Ear. The external ear consists of an expanded portion known as the pinna or auricle, and of a passage, the auditory canal or meatus, leading inwards from it. The surface of the auricle is convoluted to collect and transmit the vibrations of air by which sound is produced the auditory canal conducts these vibrations to the tympanic membrane. Many animals move the auricle in the direction of the sound. Thus the horse pricks up its ears when it hears a noise, the better to judge of the direction of sounds.[[48]]

The external auditory meatus, the passage to the middle ear, is curved and is about an inch and a quarter long. Near its outer portion are a number of fine hairs slanting outwards to prevent the entrance of insects. Embedded in the deeper parts of the canal are glands which secrete the cerumen, or ear-wax, which keeps the canal moist, and helps to protect it against foreign bodies and insects. As the result of a cold, this wax may collect in sufficient quantities to block the passage, and to diminish to a considerable extent the power of hearing.

344. The Middle Ear. At the inner end of the outer ear passage is the tympanum, known as “the drum of the ear.” It is a thin, oval membrane, stretched at an angle across the deep end of the passage, which it completely closes. The tympanum is thus a partition between the passage of the outer ear and the cavity of the middle ear. On its inner side is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum. Its bony walls are lined with mucous membrane similar to that lining the nose, mouth, and throat. On the inner wall of the tympanum are two openings, the round window, or foramen rotundum, and the oval window, or foramen ovale.

The tympanic cavity communicates with the back part of the throat, by the Eustachian tube. This tube is about one and a half inches long and lined with mucous membrane similar to that of the tympanic chamber and the throat. This passage is usually closed, but is opened in the act of swallowing. In health there is no communication between the chamber of the middle ear and the outside, except by the Eustachian tube. Thus a throat cold, with redness and swelling of the mucous membrane, is usually accompanied with some degree of deafness, because the swelling may block the lumen of the tube, and thus prevent the free passage of air to and fro.

Fig. 139.—General View of the Organ of Hearing.