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Its vibration is made possible by the presence of air on both sides, and this condition is supplied, on the inner side, by the middle ear. The Eustachian tube, by providing for an equality of pressure on the two sides of the membrane, also aids in this purpose.
Jarvis gives the history of a strange and repeated hemorrhage from the nose and adjacent parts that was found to be due to maggots from the ova of a fly, which had been deposited in the nose while the patient was asleep. Tomlinson gives a case in which maggots traversed the Eustachian tube, some being picked out of the nostrils, while others were coughed up.
A helpful plan is to have a friend seat himself at the back of the hall or church, and give his opinion of the quality and projecting power of the speaker's voice. It is difficult to judge one's own voice because it is conveyed to him not only from the outside but also through the Eustachian tube and modified by the vibratory parts of the throat and head.
The anatomy of the teeth continued to be rather vague until about the middle of the next century when Eustachius, whose investigations of the anatomy of the head have deservedly brought him fame and the attachment of his name to the Eustachian canal, wrote his "Libellus de Dentibus Manual of the Teeth," which is quite full, accurate, and detailed.
Anatomists speak of an outer or external ear, a middle ear, drum, or tympanum, and an inner ear, or labyrinth. The external, middle, and internal ear are separated by dotted lines. Into it opens 5, Eustachian tube, leading from back of throat; 4, membrana tympani or drum-head, closing the middle ear off from the external ear.
The outlying remainder of the first gill-cleft is the rudiment of the external meatus. From its inner part we get the tympanic cavity, and, further inward still, the Eustachian tube. Connected with this is the development of the three bones of the mammal ear from the first two gill-arches; the hammer and anvil are formed from the first, the stirrup from the upper end of the second, gill-arch.
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.
The office of these tubes is to supply air to the cavity on the inner side of the drum-membrane, known as the middle ear. As is well known, a passage exists from the outer ear to the drum. The Eustachian tubes connect the middle ear with the upper portion of the throat from whence the air supply to the middle ear is obtained.
This in the large majority of cases will subside and drain back again into the throat through the Eustachian tube. In a fair percentage of instances, however, it will break in the opposite direction, and we have the familiar ruptured drum and discharge from the ear.
Give the purpose of the Eustachian tubes; of the rods of Corti; of the semicircular canals. Give directions for the proper care of the ear. *To illustrate the Origin of Sound.*—1. Strike a bell an easy blow and hold some light substance, as a pith ball attached to a thread, against the side, noting the result. 2. Sound a tuning fork by striking it against the table.
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