. Human physiology : designed for colleges and the higher classes in schools, and for general reading. pletes the whole processnecessary to the production of the sensation of hearing. 412. The parts of the apparatus of hearing may be seen inFig. 148. The internal portions are made rather larger thannatural, in order that the arrangement may be more clear. Ata b c is the external ear; at d is the entrance to the tube ofthe ear/; g is the drum of the ear at the end of this tube,called the membrane of the tympanum; h is the cavity of thetympanum, the chain of bones which it contains being left ou
. Human physiology : designed for colleges and the higher classes in schools, and for general reading. pletes the whole processnecessary to the production of the sensation of hearing. 412. The parts of the apparatus of hearing may be seen inFig. 148. The internal portions are made rather larger thannatural, in order that the arrangement may be more clear. Ata b c is the external ear; at d is the entrance to the tube ofthe ear/; g is the drum of the ear at the end of this tube,called the membrane of the tympanum; h is the cavity of thetympanum, the chain of bones which it contains being left out,so that the plan of the apparatus may be more clear to you;k is the Eustachian tube, which makes a communication be-tween the back of the throat and the cavity of the tympanum;n is a part of the winding passages, shaped like a snails shell,and is therefore called the cochlea; at m are three other wind-ing passages, called, from their form, semi-circular canals; andat I is the vestibule, or common hall of entrance to all these THE parts of the apparatus of hearing described. 275 FIG. VERTICAL SECTION OF THE ORGAN OF HEARING. winding passages. In the cavity of the tympanum, on the sideopposite to tbe drum of the ear, you see two holes. Theseopen into the winding passages, the larger one into their vesti-bule or entrance hall. Both of these holes are covered by amembrane, and to the membrane of the larger one is attachedthe last of the chain of bones. At o is the trunk of the nerveof hearing, and at e e is the bone that incloses these parts,which is so hard that it is called the petrous, or rock-like bone. Having given you this general view of the apparatus, I shallnow speak of each part more particularly. 413. The object of the external ear is to collect the waves ofsound, and direct them into the tube of the ear. There havebeen many speculations in regard to the use of the prominencesand ridgfs of the external ear, but they are fanciful and ground-less; and its s
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