228 HUMAN PHYSIOLOGY. 



of these bones, the malleus, being attached to the tympanic membrane, will 

 take up the vibrations much more readily than if no membrane intervened. 

 Owing to the character of the articulations, when the handle of the malleus is 

 drawn inward, the position of the bones is so changed that they form practi- 

 cally a solid rod, and are therefore much better adapted for the transmission of 

 molecular vibrations than if the articulations remained loose. As the stapes 

 bone is somewhat shorter than the malleus, its vibrations are slighter than 

 those of the tympanic membrane, and by this arrangement the amplitude of 

 the vibrations is diminished, but their force increased. 



The function of the stapedius muscle is, according to Henle, to fix the 

 stapes bone so as to prevent too great a movement from being communicated 

 to it from the incus and transmitted to the perilymph. It may be looked 

 upon, therefore, as a protective muscle. 



The function of the Eustachian tube is to maintain a free communica- 

 tion between the cavity of the middle ear and the nasopharynx. The pressure 

 of air within and without the ear is thus equalized, and the vibrations of the 

 tympanic membrane are permitted to attain their maximum, one of the con- 

 ditions essential for the reception of sound waves. The impairment in the 

 acuteness of hearing which is caused by an unequal pressure of the air in the 

 middle ear can be shown 



1. By closing the mouth and nose and forcing air from the lungs through the 

 Eustachian tube into the ear, producing an increase in pressure. 



2. By closing the nose and mouth, and making efforts at deglutition, which 

 withdraws the air from the ear and diminishes its pressure. 



In both instances the free vibrations of the tympanic membrane are inter- 

 fered with. The pharyngeal orifice of the Eustachian tube is opened by the 

 action of certain of the muscles of deglutition viz., the levator palati, the 

 tensor palati, and the palato-pharyngei muscles. 



The internal ear, or labyrinth, is located in the petrous portion of the 

 temporal bone, and consists of an osseous and a membranous portion. 



The osseous labyrinth is divisible into three parts viz., the vestibule, the 

 semicircular canals, and the cochlea. 



The vestibule is a small, triangular cavity, which communicates with the 

 middle ear by the foramen ovule; in the natural condition it is closed by the 

 base of the stapes bone. The filaments of the auditory nerve enter the vesti- 

 bule through small foramina in the inner wall, at the fovea hemispherica. 



The semicircular canals are three in number, the superior vertical, the in- 

 ferior vertical, and the horizontal, each of which opens into the cavity of the 

 vestibule by two openings, with the exception of the two vertical, which at one 

 extremity open by a common orifice. 



