218 



ELEMENTARY PHYSIOLOGY 



The auditory canal extends inward from the pinna to a distance of about 

 l^ inch. Its inner extremity is closed by a very thin membrane called the 

 drum of the ear (or the tympanic membrane), which is stretched across it 

 obliquely. The walls of the canal are formed partly by cartilage and partly 

 by bone, and are lined with a continuation of the skin. This skin, at the 

 outer portion of the canal, is provided with small hairs, and also a number of 

 glands, similar in structure to the sweat glands, which secrete the cerumen or 

 ear wax. Both the hairs, which are inclined outwards, and the wax, tend to 

 arrest dust particles, and to prevent the intrusion of insects. 



Fig. 206. The Left Tympanic Membrane Fig. 207. The Bight Bony 

 and the Auditory Bones. Labyrinth. 



m, malleus; i, incus; st, stapes ; py, pyramid from i, vestibule ; 2, fenestra ovalis; 3, 4, 



which the tendon of the muscle of the stapes and 5, semicircular canals ; 6, 7, 



emerges ; tt, tendon of a muscle cut short ; la, and 8, the cochlea ; 9, fenestra 



anterior ligament of the malleus ; Is, superior rotunda. The smaller figure below 



ligament of the malleus ; li, ligament of the shows the natural size, 

 incus ; ch, branch of the facial nerve. 



The middle ear or tympanum (Lat. a drum) is a small irregular 

 cavity in the temporal bone. It is separated from the auditory 

 canal by the tympanic membrane, and contains a chain of small 

 bones by means of which the vibrations received by the drum are 

 transmitted across the cavity to the inner ear. 



These bones are three in number, and are called respectively the malleus 

 or hammer, the incus or anvil, and the stapes or stirrup. The handle of the 

 malleus is attached throughout its length to the drum of the ear, and con- 

 sequently vibrates with it. The vibratory motion is then taken up by the incus 

 and the stapes in turn. This last bone fits into an opening in the inner wall 

 of the tympanum, thus transmitting the sound vibrations to the inner ear. 



The middle ear contains air, and communicates with the pharynx (see 

 p. 65.) by means of the Eustachian tube. By this arrangement the air 



