288 HUMAN PHYSIOLOGY 



manubrium of the hammer by a tendon bending around a 

 bony process. By its contraction the manubrium is bent 

 inward and thus stretches the drum. It is innervated by 

 the trigeminus. 



(2) Stapedius, whose tendon is attached posteriorly to the 

 head of the stapes. It is innervated by the facial. 



The functions of these muscles are not fully understood. They 

 probably exist for the purpose of rendering the conducting appa- 

 ratus more fixed when a strong sound meets the ear in order that 

 the vibrations may be made weaker and thus prevent the auditory 

 nerve from being too strongly stimulated. By means of the tensor 

 tympani the tension of the membrana tympani can be accommo- 

 dated to very high notes. 



The Eustachian tube, a narrow canal (E, Fig. 38), passes 

 from the floor of the tympanic cavity forward and downward 

 and connects the middle ear with the pharynx. The tym- 

 panic cavity and the Eustachian tube are covered with 

 mucous membrane. The opening of the Eustachian tube 

 into the pharynx is generally closed by a fold in the mucous 

 membranes. During deglutition it is opened for a brief 

 period by the contraction of the tensor muscle and the 

 levator palati mollis. By the opening of the tube the pres- 

 sure of the external air and the air in the inner ear are 

 equalized, which is absolutely necessary for the normal con- 

 duction of sound into the middle ear. If the tube is closed 

 by catarrhal swelling of its mucous membrane, disturbances 

 in hearing result. The mucous membrane of the tube is 

 lined with cilia which move the mucus toward the pharynx. 



(c) The conduction of sound in the internal ear. The 

 internal ear, or labyrinth, is a cavity in the petrous bone and 

 is filled with a fluid. In the outer wall of the cavity are the 

 fenestrae rotundis and ovalis. 



The anterior part of the internal ear is the cochlea 

 (A, Fig. 38), a spirally wound canal of two and one-half 

 turns, divided into two parts by a bony plate. As the bony 

 plate is interrupted in the cupola, the passages of the canal 

 communicate at this place (helicotrema). One of the 

 passages, the scala vestibuli, opens at the base of the cochlea 



