890 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



a more powerful but less extensive vibration. So much for the mode of 

 conduction of sound through the ossicles. 



By contraction of the muscular fibres in connection with the ear- 

 ossicles the position and tension of the tympanic membrane, as well as 

 the pressure on the lymph of the labyrinth, may be altered. Two 

 muscles are found in connection with the ear-ossicles, the tensor tympani 

 and the stapedius muscle. The tensor tympani lies in the osseous 

 groove above the Eustachian tube, and has its tendon inserted into the 

 malleus immediately above the axis. When this muscle contracts the 

 handle of the malleus is pulled inward and the tympanic membrane 

 tightened. As a consequence, the stapes is likewise pressed inward. 

 On the other hand, when this muscle relaxes the elasticity of the axial 

 ligament and of the tympanic membrane itself causes the membrane to 

 again assume its condition of equilibrium. By the increased tension of 

 the tympanic membrane a greater resistance is offered to the sympathetic 

 vibrations when the sound-waves are very intense, since it has been found 

 that stretched membranes are less susceptible to sympathetic vibrations 

 than are relaxed membranes ; and increase of the tension of the tympanic 

 membrane by contraction of this muscle, therefore, serves to protect the 

 auditory apparatus by preventing intense vibrations reaching the nerve 

 terminations. 



The stapedius muscle arises within the pyramidal eminence, and is 

 inserted into the head of the stapes. When it contracts it draws upon 

 the head of the stapes and causes the bone to assume an oblique position, 

 the posterior end of the plate being pressed deeply inward into the 

 fenestra ovalis, while the anterior edge of the plate is displaced outward. 

 The stapes is thus firmly fixed and the annular ligament surrounding 

 the fenestra ovalis becomes more tense. The fnnction of this muscle is 

 likewise directed to preventing the communication of too intense an 

 impulse from the incus to the stapes. The stapedius muscle is supplied 

 by the facial nerve and the tensor tympani by a branch of the trigeminus 

 which passes from the otic ganglion. 



The chain of bones lies within the tympanic cavity, and it is 

 evident that the vibration of the tympanic membrane will greatly vary 

 • according as the air in the tympanic cavity is in a greater or less degree 

 of condensation. If this space were entirely shut off from the atmos- 

 phere the air in it would evidently soon be absorbed, or, at any rate, 

 undergo change in its composition, and probably be replaced by fluid 

 secretions, since we know that the middle ear is lined by a secreting 

 mucous membrane. By means of the Eustachian tube the ventilation 

 of the middle ear is rendered possible. Through it secretions are con- 

 ducted out, and by it the equilibrium of pressure between the air in the 

 tympanum and the atmosphere is rendered possible. As soon as the 



