604 PHYSIOLOGY 



the ear, it allows the drum to go slack and therefore tends to prevent this 

 membrane from becoming stretched through being continually in tension. 



The stapedius muscle is innervated by a twig from the facial. Its function 

 is problematical. Some say it antagonises the tensor by decreasing the ten- 

 sion on the drum, others that it reduces the tension on the contents of the 

 internal ear by reducing the pressure of the stapes on the oval window. 

 Hartridge's view is that the function of this muscle is to cause the body of 

 the incus to engage with the spur of the malleus with sufficient force to pre- 

 vent chattering and lost motion when the vibrations are being transmitted 

 from one bone to the other. 



THE EUSTACHIAN CANAL is a tube about 35 mm. in length which 

 connects the middle ear with the pharynx. Normally it is kept closed in 

 order that the respiratory rhythm may not affect the pressure in the tym- 

 panum, and that the noise set up by the flow of air and the voice may not 

 be heard. When the canal is closed the middle ear becomes a closed chamber 

 which appears to increase the response to low tones. Since variations in 

 barometric pressure would not be communicated to the middle ear if the 

 canal were always closed, the air pressure on the two sides of the drum would 

 be found to vary. This is avoided by a periodic opening of the canal which 

 accompanies the act of swallowing. When the throat is infected the inflam- 

 mation often spreads to this canal which then becomes blocked either by 

 mucus or the swelling of its mucous lining. The air in the middle ear is 

 then gradually absorbed and the difference in air pressure on the two sides 

 of the drum decreases its response to sound, and the affected ear thus becomes 

 partially deaf. 



Temporary deafness also occurs if the barometric pressure is suddenly 

 altered by a rapid change of level (as in an aeroplane) or by the application 

 of external pressure (as in a caisson or submarine). The deafness is however 

 immediately relieved by swallowing, because the altered pressure is communi- 

 cated to the other side of the drum through the opening of the Eustachian 

 canal. 



FUNCTIONS OF TYMPANUM. The function of the tympanic 

 apparatus (consisting of drum, bones and muscles) is to transform 

 the energy of the aerial vibrations incident on the drum into a series 

 of mechanical movements of the plunger of the stapes, by which the 

 pressure within the internal ear is rapidly varied. The evidence may be 

 summarised as follows. (1) If in man the external ear be made to form a 

 gas chamber which is connected to a manometric flame, the flame shows 

 vibration when sound falls on the drum, which could only be caused if the 

 drum were set into vibration. (2) If the drum be gilded and a beam of light 

 be caused to fall on it, the extensions of the beam caused by vibrations 

 of the drum can be recorded photographically, and are found to accompany 

 the incidence of sound waves. (3) If to the chain of ossicles a light writing 

 lever be attached, the point of which travels over a rotating smoked drum, 

 when sounds fall on the drum the vibrations are recorded showing that 

 the ossicles are set into movement. (4) By opening the middle ear from 



