TEE SENSE OF HEARING. 369 



amplitude. The wire represents a single radial fibre of the tympanic mem- 

 brane, and the funnel shape of this membrane is adapted to concentrating this 

 motion of the radial fibres upon the manubrium. The same effect is illus- 

 trated by the fact that when a string or a rope is stretched between two points, 

 no matter how tightly, it always sags at its middle; the weight of the cord, 

 however slight, is sufficient to give it a curved course, and produces a corre- 

 sponding traction on the points of support. 



Eustachian Tube. — That the tympanic membrane may maintain its 

 freedom of motion, it is obviously necessary that the average atmospheric 

 pressure on each side of it should remain the same. This equality of pressure 

 is maintained through the medium of the Eustachian tube, a somewhat trumpet- 

 shaped canal which, beginning in the lower anterior walls of the tympanum, 

 runs downward, forward, and inward, and terminates in a slit in the side of 

 the upper part of the pharynx. The Eustachian tube is lined, like the walls 

 of the tympanum, with ciliated epithelium, the cilia working in such a way 

 as to carry into the pharynx such secretions as may arise from the mucous 

 membrane of the middle ear. The pharyngeal opening of the Eustachian tube 

 is probably normally closed, but it may easily be made to open by increase or 

 decrease of air-pressure within the pharynx, as may be produced by closing 

 the nose and mouth and either forcing air into the pharynx by strong expiration 

 or rarefying it by suction. In the former case the air-pressure within the 

 tympanum is increased, and in the latter it is diminished. When air is thus 

 made to enter or to leave the tympanum, a sensation of a sudden snap and 

 a dull crackling noise in the ear is experienced. The lower end of the tube 

 is normally opened during the act of swallowing, and it is at this moment that 

 the intra- and extra-tympanic air-pressures are equalized. 



Muscles of the Middle Ear. — Two muscles are devoted to adjusting the 

 tension of the auditory mechanism of the middle ear. The tensor tympani is 

 lodged within a groove which is just above and about parallel with the Eusta- 

 chian tube. It terminates externally in a long tendon which bends nearly at 

 right angles round the outer edy;e of the groove and is inserted into the 

 handle of the malleus near the neck. Contraction of the tensor tympani 

 thus pulls inward the tympanic membrane, increases its tension, and some- 

 what dampens its vibrations. At the same time a strain is put upon the 

 chain of ossicles, the toothed processes of the malleus and incus arc broughl 

 more closely together, and the foot of the stapes is pressed into the oval win- 

 dow, increasing the pressure upon the fluids of the internal ear. It is said 

 that the relaxed tympanic membrane, particularly alter section of the tensor 

 tympani muscle, is thrown into sympathetic vibration with comparative ease, 

 and is in this condition best adapted to respond to weak aerial impulses and 

 to (he periodic waves of musical notes. When the membrane is tense its 

 vibrations are damped, and it is particularly lilted to transmit noises and con- 

 sonantal sounds, and thus the muscle involved would seem important to the 

 clear transmission of ordinary speech, though its effect would lie to decrease 

 the acuteness of hearing. According to Hensen, 1 the tensor tympani muscle 



1 Hermann's Handbuch der Physiologic, 1880. 



Vol. TT.— 24 



