814 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 edge of the groove and is inserted into the 

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

 results in pulling the tympanic membrane inward and rendering it more tense 

 (PI. 2, Fig. 1). This increase of tension of the membrane seems to adapt it 

 better to the more rapid vibrations of high musical notes, but allows less ready 

 response to lower notes. It is said that the tensor tympani comes normally 

 into action at the beginning of a sound, thus tuning the membrane for the 

 note that is to follow, and then relaxes. One of its effects is probably to bring 

 closely together the toothed processes of malleus and incus at the beginning 

 of a sound, so that there shall be no loss of motion during the vibrations 

 of the membrane. The stapedius is a small muscle imbedded in the inner 

 wall of the tympanum near the fenestra ovalis. Its tendon, passing forward, 

 is inserted into the neck of the stapes. Contraction of the muscle would cause 

 a slight rotation of the stapes round a vertical axis, so that the hinder part 

 of the foot of the ossicle would be pressed more deeply into the fenestra, while 



