762 PHYSIOLOGY. 



completing its motion by gliding in the joint. The destruction of the 

 chain of bones does not necessarily cause deafness, any more than 

 perforation of the membrana tympani, as long as the stapes is pre- 

 served. If the stapes is torn out there is deafness, because the peri- 

 lymph escapes into the middle ear and it is not able to transmit 

 sound-waves to the membranous labyrinth. 



The bones of the head also conduct sounds, as is easily proved 

 by closing the ears with your fingers and putting a watch between 

 the teeth. The intervention of the bones of the skull in the trans- 

 mission of sounds is made use of in the audiphone for the deaf, 

 where a rod, which terminates in a large disk spread out to receive 

 sounds, is held between the teeth. 



Transmission of Sounds by the Air in the Middle Ear. In the 

 normal state the air enclosed in the tympanic cavity plays an insig- 

 nificant part in the transmission of sound-waves, but its interven- 

 tion is inevitable when the chain of ossicles has been destroyed. It 

 is probable that it conveys the sound to the round window. 



Eustachian Tube. The air enclosed in the middle ear is con- 

 stantly kept in an equilibrium of pressure with the external air by 

 the intermittent patency of the Eustachian tube, which extends be- 

 tween the cavity of the tympanum and the pharynx. The Eustachian 

 tube is opened in each act of deglutition, by the salpingo-pharyngeus 

 and the dilator tubal muscles. If the air is enclosed in the tympanic 

 cavity, the oxygen goes to the blood and the carbon dioxide is given 

 off, but the amount of carbonic acid given out is less than the amount 

 of oxygen removed, so that the total quantity of gases in the tym- 

 panic cavity is reduced, the air is rarefied, and the membrana tym- 

 pani, on account of the vacuum, presses upon the chain of ossicles, 

 which are immobilized and do not readily transmit any vibrations. 

 By a forced expiration, the oral and nasal cavities being closed, fol- 

 lowed by an act of deglutition, air may be driven into the tympanic 

 cavity and a crackling noise will be heard. This is Valsalva's positive 

 experiment. A forced inspiration accompanied by deglutition will 

 draw air from the cavity, again causing a crackling noise, the nega- 

 tive experiment of Valsalva. In the positive experiment of Valsalva 

 the membrana tympani bulges outward; in the negative experiment 

 it bulges inward ; and in both, from the extreme tension of the mem- 

 brane, there is a partial deafness for high-pitched sounds. 



Permanent closure of the Eustachian tube by catarrhal condi- 

 tions is the most frequent cause of deafness. Closure of the tube, 

 except in deglutition, is necessary for the transmission of sound-waves 



