288 HUMAN PHYSIOLOGY 



The mechanism by which the gaseous exchange in the 

 alveoli is carried out is thus a double one. 



IY. Breath Sounds. The air as it passes into and out of 

 the lungs produces sounds, which may be heard on listening 

 over the thorax. The character of the breath sounds are of 

 the utmost importance in the diagnosis of diseases of the 

 lungs, and must be studied practically. 



On listening over the trachea or over the bifurcation of 

 the bronchi behind (between the 4th and 5th dorsal vertebra?), 

 a harsh sound, something like the guttural ch (German ich), 

 may be heard with inspiration and expiration. This is called 

 the bronchial sound. 



If the ear be applied over a spot under which a mass of 

 air vesicles lies, a soft sound, somewhat resembling the sound 

 of gentle wind among leaves, may be heard throughout 

 inspiration and for a third or less of expiration. This is 

 called the vesicular sound. 



When the air vesicles become consolidated by disease the 

 vesicular sound is lost and the bronchial sound takes its 

 place. The cause of the vesicular character is therefore to 

 be sought in the vesicles, infundibula, or small bronchi. 



The cause of the bronchial sound has been determined by 

 experiments on horses. It has already been seen that a 

 column of fluid and the same is true of a column of air 

 moving along a tube of uniform calibre, or with the calibre 

 only slowly changing, produces no sound. Any sudden 

 alteration in calibre produces vibration and a musical sound, 

 as explained on p. 233. The first sharp constriction of the 

 respiratory tract is at the glottis, and it is here that the 

 bronchial sound is produced. If the trachea is cut below 

 the larynx and drawn freely outwards, the bronchial sound 

 at once stops, and the vesicular sound becomes lower and less 

 distinct. 



The cause of the vesicular sound is not so satisfactorily 

 explained. It is in part due to propagation of the bronchial 

 sound, altered by passing through vesicular tissue ; but it is 

 also probably due to the expansion and contraction of the 

 air vesicles drawing in and expelling air, either through their 

 somewhat narrow openings into the infundibula, or through 



