RESPIRATION 525 



o-entle wind among leaves, may be heard throughout inspira- 

 tion, 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. In the study of the cardiac circulation, 

 it was shown that a column of fluid moving along a tube of 

 uniform calibre, or with the calibre only slowly changing, pro- 

 duces no sound. The same is true of a column of air. Any 

 sudden alteration in calibre produces vibration and a musical 

 sound, as explained on p. 409. 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 be 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 

 infundibula drawing in and expelling air. The reason why the 

 sound is best heard during inspiration may be that the sound is 

 best conducted in the direction of the air stream. 



V. Rhythm of Respiration. 

 The movements of respiration are carried on in a regular 

 rhythmic manner. They may be recorded — 



1. By recording the movement of the chest wall by some 

 form of stethograph. 



2. By recording the movements of the column of air by 

 placing a glass tube in one nostril and connecting it with a 

 recording tambour {Practical Physiology). 



3. In lower animals, by connecting a strip of the diaphragm 

 to a lever. 



Their rate varies with many factors; but the average 

 number of respirations per minute in the adult horse is about 



