EESPIEATION 305 



sudden alteration in calibre produces vibration and a musical 

 sound, as explained on p. 248. 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 

 the narrow opening of the infundibula into the bronchioles. 

 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 (fig. 134). 



V. Rhythm of Respiration. These movements of respira- 

 tion are carried on in a regular rhythmic manner. Their 

 rate varies with many factors; but the average number of 

 respirations per minute in the adult horse is about ten to 

 twelve or about one to every four or five beats of the 

 heart. 



The most important factor modifying the rate of respira- 

 tion is muscular exercise. After galloping the respirations may 

 rise to over sixty per minute. 



The other modifications in the rate of breathing will be 

 better understood after studying the nervous mechanism of 

 respiration. 



In these respiratory movements the phase of inspiration 

 bears a certain proportion to that of expiration. Inspiration 

 is much more rapid than expiration (see fig. 142). As soon 

 as inspiration is completed, a reverse movement occurs, which 

 is at first rapid, but gradually becomes slower, and may be 

 followed by a pause, during which the chest remains in the 

 collapsed condition. The existence and duration of this 

 pause varies much, and it may really be considered as the 

 terminal period of expiration. Considering it in this light, 

 we may say that inspiration is to expiration as 6 is to 7. 



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