THE RESPIRATION 289 



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. 



Y. 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 male is about sixteen, 

 or about one to every four or five beats of the heart. The 

 rate of respirations may be modified by the will, and, 

 therefore, in counting the respirations in a patient, it is well 

 to prevent his being aware of what is being done. Similarly, 

 on account of this influence of the upper part of the brain, 

 the respirations should not be counted while the patient is 

 excited or nervous. 



The most important factor modifying the rate of respira- 

 tion is the age of the individual. The following table gives 

 the average rate at different ages : 



Under 1 year . . 44 



5 years . . 26 



20 . 19 



. Adult .... 16 



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. 134). As soon 

 as inspiratian 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. 



YI. Nervous Mechanism of Respiration. The rhythmic 

 movements of respiration require the harmonious action of 



19 



