474 EESPIRATION 



during inspiration, while an ordinary rubber bulb yields a positive 

 pressure during this period, and forces the writing lever upward. 



The inspiratory movement sets in gradually. It acquires a con- 

 siderable speed in its intermediate phase but again becomes slow 

 toward its end. Expiration is slow at first, rather rapid in its inter- 

 mediate phase and decidedly slow toward its end. In general, 

 therefore, inspiration sets in more abruptly than expiration and occu- 

 pies a somewhat shorter time than expiration, the relationship between 

 these periods being as 10 : 14. Furthermore, while the expiratory 

 movement follows immediately upon the completion of inspiration, 

 the next inspiratory motion is not begun until a few moments later. 

 It is to be noted, however, that a true pause is not developed at this 

 time in spite of the fact that the muscles are perfectly quiescent. 

 This must necessarily be so, because the elastic forces are at play even 

 during this interim of relative "rest," tending to retain the chest in a 

 semi-active or "set" condition. It is also a matter of common experi- 

 ence that the action of the respiratory muscles may be arrested at 



FIG. 246. PNEUMATOGRAM RECORDED BY CONNECTING THE INTHAPLEURAL SPACE WITH 



A MEMBRANE MANOMETER. 



J, Insp. movement; E, expir. movement. The figures indicate the negative pressure 

 recorded during these resp. cycles. 



any time during the respiratory cycle. Ordinarily, however, the 

 desire to resume this activity becomes imperative in less than a minute, 

 owing to the accumulation of excessive amounts of carbon dioxid' 

 But if the system is first thoroughly ventilated and surcharged with 

 oxygen by a series of forced respiratory movements, the breath may 

 be held for a much longer period of time. Professional divers, for 

 example, are capable of remaining under water for several minutes. 

 The number of respirations in a given period of time vary with 

 the conditions under which the animal lives. In the adult human 

 being from 16 to 20 cycles are completed in the course of one minute, 

 their average number being 18. It is also to be noted that the respira- 

 tory frequency is greater when the person assumes the erect position 

 than when recumbent or sitting down. Naturally, this change is 

 in accordance with the oxygen requirement, the mere act of rising 

 necessitating a greater muscular activity and metabolism. An impor- 

 tant influence is also exerted by age, as may be gathered from the 

 following compilation: 



