REGULATION OF RESPIRATORY MOVEMENTS 1091 



up the vagus nerves. To test the truth of this hypothesis it is necessary 

 to study the two phases of respiration separately ; that is, to see first the 

 result on the respiratory impulses of distension of the lungs, and, secondly, 

 the result of a sudden collapse or a contraction caused by sucking air out 

 of the lungs. The effects of distension or collapse of the lung may be shown 

 by simply closing the trachea at the end of inspiration or of expiration. The 

 results of such an experiment are shown in Fig. 510. 



A still more marked effect is produced if the lungs, by means of a tube 

 in the trachea, be artificially inflated or if air be sucked out of them. The 

 inflation produces an instantaneous and complete relaxation of the dia- 

 phragm (Fig. 511) which by clamping the tracheal tube may be prolonged 



Pos. ventilation 



Diaphragm 

 Seconds 



FIG. 511. Positive ventilation. (HEAD.) 



Under the influence of positive ventilation, the inspiratory contractions of the 

 diaphragm become less and less till they disappear completely. 



Neg. ventilation 

 Diaphragm 



FIG. 512. Negative ventilation. (HEAD.) 



At a negative ventilation was commenced. The expiratory relaxation of the 

 diaphragm is seen to become more and more incomplete, until it finally enters into 

 continued contraction. 



for several sconds, while sucking air out of the lungs causes a tonic contrac- 

 tion of the diaphragm (Fig. 512). Somewhat similar results may be obtained 

 by repeatedly inflating or deflating the lungs (positive and negative ventila- 

 tion). The effects here are complicated by the fact that one is dealing in 

 both cases with alternating movements of the lungs, of expansion and con- 

 traction, both of which will have an influence on the respiratory centre. 

 Moreover repeated forcible inflation of the lungs increases the ventilation 

 of the pulmonary alveoli, thus lowering the normal carbon dioxide tension 

 of the lungs. As a result of repeated ventilation we may obtain a condition 

 of respiratory standstill. In this condition, however, as we shall see later, 

 the determining factor is rather chemical than mechanical. 



These inhibitory and augmentor effects of changes in the volume of 

 the lung must also result from the normal movements of these organs in 



