300 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



On using negative ventilation the respiration is modified in a very 

 different manner. Each suction causes a strong inspiratory contraction, 

 which ceases as soon as the suction stops, but leaves behind it a con- 

 dition of raised inspiratory activity, evidenced by the imperfect relaxation 

 of the diaphragm. As the suctions are repeated, the augmentor effects 



on the inspiratory activity are summated ; 

 the relaxation of the diaphragm between 

 each stroke becomes less and less, until finally 

 there is a cessation of movements of the 

 diaphragm, which remains in a state of tonic 

 or tetanic contraction, far exceeding in height 

 any normal inspiration (inspiratory apnoea). 



Compound ventilation produces an apncea 

 intermediate between the other two forms, 

 the inspiratory contractions and expiratory 

 elongations becoming smaller until they 

 finally disappear; and the lever traces a 

 line of varying height, according to the 

 relative value of the two stimuli (alternate 

 inflation and suction). 



Eosenthal ascribed the apnoea produced by 

 positive ventilation (i.e. by artificial respira- 

 tion as ordinarily practised) to the excessive 

 aeration of the blood thereby induced; whereas 

 we have just seen that the causation of the 

 apnoea can be adequately accounted for by 

 the periodically recurring excitations of the 



FIG. 176. Diagrammatic repre- 

 sentation of effects of different 

 kinds of ventilation, a, normal 

 breathing ; b, apncea with posi- 

 tive ventilation ; c, apncea with 

 negative ventilation ; d, apncea 



with compound ventilation. inhibitory fibres of the vagus, consequent en 

 Head. the repeated inflations of the lungs. 



A cogent objection to Rosenthal's interpretation is to be found in 

 the fact, that the arterial blood is normally so nearly saturated with 

 oxygen that increased ventilation can cause very little increase in the 

 amount of oxygen carried by the blood. Moreover, it has been shown 

 that towards the end of an apnoeic pause, before the respiratory move- 

 ments have recommenced, the blood becomes of a venous hue on both 



FIG. 177. Apnceic pause, caused by positive ventilation with hydrogen. Head. 



sides of the heart. The oxygenation of the blood is in fact less than 

 in a normal animal. The most striking proof, however, that the ^apncea 

 is determined solely by recurring stimulation of the vagus fibres, is 

 afforded by the fact that a distinct apnoeic pause can be produced by 

 positive or compound ventilation with some inert gas, such as hydrogen 

 or nitrogen, which is powerless to support life (see Fig. 177). 



It might be thought from these experiments that it would be 



