MECHANICAL APNGEA. 



299 



collapse of the lung or a momentary suction of air out of the air tubes, 

 as well as the diminished activity caused by inflation or by closing the 

 trachea at the end of inspiration, outlast the events which give rise to 

 them by a considerable time. Thus, a momentary suction will give 

 rise to an increased tone of the diaphragm which lasts for four or five 

 seconds, or during the five or six respirations following the suction. 

 These after-effects are designated by Head the "positive after-action!' 

 Now this observer, in confirmation of observations by Gad, has 



FIG. 174. Effects of positive ventilation with air on respiratory movements. (The upper 

 lino shows the passive movements of the sternum, the middle line the strokes of the 

 pump, and the lower line the respiratory movements of the slip of diaphragm.) Head. 



shown that, by repeating the stimulation at sufficiently short intervals, 

 we may have a summation of the effects (the positive after-action), so 

 that finally there is a standstill of respiration in a condition of expira- 

 tion, or of continuous inspiration, or in a position halfway between the 

 two. In order to carry out the series of rhythmic inflations or suctions, 

 Head employed a double-action pump devised by Hering. When the whole 

 pump is in action the first stroke pumps a certain quantity of air or 

 gas into the lungs, and the next stroke removes exactly the same 

 quantity. By connecting 

 one-half only of the pump 

 with the trachea, it is possible 

 to produce only periodic in- 

 flations (positive ventilation), 

 while by using only the other 

 half the ventilation consists 

 simply of periodic suctions 

 (negative ventilation), the 

 lungs in each case tending 

 to return to the previous 

 condition between the strokes 

 of the pump, in virtue of the 

 elasticity of the chest wall. 



When positive ventilation 

 is employed (Fig. 173), every 

 inflation not only produces an expiratory pause during the time that it 

 lasts, but this depressing influence on the inspiratory activity of the 

 centre continues even after the lungs have returned to their normal 

 volume. Hence the subsequent contractions suffer from the inhibition, 

 and the inspiratory movements get smaller and smaller with every 

 stroke of the pump, until they entirely disappear, the pause continuing 

 for a time even after the ventilation is stopped ; this condition is 

 expiratory apncea. 



