400 RESPIRATION. 



ever, rapidly fail in strength arid become irregular, though the heart con- 

 tinues to beat for some seconds after the respiratory movements have ceased. 



If the chest of an animal be opened under artificial respiration, and 

 asphyxia brought on by cessation of the respiration, it will be seen that the 

 heart during the second and third stages becomes completely gorged with 

 venous blood, all the cavities as well as the large veins being distended to 

 the utmost. If the heart be watched to the close of the events, it will be 

 seen that the feebler strokes which come on toward the end of the third stage 

 are quite unable to empty its cavities ; and when the last beat has passed 

 away its parts are still choked with blood. The veins spurt out when 

 pricked ; and it may frequently be observed that the beats recommence 

 when the over-distention of the heart's cavities is relieved by puncture of 

 the great vessels. When rigor mortis sets in after death by asphyxia, the 

 left side of the heart is more or less emptied of its contents ; but not so the 

 right side. Hence, in an ordinary post-mortem examination in cases of 

 death by asphyxia, while the left side is found comparatively empty, the 

 right appears gorged. 



The various phenomena of asphyxia are probably brought about in the 

 following way : 



The increasingly venous character of the blood augments the action of 

 the vasomotor centres, both the medullary centre and the subsidiary centres 

 in the spinal cord, and thus leads to a constriction of the small arteries, 

 especially of the splanchnic area. This is the chief cause of the markedly 

 increased blood-pressure; though the venous blood may possibly also act 

 directly on peripheral vasomotor mechanisms, or what is more likely, may 

 increase the peripheral resistance in the capillaries themselves, since there 

 are reasons for thinking ( 171) that venous blood rich in carbonic acid 

 meets with more friction, and passes less easily through the capillaries than 

 does blood less venous in character. 



This increased peripheral resistance and the high blood-pressure to which 

 it gives rise, while tending to increase the distention of the left ventricle and 

 so indirectly helping to augment the force of the heart's beat, soon becomes 

 a direct obstacle to the heart emptying itself of its contents. On the other 

 hand, the labored respiratory movements favor the flow of venous blood 

 toward the heart, which in consequence becomes more and more full. This 

 repletion is, moreover, assisted by the marked infrequency of the beats which 

 is soon developed. This in turn depends in part on the cardio-inhibitory 

 centre in the medulla being stimulated by the venous blood, but, as we 

 have previously seen, cannot be wholly accounted for in this way. The 

 increased resistance in front, the augmented supply from behind, and the 

 long pauses between the strokes, all concur in distending the heart more 

 and more. 



When the large veins have become full of blood, the inspiratory move- 

 ments can no longer have their usual effect in facilitating the venous flow 

 into the right auricle. The chief effect of the chest movement, as far as the 

 circulation is concerned, is to widen and so to increase the capacity of the 

 pulmonary vessels, and at the same time to diminish the pressure around the 

 large arteries ; hence the marked sinking of the blood-pressure during each 

 inspiratory movement. 



The distention of the cardiac cavities, at first favorable to the heart-beat, 

 as it increases becomes injurious; and the cardiac tissues after a while be- 

 come enfeebled by the action of the venous blood, so that the strokes of the 

 heart become weaker and irregular. 



On account of this increasing feebleness of the heart's beat, accompanied 

 by more or less irregularity, the blood-pressure, in spite of the continued 



