CHAP, ii.] RESPIRATION. G25 



vionsly placed under urari, so that the respiratory impulses cannot. 

 manifest themselves by any muscular movements, the rise of the 

 pressure curve, as we have already said, is at first steady and 

 unbroken, but after a variable period Traube's curves make their 

 appearance. As during the third stage the pressure sinks, these 

 undulations pass away. 



The heart-beats are at h'rst somewhat quickened, but speedily 

 become slow, at the same time as we have seen not notably losing 

 force, so that the pulse-curves on the tracing are exceedingly 

 bold and striking. But the boldness of the curve of the mercury 

 manometer, is, it must be remembered, partly the mere result of 

 the slowness of the rhythm ; the mercury has time to fall largely 

 between each two beats (Fig. 86. 3 & 4). Even while the blood- 

 pressure is sinking, and when the cardiac stroke is now certainly 

 lessening in vigour, the slowness of the cardiac rhythm is still 

 sufficient to maintain somewhat these characters of the curve. 

 The strokes at last, however, rapidly fail in strength and become 

 irregular, though the heart continues 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 towards 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 spirt out 

 when pricked : and it may frequently be observed that the beats 

 recommence when the over-distension of the heart's cavities is 

 relieved by puncture of the great vessels. When rigor mortis 

 sets in after death b}^ 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. 



These various phenomena of asphyxia are probably brought 

 about in the following way. 



The increasingly venous character of the blood augments the 

 action of the vaso-motor 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 vaso-motor mechanisms, or, what is more likely, may 

 increase the peripheral resistance in the capillaries themselves, 

 since there are reasons for thinking ( 185) that venous blood 

 rich in carbonic acid meets with more friction, and passes less 



F. 40 



