CHAP, ii.] RESPIRATION. 379 



action of the general vaso-motor centre, and thus leads to a general 

 constriction of the small arteries This is the cause of the 

 markedly increased blood-pressure; though, as we have already 

 said, the venous blood may also act directly on the other spinal 

 vaso-motor centres and possibly on peripheral vaso-motor me- 

 chanisms or on the muscular arterial coats, or may even affect the 

 peripheral resistance by modifying the changes in the capillary 

 regions. 



This increased peripheral resistance, while indirectly help- 

 ing to augment the force of the heart's beat, is a direct obstacle 

 to the heart emptying itself of its contents. On the other hand, 

 the increased respiratory movements favour the flow of venous 

 blood towards the heart, which in consequence becomes more and 

 more full. This repletion is moreover assisted by the marked 

 infrequency of the beats. This in turn depends in part on the 

 cardio-inhibitory centre in the medulla being stimulated by the 

 venous blood ; since when the vagi are divided the infrequency is 

 much less pronounced. It does not however disappear altogether ; 

 and we are therefore driven to suppose it is in part due to the 

 venous blood acting in an inhibitory manner directly on the heart 

 itself. 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 

 movements can no longer have their usual effect in increasing the 

 blood-pressure. The whole force of the chest movement, as far as 

 the circulation is concerned, is spent in diminishing the pressure 

 around the large arteries ; and hence the sinking of the blood- pres- 

 sure during each inspiratory movement. 



The distension of the cardiac cavities, at first favourable to the 

 heart-beat, as it increases becomes injurious. At the same time 

 the cardiac tissues, which at first probably are stimulated, after a 

 while become exhausted by the action of the venous blood ; and 

 the strokes of the heart become feebler as well as slower. 



On account of this increasing slowness and feebleness of the 

 heart's beat, the blood-pressure, in spite of the continued arterial 

 constriction, begins to fall, since less and less blood is pumped into 

 the arterial system ; the boldness of the pulse-curves at this stage 

 being chiefly due to the infrequency of the strokes. As the 

 quantity which passes from the heart into the arteries becomes 

 less second by second, the pressure gets lower and lower, the 

 descent being assisted by the exhaustion of the vaso-motor centre, 

 until almost before the last beats it has sunk to zero. Thus at the 

 close of asphyxia, while the heart and venous system are distended 

 with blood, the arterial system is less than normally full. 



