4 6 



THE MECHANISM OE THE CIRCULATION. 



slight convulsive spasms, owing to the production of acute cerebral 

 anaemia. 



In the vertical feet-down posture, when the full hydrostatic effect of 

 gravity opposes the return of blood from the parts below the heart, the 

 efficiency of the respiratory pump becomes of vital importance. Thus 

 the establishment of forcible artificial respiration by raising the intra- 

 thoracic pressure, while the animal is in the feet- 

 down position, forces the arterial pressure to zero. 

 In the horizontal posture the fall of arterial pressure 

 is considerable, but not so dangerous. Compression 

 of the abdomen immediately restores the circulation. 

 This compensates for the positive pressure in the 

 thorax, and fills the heart with blood. 1 



Fig. 30. — A, effect on 

 aortic pressure of 

 artificial ventilation 

 of the lungs in the 

 vertical feet- down 

 posture ; B, compen- 

 satory effect of abdo- 

 minal compression. 

 — Hill and Barnard. 



When the larynx is blocked by some hard impacted 

 mass of food, too hastily swallowed, death is hastened 

 by the violent expiratory spasms, which raise the intra- 

 thoracic pressure, and lead to deficient filling of the heart. 

 So, in the fatal crushes of panic-stricken crowds, death 

 is produced by compression of the thorax and circulatory 

 failure. The women and children with collapsible chests 

 are the first to die, while the men with rigid chests 

 escape. 



The case of Colonel Townsend, as reported by Cheyne, 2 

 has always aroused considerable interest. Colonel Towns- 

 end was in the last stage of some form of nephritic 

 disease, and life was at its lowest ebb. Cheyne reports 

 the experiment he witnessed together with two other 

 medical men. " The man could die or expire when he pleased, and yet, by 

 an effort or somehow, he could come to life again. He composed himself on 

 his back, and lay in a still posture some time. While I held his right hand, 

 Dr. Baynard laid his hand on his heart, and Mr. Skrine held a clear looking- 

 glass to his mouth. I found his pulse sink gradually, till at last I could not 

 feel any, by the most exact and nice touch. Dr. Baynard could not feel the 

 least motion in his heart, nor Mr. Skrine the least soil of breath on the bright 

 mirror he held to his mouth ; then each of us by turns examined his arm, 

 heart, and breath, but could not by the nicest scrutiny discover the least 

 symptom of life in him. We began to conclude that he had indeed carried the 

 experiment too far, and at last were satisfied he was actually dead, and were 

 just ready to leave him. As we were going away, we observed some motion 

 about the body, and upon examination found his pulse and the motion of his 

 heart gradually returning; he began to breathe gently and speak softly." He 

 died the same evening. 



The explanation of this case, which appeared so full of mystery to 

 Cheyne, is probably to be found in the following facts. The circulation was 

 failing, the blood pressure very low, and the vascular tone probably almost 

 entirely lost ; such was evidenced by his thready and feeble pulse. The heart- 

 beat was so feeble, that the vis-a-tergo was insufficient, even in the recumbent 

 posture, to return the blood to the auricle without the aid of respiration. 

 When, therefore, Colonel Townsend held his breath, the circulation ceased, 

 while the empty heart continued to beat, but so feebly as to be imperceptible. 

 If anaemia of the bulbar centres is slow in origin, spasm is never provoked 

 by the total cessation of the circulation. On asphyxiating a curarised animal, 

 the heart frequently is found to continue to beat for fifteen or even twenty 



1 Hill and Barnard, Journ. Physiol., Cambridge and London, 1897, vol. xxi. p. 



2 Cheyne, "English Malady," London, 1733, p. 307. 



333. 



