218 CIRCULATION. 



artery will indicate a great increase in the pressure of blood, 

 and a corresponding increase in the movements of the mer- 

 cury will be noted at each action of the heart ; indicating 

 that the organ is acting with an abnormal vigor. If respira- 

 tion be still discontinued, the engorgement becomes intense, 

 the heart at each diastole being distended to its utmost capa- 

 city. It now becomes incapable of emptying itself; the con- 

 tractions become very unfrequent, perhaps three or four in a 

 minute, and are progressively enfeebled. The organ is dark, 

 almost black, owing to the circulation of venous blood in its 

 substance. If respiration be not resumed, this distention 

 continues, the contractions become less frequent and more 

 feeble, and in a few minutes entirely cease. 



The arrest of the action of the heart, under these circum- 

 stances, is chiefly mechanical. The un aerated blood passes 

 with difficulty through the capillaries of the system, and as 

 the heart is constantly at work, the arteries become immensely 

 distended. This is proven by the great increase in the arte- 

 rial pressure, -while these vessels are full of black blood. If 

 we now closely examine the heart and great vessels, we are 

 able to note distinctly the order in which they become dis- 

 tended. These phenomena were particularly noticed and de- 

 scribed by Prof. Dalton, and they demonstrate conclusively 

 that in asphyxia the obstruction to the circulation commences, 

 not in the lungs, as is commonly supposed, but in the capil- 

 laries of the system, and is propagated backwards to the heart 

 through the arteries. 



" The obstacle to the passage of venous blood through the 

 capillaries, therefore, is partial, not complete. But it is still 

 sufficient to produce an immediate backward engorgement of 

 the arterial system. Then the aorta becomes distended at its 

 origin, and the left ventricle and left auricle in succession, 

 being unable to relieve themselves of blood through the arte- 

 rial system, become distended in a similar manner. During 

 this time the same kind of engorgement takes place in the pul- 

 monary artery and the right cavities of the heart ; though 



