44 CIRCULATION OF THE BLOOD 



the muscles, being no longer supplied with fresh blood, soon lose their 

 contractility and respiration ceases. Circulation, also, is impossible if 

 respiration is permanently arrested. When respiration is imperfectly 

 performed, the action of the heart is slow and labored. The effects of 

 arrest of respiration are marked in all parts of the circulatory system, 

 arteries, capillaries and veins ; but the disturbances thus produced all 

 react on the heart. 



If the heart is exposed in a living animal and artificial respiration 

 is kept up, although the pulsations are increased in frequency and 

 diminished in force, after a time they become regular and continue thus 

 so long as air is adequately supplied to the lungs. Under these condi- 

 tions, respiration is under control and the effects of its arrest on the 

 heart can easily be studied. If respiration is interrupted, the following 

 changes in the action of the heart are observed : For a few seconds 

 pulsations go on as usual ; but in about a minute they begin to diminish 

 in frequency. At the same time, the heart becomes engorged with 

 blood and the distention of its cavities rapidly increases. For a time 

 its contractions are competent to discharge the entire contents of the 

 left ventricle into the arterial system, and a cardiometer applied to an 

 artery then indicates an increase in the blood-pressure. A correspond- 

 ing increase in the movements of the mercury will be noted at each 

 contraction of the heart, indicating that the organ is acting with abnor- 

 mal vigor. If respiration is still interrupted, the engorgement becomes 

 intense, the heart at each diastole being distended to its utmost capacity. 

 It now becomes incapable of emptying itself, the contractions become 

 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 is not resumed, this 

 distention continues, the contractions become less frequent and more 

 feeble and in a few minutes they cease. 



The arrest of the action of the heart, under these conditions, is 

 chiefly mechanical. The unaerated blood passes with difficulty through 

 the capillaries of the system, and as the heart is constantly at work, the 

 arteries become greatly distended. This is shown by the great increase 

 in the pressure while the arteries are filled with black blood. If, now, 

 the heart and great vessels are closely examined, the order in which 

 they become distended is readily observed. These phenomena show 

 that in asphyxia the obstruction to the circulation begins, not in the 

 lungs, but in the capillaries of the system and is propagated backward 

 to the heart through the arteries. The distention of the heart in as- 

 phyxia is therefore due to the fact that unaerated blood cannot circulate 

 freely in the systemic capillaries. When thus distended, the heart 



