270 THE CHANGES OP THE CIRCULATION DURING ASPHYXIA. 



systemic veins. The blood itself is almost black, and is deprived of almost 

 all its oxygen, while its haemoglobin is nearly all in the condition of 

 reduced haemoglobin, while ordinary venous blood contains a considerable 

 amount of reduced and oxyhaemoglobin. The blood of an asphyxiated 

 animal practically contains none of the latter, and much of the former. 



It is important to study the changes in the circulation in connection 

 with the outward phenomena exhibited by an animal during suffocation. 



We may measure the blood-pressure in any artery of an animal while 

 it is being asphyxiated, or we may open its chest, maintain artificial respi- 

 ration, and place a manometer in a systemic artery, e.g., the carotid, 

 and another in a branch of the pulmonary artery. In the latter case, 

 we can watch the order of events in the heart itself, when the artificial 

 respiration is interrupted. It is well to study the events in both cases. 



If the blood-pressure be measured in a systemic artery, e.g., the 

 carotid, it is found that the blood-pressure rises very rapidly and to a 

 great extent during the first and second stages ; the pulse-beats at first 

 are quicker, but soon become slower and more vigorous. During the 

 third stage it falls rapidly to zero. The great rise of the blood-pressure 

 during the first and second stages is chiefly due to the action of the 

 venous blood on the general vaso-motor centre, causing constriction of 

 the small systemic arteries. The peripheral resistance is thus greatly 

 increased, and it tends to cause the heart to contract more vigorously, 

 but the slower and more vigorous beats of the heart are also partly 

 due to the action of the venous blood on the cardio-inhibitory centre in 

 the medulla. 



If the second method be adopted, viz., to open the chest, keep up 

 artificial respiration, and measure the blood-pressure in a branch of the 

 pulmonary artery, as well as in a systemic artery, e.g., the carotid we 

 find that when the artificial respiration is stopped, in addition to the 

 rise of the blood-pressure indicated in the carotid manometer, the 

 cavities of the heart and the large veins near it are engorged with 

 venous blood. There is, however, but a slight comparative rise in the 

 blood-pressure in the pulmonary artery. This may be accounted for, 

 either by the pulmonary artery not being influenced to the same extent 

 as other arteries, by the vaso-motor centre, or by its greater distensibility 

 (Lichtheim compare 88). But, as the heart itself is supplied through 

 the coronary arteries with venous blood, its action soon becomes 

 weakened, each beat becomes feebler, so that soon the left ventricle 

 ceases to contract, and is unable to overcome the great peripheral 

 resistance in the systemic arteries, although the right ventricle may 

 still be contracting. As the blood becomes more venous, the vaso- 

 motor centre becomes paralysed, the small systemic arteries relax, and 

 the blood flows from them into the veins, while the blood-pressure in 



