CH. XXIV.] 



ASPHYXIA. 



371 



respiratory movements, arrives at the right side of the heart. 

 When it reaches the pulmonary arterioles it gives rise to a 

 certain amount of constriction of them as of the systemic vessels. 

 The obstruction to the circulation through the lungs thus produced 

 assists in bringing about a distended condition of the right heart 

 and pulmonary artery, and, on the other hand, produces a diminished 

 blood-flow through the pulmonary veins into the left side of the 

 heart. The main cause, however, of the distended state of these 

 parts, is due to the fact that the suction action of the left ventricle 

 diminishes as asphyxia progresses, and so the blood accumulates 

 in the right heart and veins. In the third stage of asphyxia the 

 left side of the heart therefore gets into the empty condition in 

 which it is found after death. (See figs. 329 and 330.) 



In the first and second stages of asphyxia the arterial blood- 

 pressure continuously rises until it reaches a point far above the 



Fig. 3.5 1. Blood-pressure tracing during asphyxia, much reduced in size. The tracing was 

 taken by a manometer connected with the femoral artery of a dog under curare. 

 Artificial respiration was discontinued at X. Both vagi had been previously divided. 

 If the vagi are not divided, the rise of pressure is much less, and the heart beats 

 very slowly. This enables the heart to last longer, and is due to excitation of the 

 cardio- inhibitory centre by venous blood. (Starling.) 



normal, and in the third stage blood-pressure falls rapidly. A 

 tracing of the arterial pressure is shown in fig. 331. 



Effects of Breathing Gases other than the Atmosphere. 



The diminution of oxygen has a more direct influence in the 

 production of asphyxia than the increased amount of carbonic 

 acid. Indeed the fatal effect of carbonic acid in the blood when 

 a due supply of oxygen is maintained, resembles rather the action 

 of a narcotic poison than it does asphyxia. 



Then again we must carefully distinguish the asphyxiating 

 effect of an insufficient supply of oxygen from the directly 

 poisonous action of such a gas as carbonic oxide, which is 

 contained to a considerable amount in common coal-gas. The 

 fatal effects often produced by this gas (as in accidents from 



B B2 



