CH. XXIV.] ASPHYXIA 373 



passes through the arterioles, and, favoured by the laboured respira- 

 tory movements, arrives at the right side of the heart, which it 

 fills and distends ; the right side of the heart is becoming feebler at 

 the same time, and therefore unable to effectively discharge its blood 

 through the pulmonary circuit. Simultaneously the left ventricle is 

 also becoming weakened, and therefore its suction action diminishes. 

 In this way the blood is dammed back 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. Some consider that the early onset of rigor mortis in the 

 left ventricle may be in part a cause of its contracted and empty 

 condition. 



In the first and second stages of asphyxia, the arterial pressure 

 rises until it reaches a point far above the normal ; this is due to the 

 constriction of the arterioles. The fall of pressure in the last stage 

 is mainly due to heart failure. If the vagi are not divided previously, 

 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. The accompanying photo- 

 graph of a tracing, which I owe to Prof. C. J. Martin, shows these 

 effects ; it has been somewhat reduced in size for purposes of repro- 

 duction. The lower tracing is that of venous pressure taken with 

 a salt solution manometer from the jugular vein. It will be noticed 

 that the fall of arterial pressure in the last stage is accompanied 

 with a great rise of venous pressure due to the venous congestion 

 just described. 



Effects of Breathing Gases other than the Atmosphere. 



The diminution of oxygen has a less direct influence in the production of 

 asphyxia than the increased amount of carbonic acid. Nevertheless, 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 burning charcoal 

 stoves in small, close rooms) are due to its entering into combination with the 

 haemoglobin of the blood-corpuscles, and thus expelling the oxygen. Hydrogen 

 may take the place of nitrogen if the oxygen is in the usual proportion, with no 

 marked ill effect. Sulphuretted hydrogen interferes with the oxygenation of 

 blood. Nitrous oxide acts directly on the nervous system as a narcotic. Certain 

 gases, such as carbon dioxide in more than a certain proportion ; sulphurous and 

 other acid gases, ammonia, and chlorine produce spasmodic closure of the glottis, 

 and are irrespirable. 



Alterations in the Atmospheric Pressure. 



The normal condition of breathing is that the oxygen of the air breathed should 

 be at the pressure of i of the atmosphere, viz., | of 760 mm. of mercury, or 152 mm., 



