THE RESPIRATORY MECHANISM. 423 



the formation of carbon-monoxide haemoglobin which has a 

 different color from oxyhaemoglobin. A dilute solution of 

 reduced haemoglobin absorbs all the rays of light in one 

 region about the green of the solar spectrum, and so pro- 

 duces there a dark band; a thin layer of the blood of an 

 asphyxiated animal does the same. Dilute solution of oxy- 

 haemoglobin absorbs the rays in two narrow regions of the 

 solar spectrum at the confines of the yellow and green, and 

 arterial blood does the same. Dilute solution of carbon- 

 monoxide haemoglobin, or blood which has been exposed to 

 this gas, also absorbs the light in two narrow bands of the 

 >.olar spectrum; but these are a little nearer the blue end of 

 the spectrum than the absorption bands of oxyhaemoglobin. 

 Pure blood serum saturated with oxygen gas or with carbon 

 monoxide does not specially absorb any part of the spectrum; 

 therefore the absorptions when haemoglobin is present must 

 be due to chemical compounds of those gases with that body. 



Since carbon-monoxide haemoglobin has a bright-red color, 

 we find, in the Bodies of persons poisoned by that gae, the 

 blood all through the Body cherry-red; the tissues being 

 unable to take carbon monoxide from haemoglobin in the 

 systemic circulation. Hence the curious fact that, while 

 death is really due to asphyxia, the blood is almost the color 

 of arterial blood, instead of very dark purple, as in ordinary 

 cases of death by suffocation. Experiments with animals 

 show that in poisoning by carbon monoxide persistent ex- 

 posure of the blood to oxygen, by means of artificial respira- 

 tion, will cause the poisonous gas to be slowly replaced again 

 by oxygen; hence if the heart has not yet quite stopped 

 beating, artificial respiration, kept up patiently, should be 

 employed in the case of poisoning by carbon monoxide unless 

 transfusion of blood be possible. 



The Phenomena of Asphyxia. As soon as the oxygen 

 in the blood falls below the normal amount the breathing 

 becomes hurried and deeper, and the extraordinary muscles 

 of respiration are called into activity. The dyspnoea be- 

 comes more and more marked, and this is especially the case 

 with the expirations which, almost or quite passively per- 

 formed in natural breathing, become violently muscular. At 

 last nearly all the muscles in the Body are set at work; the 

 rhythmic character of the respiratory acts is lost, and general 

 convulsions occur, but, on the whole, the contractions of the 



