1 66 RESPIRATION 



great significance of this "acclimatization" will be discussed in 

 succeeding chapters. 



The other gas, besides CO, which enters into molecular com- 

 bination with haemoglobin is nitric oxide. But as free nitric oxide 

 combines at once with the oxygen in air to form yellow "nitrous 

 fumes," and these are intensely irritant and produce very danger- 

 ous inflammation, nitric oxide poisoning in the same sense as CO 

 poisoning is impossible. Sir Humphrey Davy nearly killed him- 

 self when he attempted to breathe nitric oxide (NO) at the time 

 when he discovered the effects of nitrous oxide, or "laughing gas" 

 (N 2 O). NO haemoglobin is, however, formed to some extent in 

 the living body during poisoning by nitrites, as was discovered by 

 Makgill, Mavrogordato, and myself ; 24 and some time after death 

 from nitrite poisoning the whole of the haemoglobin becomes 

 combined with NO. Hence the body is red, just as in a fatal case of 

 CO poisoning, so that the case might easily be mistaken for CO 

 poisoning on mere spectroscopic examination of the blood. The 

 condition can be distinguished at once by the fact that the blood 

 and tissues remain red on boiling, just as in the case already al- 

 luded to of salted meat. 



Another cause of an anoxaemia analogous to that of CO poison- 

 ing is present in the case of the action of poisons which produce 

 methaemoglobin in the living body. The first of these to be dis- 

 covered was chlorate of potash, which in former times, before the 

 dangerous properties of chlorates were realized, used to be ad- 

 ministered freely as an oxidizing agent, and has even been recom- 

 mended as an antidote for the anoxaemia of high altitudes. The 

 discovery that in a fatal case of diphtheria treated with chlorate 

 of potash the blood contained much methaemoglobin drew atten- 

 tion to the possible dangers from anoxaemia in poisoning by any 

 of the numerous substances which are capable of producing me- 

 thaemoglobin in the living body. 



The possibilities of anoxaemia being produced were investi- 

 gated by Makgill, Mavrogordato, and myself. As ferricyanide 

 does not penetrate the walls of the red corpuscles, and chlorates 

 do not do so in the animals we were using, we used chiefly nitrites 

 for the experiments; and we did so for the reason, partly, that 

 nitrites have other important physiological actions besides that of 

 producing methaemoglobin (in reality a mixture of methaemo- 

 globin with a certain proportion of NO haemoglobin). Having 

 discovered the dose required to produce death we then, as soon 



34 Makgill, Mavrogordato, and Haldane, Journ. of Physiol., XXI, p. 160, 1897. 



