170 THE PATH OF SCIENCE 



its behavior when it gets to the tissues had not even been 

 glimpsed. In the eighteen eighties C. A. MacMunn brought 

 for^vard evidence that the tissues themselves, apart from the 

 blood, contain substances resembling hemoglobin. These he 

 named histohaematin and myohaematin. The great Hoppe- 

 Seyler said that MacMunn's substances were simply decom- 

 position products of the hemoglobin of the blood. MacMunn 

 defended himself: he had shown in his very first paper that 

 his substances were present in the tissues of insects, which 

 have no hemoo^lobin in their blood. This miQ;ht have seemed 

 conclusive, but Hoppe-Seyler refused to consider the evidence 

 from insects. He simply printed a note alongside MacMunn's 

 last paper saying that he considered all further discussion of 

 the subject superfluous. People accepted his opinion, and 

 little more was heard of histohaematin, myohaematin, or 

 MacMunn. 



It was not until the twenties of the present century that 

 D. Keilin of Cambridge showed that MacMunn had been 

 right and Hoppe-Seyler wrong. It would appear that 

 throughout the plant and animal kingdoms every cell that 

 gets its energy by the ordinary process of combustion con- 

 tains MacMunn's substances (or cytochrome, to use Keilin's 

 word). MacMunn had really been studying something far 

 more fundamental than Hoppe-Seyler. The latter was inter- 

 ested in the vehicle by which oxygen is transported to the 

 tissues in certain animals; MacMunn, on the contrary, was 

 on the verge of discovering what happens to oxygen when it 

 actually gets to cells, whether by Hoppe-Seyler's vehicle or 

 not. We realize nowadays that cell respiration is a matter 

 of enormous complexity. The oxygen by no means simply 

 diffuses into cells and combines with combustible substances. 

 It first combines with cytochrome and is then handed on by 

 this cellular respiratory pigment to combine with the hydro- 

 gen of combustible substances, each stage of the process being 

 made possible by the presence of particular intracellular fer- 

 ments. Knowledge of the processes of cellular respiration is 

 growing rapidly. It is strange to think that if MacMunn had 



