BLOOD AND LYMPH. 565 



the hematinic acids are to be regarded as the oxidation products of 

 hemopyrrole. On this basis we can assign to hemin, the hydrochloric 

 ester of hematin, the following structural formula: 



CH 3 . C C CH = C(OH) C = C CH = CH C C CH 3 



II II II II II 



HC CH O FeCl HC CH 



\ / 



NH 



\ / 

 NH 



CH 3 . C C CH=C(OH) C=C CH=CH C C CH 3 



II II II II 



HC CH HC CH 



\ / \ / 



NH NH 



The correctness of the above formula has not been established in all its 

 details. 1 It should serve merely to give us an approximate picture of the 

 structure of hematin. We will state in this connection that the question 

 has been discussed often, whether the hematin of different species of ani- 

 mals, or even of animals in the same species, has a uniform composition, 

 or whether we shall have to assume the existence of different hematins. 

 This question arose from the fact that different observers claimed to isolate 

 hemins of different compositions so that the hematins from which they 

 were made must have been different. The most recent investigations, 

 however, make it seem more probable that there is but one hematin. 2 

 The observed differences in the composition of hemin may be explained 

 partly by the different ways in which the substance was prepared, and 

 partly by the tendency that hemin has of crystallizing out together with 

 a portion of the solvent. 



From the experiments performed in the attempt to explain the consti- 

 tution of hematin, interesting relations have been discovered between it 

 and a color-principle, which for a long time has been assumed to exert 

 quite similar biological functions. We refer to chlorophyll, the pigment 

 of green plants; although it would be perhaps better to include under the 

 name chromophyll all the different pigments of the vegetable kingdom 

 which exert parallel functions. At present, however, chlorophyll, the 

 green pigment, is the only one of such substances which has been studied 

 exhaustively. It is hardly to be doubted that the other pigments in the 

 vegetable kingdom which play the same part in plant economy as that 

 of chlorophyll have quite similar compositions. We have already 

 stated that we are not justified in regarding the function of chlorophyll 

 as parallel to that of hemoglobin, or to hemochromogen. It appears, 



1 Cf Lecture XVII, p. 396. 



2 William Kiister: Z. physiol. Chem. 29, 185 (1900); 40, 391 (1904). K. A. H. 

 Morner: ibid. 41, 542 (1904). 



