BLOOD. 335 



the substance used. Thus, a solution of NaCl of 0.64 per cent, is isotonic with a 

 solution of sugar of 5.5 per cent, or a solution of KNO 3 of 1.09 per cent. When 

 placed in any of these three solutions red corpuscles do not take up water at 

 least not in quantities sufficient to discharge the haemoglobin. For a more 

 complete account of these relations the reader is referred to original sources 

 (Hamburger 1 ). It may be said that the term was introduced first in connec- 

 tion with plant-cells. In the animal body it happens that the isotonic relations 

 of certain substances have been worked out for the red corpuscles, but similar 

 relations must exist with reference to the other cells. Speaking generally, it 

 may be said that the composition of normal blood and lymph is isotonic to the 

 tissue-elements, and that it must be kept so to preserve the cells from injury. 

 "^ Nature and Amount of Haemoglobin. Hemoglobin is a very complex 

 substance belonging to the group of combined proteids. (For the definition 

 and classification of proteids, as well as for the purely chemical properties of 

 haemoglobin and its derivatives, reference must be made to the section on " The 

 Chemistry of the Body.") When decomposed in various ways haemoglobin 

 breaks up into a proteid (globulin, 96 per cent.) and a simpler pigment (haema- 

 tin, 4 per cent.). When the decomposition takes place in the absence of oxygen, 

 the products formed are globulin and haemochromogen, arid the decomposition 

 seems to be of the nature of a simple dissociation. Haemochromogen in the 

 presence of oxygen quickly undergoes oxidation to the more stable hsematin. 

 Hoppe-Seyler has shown that hsemochromogen possesses the chemical group- 

 ing which gives to haemoglobin its power of combining readily with oxygen 

 and its distinctive absorption spectrum. On the basis of facts such as these, 

 hemoglobin may be defined as a compound of a proteid body with hsemochro- 

 mogen. It seems, then, that although the haemochromogen portion is the 

 essential thing, giving to the molecule of haemoglobin its valuable physiological 

 properties as a respiratory pigment, yet in the blood-corpuscles this substance 

 is incorporated into a much larger and more unstable molecule, whose behavior 

 toward oxygen is different from that of the haemochromogen itself, the differ- 

 ence being mainly in the fact that the haemoglobin as it exists in the corpus- 

 cles forms with oxygen a comparatively feeble combination which may be 

 broken up readily with liberation of the gas. 



Haemoglobin is widely distributed throughout the animal kingdom, being 

 found in the blood-corpuscles of mammalia, birds, reptiles, amphibia, and 

 fishes, and in the blood or blood-corpuscles of many of the invertebrates. 

 The composition of its molecule is found to vary somewhat in different animals, 

 so that, strictly speaking, there are probably a number of different forms 

 of haemoglobin all, however, closely related in chemical and physiological 

 properties. ^Ele t rn"entary analysis of dog's haemoglobin shows the following 

 percentage composition (Jaquet) : C 53.91, H 6.62, N 15.98, S 0.542, 

 Fe 0.333, O 22.62. Its molecular formula is given as C 758 H 1203 N 195 S 3 FeO 218 , 

 which would make the molecular weight 16,669. Other estimates are given of 

 the molecular formula, but they agree at least in showing that the molecule 

 1 Du Bois-Reymond's Archivfur Physiologic, 1886, p. 476; 1887, p. 31. 



