THE CIRCULATING LIQUIDS OF THE BODY 43 



animals, but in man consist chiefly of phosphates and chloride 

 of potassium, with a much smaller amount of sodium chloride) 

 about i per cent. There is evidence that a portion of the salts is 

 more firmly combined than the rest, so that, even after the action 

 of the most energetic laking agents, this fraction remains attached 

 to the stroma. 



Hemoglobin. Of all the solid constituents of the blood, haemo- 

 globin is present in greatest amount, constituting, as it does, no less 

 than 13 per cent., by weight, of that liquid. It is an exceedingly 

 complex body, containing carbon, hydrogen, nitrogen, and oxygen in 

 much the same proportions in which they exist in ordinary proteins 

 (p. i). Iron is also present to the extent of almost exactly one-third 

 of i per cent., and there is also a little sulphur, the amount of which 

 stands in a very simple relation to the quantity of iron (i atom of 

 iron to 3 of sulphur in dog's haemoglobin, and i atom of iron to 

 2 of sulphur in the haemoglobin of the horse, ox, and pig). Haemo- 

 globin is made up of a protein element which contains all the sulphur 

 and a pigment which contains all the iron, the protein constituting 

 by far the larger portion of the gigantic molecule, whose weight has 



FIG. 6. DIAGRAM OF SPECTROSCOPE. 



A, source of light ; B, layer of blood ; C, collimator for rendering rays parallel ; 

 D, prism ; E, telescope. 



been estimated at more than 16,000 times that of a molecule of 

 hydrogen. Since its percentage composition is still undetermined 

 with absolute precision, it is impossible to give an empirical formula 

 that is more than approximately correct. For dog's haemoglobin 

 Jaquet gives C 758 H 1203 N 195 S 8 FeO 21 8, which would make the molecular 

 weight 16,669. 



The most remarkable property of haemoglobin is its power 

 of combining loosely with oxygen when exposed to an atmo- 

 sphere containing it, and of again giving it up in the presence of 

 oxidizable substances or in an atmosphere in which the partial 

 pressure of oxygen (pp. 248, 254), has been reduced below a 

 certain limit. It is this property that enables haemoglobin to 

 perform the part of an oxygen-carrier to the tissues, a function 

 of the first importance, which will be more minutely considered 

 when we come to deal with respiration. 



The bright red colour of blood drawn from an artery or of 

 venous blood after free exposure to air is due to the fact that the 



