IRON 45 



iron occurring, on the contrary, in the cecum and large intestine, 

 although part of the small intestine may also participate in this 

 function. 



It is one thing to show that inorganic salts of iron are absorbed and' 

 it is another to show that they may be utilized in the building up of 

 hemoglobin. The iron in hemoglobin is very firmly and intimately 

 combined, and cannot be detected by the reagents ordinarily employed 

 for this purpose, such as ammonium sulphide or potassium ferro- 

 cyanide. In fact the iron in hemoglobin resists the action of boiling, 

 concentrated potassium hydroxide and boiling hydrochloric acid. Only 

 by dissolving the hematin radical (which is the iron-containing moiety 

 of the hemoglobin molecule) in concentrated sulphuric acid is the 

 iron split off and the hematin changed into iron-free hematin, or 

 Hematoporphyrin. 



Most of the iron in our diet is in the form of hemoglobin or other 

 organic compounds of iron from which free ionic iron is not readily 

 split off. The yolk of eggs is very rich in iron, as might be anticipated 

 from the fact that the yolk of an egg must contain all of the constit- 

 uents necessary to form the hemoglobin of the developing embryo. 

 The iron-compound in yolks of eggs is not hemoglobin, but some 

 antecedent of hemoglobin. On extracting the yolk of a hen's egg with 

 alcohol or ether, none of the iron goes into the extract. The residue, 

 which contains all of the iron, is a mixture of proteins and nucleo- 

 proteins. The iron cannot be extracted from this residue by alcohol 

 and hydrochloric acid, although inorganic salts of iron readily yield 

 up iron to these reagents. During the digestion of iron-containing 

 protein by Pepsin in the stomach, the part containing iron does not go 

 into solution and its digestion is not accomplished until it reaches the 

 small intestine and comes in contact with the digestive fluid secreted 

 by the pancreas. It is not digestible by pepsin and in this and in other 

 respects corresponds in its behavior to the class of bodies which the 

 reader will later learn to recognize as nucleins. The ordinary tests for 

 iron are given by this substance, to which von Bunge gave the name 

 "Hematogen," but not so readily as by inorganic salts of iron. On 

 adding ammonium sulphide to an ammoniacal solution of this nuclein 

 a greenish coloration is produced, which only slowly changes to black 

 on standing. In other words ionized iron is at first only present in 

 traces and is slowly split off from the compound under the prolonged 

 influence of the reagents. The compound thus behaves in a manner 

 very like that of the protein salts of the heavy metals, for instance 

 casein salts of silver, mercury and so forth to which the reader's atten- 

 tion will be directed in a later chapter. There is little reason to doubt 

 that hematogen is simply a protein salt of iron in which the protein is 

 acting the part of a weak acid, or else a double salt of protein and an 

 inorganic salt of iron. Protein compounds of this type yield no metal- 

 ions in solution, or at the most, only traces of them. 



Since compounds such as these are the only^forms in which we 



