THE CHEMISTRY OF THE ANIMAL BODY. 527 



Magnesium, Mg = 24.3. 



This is the second in importance of the alkaline earths. It is present 

 wherever calcium is found, but in comparison with calcium it has been little 

 investigated. It occurs principally as phosphate, but is found as carbonate 

 in herbivorous urine. Of the total quantity of magnesium in the dog, Heiss 

 found that 71 per cent, belonged to the bones. It is found decidedly pre- 

 dominating over calcium in muscle, but is less in quantity than calcium in 

 the blood. 



Magnesium Phosphates. — Magnesium tertiary phosphate, Mg 3 (P0 4 ) 2 , is 

 found in the ash of bones to the extent of about 1 per cent., is present in blood 

 and especially in muscle, probably in combination with protcid, and contrib- 

 utes to the functional activity of protoplasm. It is continuously excreted 

 by the walls of the intestinal canal. The primary and secondary phosphates 

 of magnesium are found in carnivorous urine, solution of the latter being 

 aided by the presence of primary alkali phosphate and sodium chloride. 

 Tertiary phosphate of magnesium is insoluble in water, the secondary very 

 slightly so, the primary quite soluble ; but all are soluble in acids. In the am- 

 moniacal fermentation of the urine, ammonium magnesium phosphate, MgNH 4 - 

 P0 4 , is precipitated as a fine crystalline powder insoluble in alkalies. When- 

 ever this fermentation takes place, whether in the bladder or, by similar 

 reaction, in the intestines (herbivora especially), stones are formed. 1 



Magnesium Carbonates. — The neutral carbonate, MgC0 3 , is insoluble in 

 water, but soluble in water containing carbonic oxide, forming secondary or acid 

 carbonate, MgH 2 (C0 3 ) 2 . This latter occurs in herbivorous urine. 



Magnesium in the Body. — Considerations regarding the absorption of 

 calcium apply likewise to magnesium. It is absorbed by the iutestine as inor- 

 ganic and probably as organic combinations. If growing rabbits be fed on 

 a diet poor in calcium salts, but containing magnesium carbonate, the bones 

 may be brought to contain double the normal quantity of magnesium, but the 

 skeletal development remains far behind that of a normal rabbit, and there- 

 fore magnesium can in no sense be considered a substitute for calcium. 2 The 

 magnesium salts, whether phosphate or carbonate, being more soluble than the 

 calcium salts, occur in the urine in greater abundance. Indeed, in carniv- 

 orous urine the major part of excreted magnesium is found in the urine, the 

 balance being given off through the intestinal wall to the feces. In starvation 

 the source of the excreted magnesium is from the bones, and especially from 

 destruction of its combination in proteid metabolism. 



Iron, Fe = 56. 



This is the one heavy metal which is an absolute necessity for the organ- 

 ism. About three grams occur in the average man. It has been demon- 

 strated of certain bacteria that they will not develop in the absence of iron, 



1 For example in man see C. Th, Mdrner: Zeitschrift fw ]>hysiologische Chemie, 1897, Bd. 

 22, S. 522. 



2 Weiske : Zeitschrift fur Biologie, 1894, Bd. 31, S 437. 



