EXCKETIONS 503 



constantly maintained at about 5 :3. It is well known that foods rich in 

 potassium, such as meat and potatoes, require more salt than other foods. 

 The quantity of both of these elements excreted depends chiefly upon the 

 food. In starvation or during fever the potassium of the urine may be in 

 excess owing to a destruction of the body's own tissues. 



Calcium and Magnesium. Since the larger part of the calcium and 

 magnesium eliminated are excreted in the feces it is always necessary to 

 have data on the fecal excretion of these elements to make satisfactory de- 

 ductions (see discussion on page 511). Under different conditions of diet 

 the calcium excretion in the urine may vary from 0.1 to 0.5 gram calcu- 

 lated as CaO, and the magnesium from 0.1 to 0.3 gram calculated as MgO 

 depending upon the diet ; sometimes the calcium is in excess in the urine 

 and sometimes the magnesium. In a series of 25 healthy adults Xelson 

 and Burns found the calcium in excess in IT and the magnesium in 8. 

 The figures for the CaO ranged from 0.13 to 0.49 gram and for the MgO 

 from 0.12 to 0.30. In this connection they state that either calcium or 

 magnesium may be excreted by way of the urine in the larger amount, 

 in the normal individual. Whichever element predominates does so con- 

 stantly, or very nearly so, and seems to be independent of the character 

 of the food ingested. The excretion of calcium and magnesium does not 

 necessarily run parallel pathologically, since there may be a retention 

 of magnesium in certain bone disorders accompanied by a loss of calcium ; 

 for example, osteomalacia. Very little is known, however, about the 

 pathological excretion of these elements. The lime salts absorbed are in 

 great part excreted again into the intestine, and the quantity in the urine 

 is therefore no measure of their absorption. The introduction of readily 

 soluble lime salts or the addition of hydrochloric acid to the food may 

 therefore cause an increase in the quantity of lime in the urine, while 

 the reverse takes place on the addition of alkali phosphate to the food. 

 In other words, the balance between the acid- and base-forming elements 

 in the foods has a very important bearing upon the excretory path of these 

 elements and phosphorus. 



Iron. Iron exists in the urine only in very small amount (1 to 5 mg. 

 per day) and that in organic form. It is largely eliminated by the intes- 

 tine. 



Feces 



It has long been the common notion that feces are composed of the 

 residues of undigested food. In health, however, this is far from the 

 truth. It is easy to comprehend that the nitrogenous waste products of 

 the urine are derived from the catabolism of protein in the body, but 

 since the intestinal canal is a long tube open at both ends through which 

 undigested material may pass, it has been difficult to appreciate that 



