STRUCTURE AND FUNCTION OF THE KIDNEYS. 371 



tractives are chiefly interesting as being closely connected with urea, and 

 mostly yielding that substance on oxidation. Leucin and tyrosin can 

 scarcely be looked upon as normal constituents of urine. 



(7) Saline Matter. (a) The sulphuric acid in the urine is com- 

 bined chiefly or entirely with sodium or potassium; forming salts which 

 are taken in very small quantity with the food, and are sqarcely found 

 in other fluids or tissues of the body; for the sulphates commonly enu- 

 merated among the constituents of the ashes of the tissues and fluids are 

 for the most part, or entirely, produced by the changes that take place 

 in the burning. Only about one-third of the sulphuric acid found in 

 the urine is derived directly from the food (Parkes). Hence the greater 

 part of the sulphuric acid which the sulphates in the urine contain, 

 must be formed in the blood, or in the act of secretion of urine; the 

 sulphur of which the acid is formed being probably derived from the de- 

 composing nitrogenous tissues, the other elements of which are resolved 

 into urea and uric acid. It may be in part derived also from the sul- 

 phur-holding taurin and cystin, which can be found in the liver, lungs, 

 and other parts of the body, but not generally in the excretions; and 

 which, therefore, must be broken up. The oxygen is supplied through 

 the lungs, and the heat generated during combination with the sulphur, 

 is one of the subordinate means by which the animal temperature is 

 maintained. 



Besides the sulphur in these salts, some also appears to be in the 

 urine, uncombined with oxygen; for after all the sulphates have been 

 removed from urine, sulphuric acid may be formed by drying and burn- 

 ing it with nitre. From three to five grains of sulphur are thus daily 

 excreted. The combination in which it exists is uncertain: possibly it 

 is in some compound analogous to cystin or cystic oxide (Fig. 258). Sul- 

 phuric acid also exists normally in the urine in combination with phenol 

 (C 6 H 6 0) as phenol sulphuric acid or its corresponding salts, with so- 

 dium, etc. 



(b) The phosphoric acid in the urine is combined partly with the al- 

 kalies, partly with the alkaline earths about four or five times as much 

 with the former as with the latter. In blood, saliva, and other alkaline 

 fluids of the body, phosphates exist in the form of alkaline, neutral, or 

 acid salts. In the urine they are acid salts, viz., the sodium, ammo- 

 nium, calcium, and magnesium phosphates, the excess of acid being 

 (Liebig) due to the appropriation of the alkali with which the phos- 

 phoric acid in the blood is combined, by the several new acids which are 

 formed or discharged at the kidneys, namely, the uric, hippuric, and 

 sulphuric acids, all of which are neutralized with soda. 



The phosphates are taken largely in both vegetable and animal food; 

 some thus taken are excreted at once; others, after being transformed 

 and incorporated with the tissues. Calcium phosphate forms the prin- 



