EXCRETION. 479 



the liberated iodine communicates a strong mahogany color to the urine 

 thus treated. 



Mucus. Mucus in the urine consists principally of the epithelial 

 debris from the mucous surface of the urinary passages. Particles of 

 epithelium, in greater or less abundance, may be detected in most sam- 

 ples of urine, especially if it has remained at rest for some time, and the 

 lower strata are then examined (fig. 303). As urine cools, the mucus is 

 sometimes seen suspended in it as a delicate opaque cloud, but generally 

 it falls. In inflammatory affections of the urinary passages, especially 

 of the bladder, mucus in large quantities is poured forth, and speedily 

 undergoes decomposition. The presence of the decomposing mucus 

 excites chemical changes in the urea, whereby carbonate of ammonium 

 is formed, which, combining with the excess of acid in the superphos- 

 phates in the urine, produces insoluble neutral or alkaline phosphates 

 of calcium and magnesium, and phosphate of ammonium and magne- 

 sium. These, mixing with the mucus, constitute the peculiar white, 

 viscid, mortar-like substance which collects upon the mucous surface of 

 the bladder, and is often passed with the urine, forming a thick tena- 

 cious sediment. 



Extractives. In addition to those already considered, urine con- 

 tains a considerable number of nitrogenous compounds. These are 

 usually described under the generic name of Extractives. Of these, the 

 chief are: (1) Kreatinin (C.H 7 N 3 0), a substance derived almost en- 

 tirely from muscle taken as food, crystallizing in colorless oblique 

 rhombic prisms; a fairly definite amount of this substance, about 15 

 grains (1 grin.), appears in the urine daily, so that it must be looked 

 upon as a normal constituent; it is increased by increasing the ni- 

 trogenous constituents of the food; (2) Xanthin (C 5 N 4 H 4 2 ), when 

 isolated, is an amorphous powder soluble in hot water; (3) Sarkin, or 

 hypo-xanthin (C 5 N 4 H 4 0); (4) Oxaluric acid (C S H 4 N 2 4 ), in combi- 

 nation with ammonium in the urine of the new-born child; (5) Allantoin 

 (CJleN^Os). All these extractives 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. 



Saline Matter. (a) The Sulphuric acid in the urine is combined 

 chiefly or entirely with spdium or potassium; forming salts which are 

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

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

 ated 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,, 



