SECRETION OF URIXE. 



641 



secretion, is termed Urea. This maybe readily separated from Urine, in the 

 form of transparent colourless crystals ; which have a faint and peculiar, but 

 not urinous odour: and, as already mentioned, it is distinctly traceable in the 

 Blood, where it rapidly accumulates, if its continual elimination be in any way 

 interfered with. It is very soluble in water, and combines with acids without 

 neutralizing them. In its chemical composition, it is identical with cyanate 

 of ammonia ; this composition being 2 Carbon, 4 Hydrogen, 2 Nitrogen, and 

 2 Oxygen, a formula much more simple than that of almost any other 

 organic substance. The amount of Urea excreted in twenty-four hours has 

 been made the subject of examination by Lecanu;* and the following are his 

 results, as deduced from a series of 120 analyses : 



Minimum. 



Wean. 



Maximum. 



By men ....... 



By women 



By old men (84 to 86 years) 



By children of eight years 



By children of four years .... 



It is very interesting to perceive, in this table, how large an amount of Urea 

 is excreted by children ; and how small a quantity, in proportion to their bulk, 

 by old men. This corresponds precisely with the rapidity of interstitial 

 change at different periods of life. (See 812.) Moreover, as this continual 

 disintegration is very much accelerated by increased vital activity of the Tis- 

 sues, the amount of Urea undergoes a like augmentation ; so that other cir- 

 cumstances being equal the amount of Urea excreted may fairly serve as a 

 measure of the waste of the tissues, and consequently of the degree in which 

 they have been exercised. This will be especially the case in regard to the 

 Muscular Tissue ; which constitutes so large a part of the fabric. In some 

 experiments recently made on the influence of various causes upon the con- 

 stitution of Urine, Dr. Lehmann found that, by the substitution of violent for 

 moderate exercise, the quantity of Urea was raised from 32k to 45^ parts; 

 and Simon found that, by two hours' violent exercise, the proportion of the 

 urea in the urine passed half an hour subsequently was double that contained 

 in the morning urine. If such increased tvaste be not compensated by in- 

 creased nutrition, a diminution in the bulk of the body is the necessary con- 

 sequence. 



845. The next important ingredient, Uric or Lithic Acid, exists much 

 more largely in the Urine of the lower Vertebrata, than in that of Mammalia ; 

 thus the nearly solid urinary excretion of Serpents, and the semi-fluid urine 

 of Birds, is almost entirely composed of this acid, in combination with Am- 

 monia. Its presence has not yet been detected in healthy blood; but when 

 it is imperfectly eliminated, we are assured of its accumulation in the circu- 

 lating fluid, by its deposition, in combination with Soda, in the neighbourhood 

 of the joints, forming Gouty concretions, or Chalk-stones. Pure Lithic acid 

 crystallizes in fine scales of a brilliant white colour, and silky lustre ; it is 

 tasteless and inodorous, and is so sparingly soluble in water, that at least 

 10,000 times its own weight is required to dissolve it. As it exists in a state 

 of perfect solution in healthy Urine, it must be in combination with some 

 base; and that this is the case, is at once proved by the fact, that it is precipi- 

 tated immediately on the addition of a small quantity of any acid, even the 

 Carbonic. It is generally believed, that the base is Ammonia ; but it has 

 recently been affirmed by Liebig,t that the Uric acid (with the Hippuric) is 

 held in solution by the Phosphate of Soda, which, from being bibasic or 

 alkaline, is rendered acid, by yielding up a part of its soda to these organic 



* Journal de Pharmacie, torn. xxv. 



54* 



f Lancet, June 8, 1844. 



