534 OF SECRETION. 



in the state of a Lactate ; whilst in the urine of Herbivorous animals it is 

 combined with Hippuric acid. In its chemical composition, it is identical 

 with cyanate of ammonia ; and its composition is 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 its results, as deduced from a series of 120 analyses. 



Minimum. Mean. Maximum. 



By men 357-51 grs. 433-13 grs. 510-36 grs. 



y women .... 153-25 295-15 437-06 



By old men (84 to 86 years) . 61-08 125-22 295-15 



By children of eight years . 161-78 207-99 254-20 



By children of four years . . 57-28 59-55 81-83 



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 646). Moreover, as this continual disinte- 

 gration is very much accelerated by increased vital activity of the Tissues, the 

 amount of Urea undergoes a like augmentation ; so that other circumstances 

 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 constitution of 

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

 exercise, the quantity of Urea was raised from 32 to 45| parts. If such in- 

 creased waste be not compensated by increased nutrition, a diminution in the 

 bulk of the body is the necessary consequence. 



673. 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 

 its elimination is checked, we are assured of its accumulation in the 

 circulating fluid, by its deposition, in combination with Soda, in the neigh- 

 bourhood 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 

 precipitated 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 Dibasic or 

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

 acids, which are thereby rendered soluble. It is in this manner that he partly 

 explains the usually acid reaction of healthy urine ; the other causes of which 

 will be presently noticed. If there be an undue proportion of Lithic acid in 

 the urine, it will be precipitated on cooling ; because it is less soluble in a 

 cold than in a warm solution of phosphate of soda; and the same result will 



* Journal de Pharmacie, torn. xxv. 

 f Lancet, June 8, 1844. 



