778 SPECIAL PHYSIOLOGY. 



though towards the end of labor, and especially in the period of rest 

 afterwards, it is greatly increased. Gelatin, which seems never to be 

 directly assimilated by the tissues, but rather to save them from oxi- 

 dation, is readily, perhaps directly, converted into urea. Water, 

 especially if taken with food, causes an increase in the ureal excre- 

 tion, and also in that of the saline constituents of the urine. Dimin- 

 ished temperature and increased barometric pressure, are said to in- 

 crease the quantity of urea. In most cases, the urea is not eliminated 

 until some hours after its actual formation in the system, or until the 

 determining cause of its increase has taken effect. The quantity ex- 

 creted is greater during the day than in the night. Common salt, 

 phosphoric acid, theobromine, urea itself, uric acid, and cantharides, 

 are found to increase the amount of urea excreted, whilst tea, but es- 

 pecially coffee, alcohol, turpentine, and digitalis, diminish it. It is 

 remarkable that such large quantities of nitrogen and carbon are 

 eliminated from the system in the form of urea a comparatively in- 

 active chemical substance; whereas carbonate of ammonia, a com- 

 pound readily produced from the elements of urea, and an extremely 

 irritating and noxious substance, is not formed in quantity, in the 

 animal economy. In certain diseases of the kidneys, the urea is not 

 excreted, but, the blood becoming vitiated, ursemic poisoning occurs, 

 characterized by symptoms, such as convulsions and coma, referrible 

 to the nervous centres, and often fatal. It was formerly supposed 

 that the urea itself is the toxic agent, but possibly it is the carbonate 

 of ammonia derived from the decomposition of the urea. In these 

 cases, ammonia is found in the breath, and, after death, in the blood; 

 the injection of that substance into the veins of an animal also causes 

 similar symptoms. A dilute solution of urea, to which a small quan- 

 tity of mucus or other animal substance is added, readily ferments, 

 and, evolving a pungent odor, forms carbonate of ammonia; this kind 

 of fermentation may take place very rapidly, even within the bladder. 

 The amount of urea increases in all those diseases which are accom- 

 panied by an increase of tissue change, such as active inflammation of 

 the lungs, or of the membranes of the brain, and in fevers generally, 

 even though less food and exercise are taken than in health. In 

 fever, the quantity has been found to be double the ordinary amount, 

 viz., 1065 grains daily; in pyaemia, it has reached 1285 grains. 

 <(Vogel and Warnecke.) During recovery, the quantity excreted falls 

 for a time, although more food and exercise are taken. 



Uric acid is found not only in the urine, but also in the blood, and 

 in most organs of the body. It contains 33.3 per cent, of nitrogen, 

 ;and has the following composition: C 6 H 4 N 4 3 ; it is therefore regarded 

 as resulting from a less complete oxidation of the nitrogenous compounds 

 of the food and of the body than that which produces urea. The for- 

 mation of urea in the system is supposed, by some, to be normally 

 preceded by that of uric acid. The former may be easily produced 

 from the latter, by processes in which oxidation forms a part; when an 

 alkaline urate is digested with portions of liver, at a certain tempera- 

 ture, urea is formed at the expense of the uric acid. Animals to which 

 uric acid is administered with the food, excrete an unusually large 



