PASSAGE OF SUBSTANCES INTO THE URINE. 785 



they are converted in the system, into carbonates, which enter the 

 urine. Hence the alkaline condition of this fluid, caused by succulent 

 vegetable diet, and the alkaline urine of the Herbivora. Again, as 

 already mentioned, benzoic acid and the allied cinnamic acid, are first 

 converted into hippuric acid. Organic compounds containing sulphur, 

 produce sulphates in the urine. A great number of substances, on 

 being taken into the stomach, do not reappear in the urine, such as 

 ether, thein, caffein, theobromin, asparagin, amygdalin, musk, cam- 

 phor, and certain coloring matters, such as cochineal and chlorophyll. 

 Alcohol, though chiefly decomposed in the system, may partly appear 

 in the urine. Of the metallic salts, such as arsenic and antimony, the 

 bases of which can, of course, undergo no change in the body, some ap- 

 pear with great facility in the urine ; whilst others enter that fluid with 

 difficulty, or only in minute traces even after long periods of administra- 

 tion; such are gold, silver, mercury, lead, bismuth, zinc, and iron. 

 Alumina is absorbed with difficulty, or not at all, hence it does not 

 appear in the urine. 



Water is eliminated with great rapidity from the kidneys. In large 

 quantities, as already stated, it causes, by stimulating the excreting 

 power of the uriniferous tubes, an absolute increase in the amount of 

 urea separated from the body; though relatively, owing to its dilution, 

 a given quantity of urine contains less urea. The diminution in the 

 quantity of urea and uric acid excreted by the kidneys, caused by many 

 agents, such as coffee, tea, alcohol, and tobacco, articles so widely and 

 instinctively adopted by mankind as dietetic substances, has been ex- 

 plained, by supposing that they interfere with, or retard, the meta- 

 morphoses of the albuminoid and fatty tissues, and so preserve them 

 from waste. In this way, when taken in moderation, they conserve 

 the strength. The action of creatin and creatinin, so abundant in beef- 

 tea and beef-juice, may be similar. Thein and caffein resemble those 

 substances very closely in composition. 



The rapidity with which water and substances soluble in it, pass into 

 the urine, after being taken into the stomach, formerly led to the idea 

 that direct channels of communication, passages, or ducts, existed be- 

 tween the stomach and the kidneys, or some other part of the urinary 

 apparatus. Many investigations were undertaken, some even with 

 pretended success, for the purpose of discovering such passages. No 

 such communications, however, exist. Soluble substances pass from 

 the stomach into the circulation, by venous absorption, and are then, 

 after traversing the lungs, conveyed by the renal arteries to the kid- 

 neys, in which, by porous diffusion or dialysis, they enter the urinif- 

 erous tubules, and so reach the urinary passages. The rate at which 

 this circuitous route through the vascular system, from the stomach to 

 the kidneys, occurs, is adduced as one proof of the rapidity of the cir- 

 culation of the blood (p. 618). When the stomach is empty, after long 

 abstinence, the time is 1 minute; 4 hours after a meal, it is 2 minutes; 

 1J hour after, 6J minutes; 1 hour after, 14 minutes; arid 25 minutes 

 after, 16 minutes. If the test substance be taken with the food, it 

 requires 40 minutes for it to appear in the urine. (Erichsen.) 



The affinity of certain substances for the living tissues influences 



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