482 VICTOR C. MYERS 



secretion of these glands under various conditions may be expected to 

 throw light upon the processes involved in the metabolism of the above 

 substances. With a knowledge of the principal constituents of the urine 

 and a partial understanding, at least, of their history in the body, the 

 appearance of any unusual substance or the presence of a normally 

 occurring constituent in an amount inconsistent with the attending con- 

 ditions may bring to light derangements of body functions. 



The mechanism of kidney secretion has been a much controverted 

 question. The view (modilied Heidenhain) which has been most, gen- 

 erally held for some years past is that the renal cells actively participate 

 in the secretion, the water and inorganic salts being eliminated in the 

 capsular region, while the urea, creatinin, uric acid, etc., find their exit 

 through the uriniferous tubules. Quite recently our conception of urine 

 secretion has undergone material change partly as a result of advances 

 in our knowledge of physical chemistry and partly from added anatomical 

 data. From a study of the blood vessel structure of the glomerulus, it is 

 apparent that the blood pressure in the glomerular capillaries must be 

 high, much higher than that of the fluid in the capsule. According to 

 the "modern theory" (Cushny (&)), the secretion of urine consists of two 

 distinct processes differing not only in site but also in nature. The first 

 of these, the filtration, occurs in the glomerulus, and is purely physical ; 

 the second, the reabsorption, occurs in the tubules, and depends upon the 

 vital activity of the epithelium. By the first process the protein colloids 

 of the blood plasma are filtered off. By the second process water and so- 

 called threshold bodies such as chlorids and sugar are largely reabsorbed, 

 while no-threshold substances, such as urea, are rejected and can only 

 escape by the ureter. 



That the cells of the tubules actively participate in the secretion of 

 urea, however, seems apparent from recent experiments of Oliver. With 

 the aid of xanthydrol he has shown that urea is present in the cells of 

 the proximal convoluted tubules in a concentration higher than that of 

 the blood or that of the cells of any of the other kidney tubules, which 

 condition can only be reconciled to an assumption of an active secretion 

 (excretion) on the part of these cells. 



Physical Properties. Volume. The volume of urine eliminated de- 

 pends in great part upon the volume of fluid ingested. Under normal con- 

 ditions 1000 c.c. may be taken as the average volume of, urine excreted in 

 24 hrs. This, however, is subject to great variations under both normal 

 and pathological conditions. 



The volume of urine is diminished by conditions which cause an 

 increased elimination of water through other channels, for example 

 through the alimentary tract during diarrhea and vomiting, or through 

 the skin as perspiration. On the other hand during cold weather, when 

 cutaneous evaporation is reduced, the volume of urine is increased. Thus 



