370 HAND-BOOK OF PHYSIOLOGY. 



for believing that part of the increase of the secretion is due to the direct 

 stimulation of the cells by the urea contained in the blood. 



To sum up, then, the relation of the two functions: (1.) The process 

 of nitration, by which the chief part if not the whole of t\\Q fluid is elim- 

 inated, together with certain inorganic salts, and possibly other solids, is 

 directly dependent upon blood-pressure, is accomplished by the renal 

 glomeruli, and is accompanied by a free discharge of solids from the 

 tubules. (2.) The process of secretion proper, by which urea and the 

 principal urinary solids are eliminated, is only indirectly, if at all, de- 

 pendent upon blood-pressure, and is accomplished by the cells of the con- 

 voluted tubes. It is sometimes accompanied by the elimination of copious 

 fluid, produced by the chemical stimulation of the epithelium of the same 

 tubules. 



SOURCES OF THE NITROGENOUS URINARY SOLIDS. 



Urea. In speaking of the method of the secretion of urine, it was 

 assumed that the part played by the cells of the uriniferous tubules was 

 that of mere separation of the constituents of the urine which existed 

 ready-formed in the blood: there is considerable evidence to favor this 

 assumption. What may be called the specially characteristic solid of the 

 urine, i.e., urea (as well as most of the other solids), may be detected in 

 the blood, and in other parts of the body, e.g., the humors of the eye (Mil- 

 Ion), even while the functions of the kidneys are unimpaired; but when 

 from any cause, especially extensive disease or extirpation of the kidneys, 

 the separation of urine is imperfect, the urea is found largely in the blood 

 and in most other fluids of the body. 



It must, therefore, be clear that the urea is for the most part made 

 somewhere else than in the kidneys, and simply brought to them by the 

 blood for elimination. It is not absolutely proved, however, that all the 

 urea is formed away from these organs, and it is possible that a small 

 quantity is actually secreted by the cells of the tubules. The sources of 

 the urea, which is brought to the kidneys for excretion, are stated to be 

 two. 



(1.) From the splitting up of the Elements ef the Nitrogenous Food. 

 The origin of urea from this source is shown by the increase which ensues 

 on substituting an animal or highly nitrogenous for a vegetable diet; in 

 the much larger amount nearly double excreted by Carnivora than 

 Herbivora, independent of exercise; and in its diminution to about one- 

 half during starvation, or during the exclusion of non-nitrogenous prin- 

 ciples of food. Part, at any rate, of the increased amount of urea which 

 appears in the urine soon after a full meal of proteid material may be 

 attributed to the production of a considerable amount of leucin and ty- 

 rosin as by-products of pancreatic digestion. These substances are car- 



