STRUCTURE AND FUNCTION OF THE KIDNEYS. 379 



due to the direct stimulation of the cells by the urea contained in the 

 blood. 



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

 of filtration, by which the chief part, if not the whole, of the fluid is 

 eliminated, 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, 

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

 voluted tubes, and is sometimes (as in the case of the elimination of urea 

 ;and similar substances) 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, 

 ^ven 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, may be stated 

 to be the two. 



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

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

 sues 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 nitrogenous principles 

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

 pears in the urine soon after a full meal of proteid material may be attrib- 

 uted to the production of a considerable amount of leucin and tyrosin 

 by the pancreatic digestion. These substances are carried by the portal 



