216 CIRCULATING FLUIDS : 



structure of the organ -, it cannot be regarded as a product of 

 the development of the cells, or of the metabolic power of the 

 cells acting on the plasma ; but the water is separated in much 

 the same manner as the various gases of the blood are re- 

 moved by the lungs. 



But Avhether the salts which are separated by the kidneys, the 

 combinations of chlorine, and of phosphoric, sulphuric and lactic 

 acids, are, so to speak, mechanically carried away in the water 

 in which they are held in solution, and which permeates the 

 textures of the kidney, or whether their separation is to be re- 

 garded as a true secretion of the renal cells, due to their organ- 

 ised development, is a point which I have no means of ascer- 

 taining. An accurate analysis of the kidneys would soon show 

 whether the salts which have been mentioned do or do not 

 belong to the constitution of the renal cells, a point which the 

 analysis of Berzelius has left undecided. These salts, most of 

 which preexist in the blood, at all events find their way into 

 the renal cells, and either are or are not connected with their 

 peculiar vital development. The former is far the more pro- 

 bable; and in that case the secretion of the salts would not 

 be a mefe mechanical act, but would be due to organic causes. 



The kidneys separate hsemaphsein from the colouring mat- 

 ter produced by the metamorphosis of the blood-corpuscles, 

 and the proportion in which they separate it is larger than 

 the proportion contained in the plasma, a circumstance which 

 is obvious from the colour of the urine being generally deeper 

 than that of the liquor sanguinis. Hence it is very probable that 

 a portion of the colouring matter is formed by the metamor- 

 phosis of the corpuscles in the peripheral system of the kidney. 

 The kidneys likcAvise separate another colouring matter, uro- 

 erythrin ; in a normal state, only in a slight proportion, but 

 in certain pathological conditions, in a comparatively large 

 quantity. Uroerythrin, in all probability, owes its origin to 

 the hfematin of the blood-corpuscles. As the proportions of 

 uric acid and of uroerythrin to urea are very small in normal 

 lu'ine, but are much increased in certain pathological conditions, 

 we must infer that, in these latter cases, the blood undergoes 

 some peculiar change. 



