12 



The formation of an acid fluid from alkaline material, — blood 

 and lymph, — is at first sight puzzling. The well-known fact 

 that the gastric secretion of all mammals is acid is a case in 

 point. Furthermore, experimental evidence shows that if an 

 alkaline solution of sodium bicarbonate be placed upon one 

 side of the membrane in an osmometer and a solution of neu- 

 tral sodium phosphate be placed upon the other side and a 

 weak electric current be sent through the solutions, the fluid 

 in contact with the positive pole will, in a short time, become 

 acid from the formation of acid sodium phosphate, while the 

 fluid in contact with the negative pole is increased in alkalinity 

 Na HCOg+Na^ HPO, = Na, COa-f Na H^ PO,. 



While it may be true to a considerable extent that the kid- 

 ney merely removes certain constituents from the blood and 

 transfers them to the urine, it has been shown that the activity 

 of the renal cells is required in the production of hippuric acid, 

 — a new product, as hippuric acid is not present in the blood. 



In general, the theory explaining the secretion of urine, 

 according to observed facts, is one which, while recognizing 

 the process as partly physical, also requires some process of 

 activity or elaboration on the part of the kidney itself. 



II. 



The urine of all mammals may be regarded, for the most 

 part, as a solution of constituents derived from the metabolism 

 of the tissues of the body. Some of these constituents, espec- 

 ially the inorganic, may appear in the urine in the same form 

 as they are taken into the body in the food, e. g., sodium 

 chloride; others, especially the organic, represent decom- 

 position products derived from the food or tissues, e. g., urea, 

 creatinin, etc. The composition of the urine may, therefore, 

 to some extent be regarded as an index of tissue activity, and 

 the examination of this secretion is of considerable importance 

 in clinical diagnosis and prognosis. 



That there is a relationship between the diet and the renal 

 excretion is shown by the examination of the urine of the three 

 great classes of animals grouped according to the food they 

 eat ; herbivora, omnivora and carnivora. Perhaps the first and 



