VARIATIONS WITH FOOD. CALCULI. 215 



425. THE EFFECT OF FOOD ON THE COMPOSITION OF THE 

 URINE. Let a number of subjects each select food of a different 

 class and eat only this for twenty-four hours, collecting all the 

 urine for the period. The following dietaries will give a variety: 



1. Largely animal. 



2. Vegetable. 



3. Rich in purins, sweetbreads, etc. 



4. Purin free milk, eggs, wheat bread, butter, cheese. 



5. Low in nitrogen. 



6. No food. 



Determine volume, specific gravity, color, reaction; amounts 

 of nitrogen, iiric acid, phosphoric acid, urea. Report results. 



URINARY CALCULI. 



The constituents of calculi are the same as those of 

 the chemical sediments, and the causes which give rise to 

 the formation of the latter will also favor the production 

 of calculi in the bladder. To these various names are 

 applied, according to their size: sand, gravel, stone, and 

 calculi, or concretions. They vary from the microscopic 

 to aggregations as large as an orange. They are generally 

 not composed of a single material, but have at the centre a 

 nucleus, and this is surrounded by layers, often of two or 

 more compounds in alternation. The nucleus may be a 

 mass of foreign matter, or it may be a clot of blood or a 

 particle of one of the sediments around which material, 

 perhaps of a different kind, can be deposited. Uric acid 

 concretions are the most common. They are brown in 

 color, rough of surface, and brittle. The form of the 

 crystals cannot be seen, but they give the murexid test. 

 They dissolve in sodium or potassium hydrate, from which 

 solutions the uric acid may be precipitated in the crystal- 

 line form by the addition of a mineral acid. Uric acid 

 calculi are formed only in an acid urine. 



