462 TEXT-BOOK OF PHYSIOLOGY 



by a diet of asparagus, plums, cranberries, etc., and by the administration of 

 benzoic and cinnamic acids. There is evidence that hippuric acid is formed 

 in the kidney from benzoic acid, its precursors, or related bodies. Various 

 compounds of this class are found in vegetable foods, a fact which may ac- 

 count for the increase in the excretion of hippuric acid on a vegetable diet. 



Indol, Skatol, Phenol, Cresol. These compounds, products of the 

 putrefactive changes in the derivatives of protein are present in variable 

 amounts, associated with potassium sulphate (see page 205). These com- 

 pounds are known as the ethereal sulphates. The extent to which they are 

 present is taken as a measure of the extent of intestinal putrefaction; their 

 presence can be determined by various tests. Of these compounds the one 

 generally tested for is potassium indoxyl sulphate or indican. If hydro- 

 chloric acid and a small quantity of sodium hypochlorite be added to sus- 

 pected urine, together with a few cubic centimeters of chloroform, the indican 

 if present will be separated by the acid into indoxyl and potassium sulphate. 

 The former compound will then be oxidized by the oxygen set free from the 

 sodium hypochlorite and form indigo blue. The chloroform will absorb the 

 indigo blue as fast as formed and when the reaction is completed will accumu- 

 late at the bottom of the test-tube. The depth of the color is indicative of 

 the quantity present and of the extent of the intestinal putrefaction. 



Inorganic Salts. Sodium and potassium phosphates, known as the 

 alkaline phosphates, are found in both blood and urine. The total quantity 

 excreted daily is about 4 grams. Calcium and magnesium phosphates, 

 known as the earthy phosphates, are present to the extent of i gram. Though 

 insoluble in water, they are held in solution in the urine by its acid constitu- 

 ents. If the urine be rendered alkaline, they are at once precipitated. 

 Sodium and potassium sulphates are also present to the extent of about 2 

 grams. The phosphoric and sulphuric acids which are combined with these 

 bases enter the body for the most part in the foods, though there is evidence 

 that they also arise by oxidation in consequence of the metabolism of proteins 

 which contain phosphorus and sulphur. Sodium chlorid is the most 

 abundant of the inorganic salts. It is derived mainly from the food. The 

 amount excreted is about 15 grams in twenty-four hours. 



THE KIDNEYS 



The kidneys are the organs engaged in the excretion of the urinary 

 constituents from the blood. Each kidney resembles a bean in shape, is 

 from 10 to 12 centimeters in length, 2 centimeters in breath, and weighs from 

 144 to 170 grams. These organs are situated in the lumbar region, one on 

 each side of the vertebral column behind the peritoneum, and extend from 

 the eleventh rib to the crest of the ilium. The anterior surface is convex, 

 the posterior surface concave. The internal border presents a deep notch 

 the hilum. The kidney is surrounded by a thin smooth membrane composed 

 of white fibrous and yellow elastic tissue; though it is attached to the surface 

 of the kidney by minute processes of connective tissue, it can very readily be 

 torn away. The substance of the kidney is dense but friable. 



Upon making a longitudinal section of the kidney it will be observed that 

 the hilum extends into the interior of the organ and expands to form a 

 cavity known as the sinus, in which are found the blood-vessels, nerves, and 



