PHYSIOLOGICAL 355 



forms part of the uric acid molecule, as will be seen on comparing 

 the formulae. Nucleic acid also contains substances more closely 

 allied to uric acid, having the same double-ring structure (purines). 

 It may be mentioned that the excreted products are very different 

 in different classes of animals ; in most mammals uric acid is further 

 oxidised to allantoin, while in birds urea is actually transformed 

 into uric acid, which becomes the chief nitrogenous waste-product. 



The sulphur-containing waste-products are also important, and 

 may also be distinguished as "endogenous" and "exogenous". The 

 chief source of sulphur is the amino-acid cystein, which contains a 

 sulfhydryl or — SH group. The liver has the power of oxidising this 

 to a sulphate or — SO4 group, and inorganic sulphates are excreted 

 in the urine. These substances have the property of combining with 

 various aromatic substances (phenol, indole) formed by the intestinal 

 bacteria and harmful to the body, whereas the ethereal sulphates 

 formed by this reaction, such as potassium indoxyl sulphate, 



/O.CgHeN 

 S0,( 



^O.K 



are harmless. Such protective syntheses are undoubtedly valuable: 

 another example is the combination between benzoic acid and the 

 amino-acid glycine to form the harmless hippuric acid, a reaction 

 that is known to go on in the kidney itself. 



All the sulphur-containing compounds absorbed by the intestine 

 and most of those liberated in the breakdown of tissue proteins will 

 pass through the liver and be largely converted to sulphates; but 

 a fraction of the endogenous sulphur will reach the kidney without 

 passing the liver, and is excreted in the reduced form as "neutral 

 sulphur" compounds of somewhat obscure composition: Phosphorus, 

 like sulphur, is both endogenous and exogenous, and is excreted in 

 the form of inorganic phosphates. 



We may now examine a little more closely the action of the 

 kidney, the organ which separates out these waste-products and 

 discharges them from the body. In its simplest aspect a kidney 

 need be little more than a filter, which allows the water and all 

 dissolved substances, salts and waste-products alike, to leave the 

 blood, but prevents the colloids from passing through. In marine 

 animals which have no need to economise either water or salts, such 

 an arrangement works very well; but in the higher vertebrates the 

 kidney is much more specialised in its action, so as to retain part of 

 the water and salts. 



The kidney consists of a large number of minute tubules which 

 form the internal or medullary part of the organ. They unite inwardly 

 to discharge the excreted fluid into the ureter, while outwardly, in 



