116 PHYSIOLOGY 



Besides these two groups of nitrogenous compounds derived from 

 the purine and pyrimidine rings, many nucleic acids yield on hydrolysis 

 a carbohydrate. Thus, Hammarsten has isolated a pentose, xylose, 

 from the nucleoproteins of the pancreas. It is supposed that the 

 nucleic acid of the thymus gland contains a hexose, since it is possible 

 to split off from it Isevulinic acid, which is one of the first products of 

 the decomposition of a hexose. The complex constitution of the 

 nucleic acids and nucleoproteins may be rendered clearer from the 

 following schema : 



Nucleo -protein 

 on digestion yields 



nuclein proteoses and peptones 

 dissolved in alkali and precipitated 

 with hydrochloric acid yields 



nucleic acid acid derivatives of protein, histones 



or protamines 

 hydrolysed yields 



Tt must not be imagined, however, that all these disintegration pro- 

 ducts are present in all nucleic acids. Thus the nucleic acid derived 

 from the pancreas, the so-called guanylic acid, yields of the purine 

 bases only guanine, and of the pyrimidine bases only thymine and uracil, 

 and every variety is met with as we analyse the nucleic acids of different 

 origin. The fact that nucleic acid is a characteristic and necessary 

 constituent of all nuclei adds interest to the divergence of its con- 

 stituent radicals from those which distinguish the proteins of the cell 

 protoplasm. Further importance is lent to this section of the chemistry 

 of the body by the close relationship which we shall have to study later 

 between the nuclein metabolism of the body and the production and 

 excretion of uric acid. 



(c) THE GLYCOPROTEINS. In the glycoproteins the prosthetic 

 group is represented by a carbohydrate radical, generally containing 

 nitrogen, such as glucosamine or galactosamine. They are split into 

 their two constituents, protein and carbohydrate radical, on prolonged 

 boiling with dilute mineral acids or by the action of alkalies. They 

 may be divided into the two main groups of mucins and mucoids. 



The mucins play a large part in the animal kingdom as protective 

 agents. They form the slimy "secretion which covers the inner surface 

 of the mucous membranes and the outer surface of many marine 



