102 



PHYSIOLOGY 



NH CO 



I I 

 CO CH 



THYMINE is 5-methyl uracil, 



NH CH 



NH CO 



I I 

 CO C.CH 3 



I II 

 NH CH 



while CYTOSINE is 6-amino-2-oxypyrimidine, 



N =C.NH 2 



I I 

 CO CH 



I II 

 NH CH 



Besides these two groups of nitrogenous compounds derived from the 

 purine and pyrimidine rings, many nucleic acids yield on hydrolysis a carbo- 

 hydrate. Thus Hammarsten has isolated a pentose from the nucleo- 

 proteins 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 : 



on digestion yields 



Nucleo-protein 

 nuclein proteoses and peptones 



dissolved in alkali and precipitated with hydrochloric acid 

 yields 



nucleic acid 

 hydrolysed yields 



acid derivatives of protein, histones or protamines 



cytosinc 



It must not be imagined, however, that all these disintegration products 

 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 thy mine 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 constituent radicals from those which dis- 

 tinguish the proteins of the cell protoplasm. Further importance is lent t<> 



