38 A TEXTBOOK OF PHYSIOLOGY 



A combination of four carbon atoms with two nitrogen atoms yields 

 a ring known as the pyrimidin ring: 



N CH 



dn 



I ' 



N CH 



Another ring which occurs is 



HC NH X 



HC W 



known as the iminazol ring. This is found in the body histidin. 



A combination of the pyrimidin with the iminazol ring yields the 

 important nucleus or ring known as the p\irin ring. The different 

 positions in this nucleus have been numbered, and it may be repre- 

 sented thus: 



Nr C 6 



C 2 5 C-N 7 



J C 8 

 N 3 -C 4 -N 9 



Each of these rings will be referred to when dealing with com- 

 pounds or groups of bodies containing them. 



