16 PRINCIPLES OF PHYSIOLOGY 



substances in the blood are purely chemical. 

 Physiological Chemistry has as its field a 

 description of the physical and chemical char- 

 acter of the substances forming the tissues 

 and existing in the fluids of the body. It 

 also gives an explanation of the chemical 

 processes occurring in the body. This depart- 

 ment of science has also made great progress 

 in recent years. At one time it was thought 

 that certain substances formed in the body 

 could be formed only in living matter. The 

 synthesis of urea by Wohler in 1828 upset this 

 notion. This experiment laid the foundations 

 of organic chemistry. Since then hundreds of 

 chemical substances found in plant or animal 

 tissues have been formed synthetically by the 

 chemist by the operation of chemical methods. 

 It is remarkable, however, that in living matter 

 such substances are formed by molecular 

 action and by hidden processes, while the 

 chemist can only form them by the agency 

 of high temperatures, and the action of power- 

 ful substances such as acids or oxydising or 

 reducing substances. It is possible that the 

 processes of nature and of the chemist may in 



