DEVELOPMENT OF BIOLOGY 



461 



which showed ' solution ' of the food to be the main factor in diges- 

 tion; although it was not clear how these changes differ from or- 

 dinary chemical ones. It was left for nineteenth-century investi- 

 gators to establish the fact that food in passing along the digestive 

 tract runs the gauntlet of a series of complex chemical substances, 

 each of which has its part to play in putting the various constit- 

 uents of the food into such a form that they can pass to the various 

 cells of the body where they are actually used. (Fig. 113.) 



Fig. 300. — Albrecht von Haller. 



On the side of respiration, a closer approach was made toward a 

 true understanding of the process. In France Lavoisier (1743- 

 1794) demonstrated that the chemical changes taking place in 

 respiration involve essentially a process of combustion, and it 

 chiefly remained for later work to show that this takes place in 

 the tissues rather than in the lungs. (Fig. 118.) 



Most of the firm foundation on which the physiology of animals 

 rests to-day has been built up by the work on Vertebrates. But 

 since the middle of the nineteenth century, when the versatile 

 Muller (1801-1858) of Germany emphasized the value of study- 

 ing the physiology of higher and lower animals alike, there has 

 been an ever-increasing tendency to focus evidence, in so far as 

 possible, from all forms of life on general problems of function. 

 This has culminated in the science of general physiology. 



