THE NATURE AND THE MEANS OF DIGESTION 35 



a solid food particle, and is probably just as definite a 

 process when a muscle-fiber of the human body nourishes 

 itself at the expense of the surrounding lymph. We owe 

 to the German chemist, Abderhalden, the clear exposition 

 of this second digestion, which is an essential feature of the 

 life-processes of the higher animals. We shall recur to the 

 subject in treating of protein metabolism. 



While the great majority of enzymes are hydrolytic and 

 favor reactions of the class which we have been discussing, 

 it must be added that there are other enzymes associated 

 with other orders of chemical change. Enzymes which 

 promote oxidations are believed to play a most important 

 part in the activities of the tissues. When reactions are 

 hydrolytic they proceed with but little evolution or 

 absorption of heat. When they are oxidative the release 

 of energy is a most characteristic attendant condition. 



