66 INTRODUCTION TO GENERAL PHYSIOLOGY 



Tables have been constructed, giving the composition of various 

 articles of food and their calorie values. 



Digestion 



The fats, carbohydrates, and proteins contained in the various 

 articles of our diet are not in the form in which they can be made 

 use of by the living cells, whether for growth or energy purposes. 

 Carbohydrate must be in the form of glucose or fructose ; protein 

 in that of amino-acids. That in which fat is required to be is 

 unknown, but it cannot be absorbed in the form found in articles of 

 food. The necessary changes in all these cases are made in the 

 alimentary canal. 



The primitive form of the alimentary canal is that of a tube 

 passing through the body, open to the exterior at both ends. At 

 the anterior end, the mouth, the food is taken in. In its passage 

 it is subjected in turn to the action of various fluids and the 

 products absorbed. Finally, the constituents which resist the 

 action of the digestive juices are expelled through the anus. The 

 processes to which the food is subjected are essentially the same 

 in all animals, so that we may take the arrangements present in 

 one of the higher vertebrates for description of the whole series of 

 events, which can be analysed in such a case much more accurately 

 than in small animals. 



We need spend but little time on the mechanical disintegration 

 necessary in the case of certain materials, on account of their being 

 united together in more or less dense masses. This is done by 

 the teeth in mammals ; in birds which eat hard grains there is 

 a muscular organ, the gizzard, which contains small stones, serving 

 to grind up the food. In the mouth cavity the food is moistened 

 by a liquid, the saliva, which is poured in along tubes leading from 

 special organs which secrete it, as the process of its formation is 

 called. In some animals, including man, saliva contains an agent 

 which brings about the conversion of starch to sugar. This is the 

 first of a series of agents acting in a similar way as the food 

 passes along the alimentary canal. They are known as " enzymes" 

 and before we proceed further we must learn something about the 

 manner in which they act. 



Enzymes 



These substances are sometimes defined as the catalysts pro- 

 duced by living cells. But what are catalysts? In brief, we may 

 say that their action is to make chemical reactions proceed at 

 a faster rate than they naturally do, and that they do this without 



