92 



THE FOOD-rREPARIXG SYSTEM 



any energy, nor, so far as we know, are built into living 

 protoplasm ; yet, if they are left out of the diet we do not 

 thrive. It is evident that some essential bodily processes 

 cannot go on well unless these vitamins are present, but 

 just what the nature of these processes may be is 

 not at present at all clear. 



The Foods Proper fall into 

 three classes: carbohydrates 

 (sugars and starches), fats, and 

 proteins. The accessories do not 

 have to be digested, they are util- 

 ized by the body in the form in 

 which they are taken in. All of 

 the nutrients except some of the 

 simpler sugars have to be digested, 

 either because in the form in which 

 they are taken in they are in- 

 soluble in water and so can not be 

 taken up by the body fluids, or 

 because they are not in such 

 chemical form that the cells can 

 use them. 



The Alimentary Tract in the 

 higher animals consists, first, of a 

 tube, into which is inserted at one 

 end the mixtures of complicated 

 materials which the animal eats 

 and within which the process of 

 preparing the food for the use of 

 the cells goes on, and second, of 

 cell-masses called glands, which manufacture the various 

 digestive secretions and pour them out into the alimen- 

 tary tube. The three food classes, the carbohydrates, 

 fats, and proteins, have to be treated individually; that 

 is to say, the processes by which carbohydrates are di- 

 gested do not serve for fats, nor is the digestion of fats 

 exactly like the digestion of proteins. So we find the 



Fio. 18. — Diagram of 

 Alimentary Canal in a 

 Higher Vertebrate (man). 

 €, esophagus; s, stomach; 

 t, small intestine; c, large 

 intestine; g^ salivary 

 gland; p, pancreas. For 

 the .sake of simplicity the 

 liver and the pa-ssages 

 of>ening from the alimen- 

 tary canal (nostrils and 

 windpipe) are omitted. 



