738 PHYSIOLOGY 



deamination and complete oxidation. If they are to be built up as 

 integral parts of living cells, to take the place of molecules which have 

 been destroyed in the wear and tear of functional activity, the change 

 must be almost equally profound. The proteins of the cells from 

 different parts of the body have different molecular constitutions. 

 Not only do they differ among themselves, but they differ very 

 largely from many of the proteins which may be taken in with the 

 food. A child is able to obtain material for the growth of his brain- 

 cells, his muscle-cells, or his liver-cells, from a diet containing protein 

 in the form of caseinogen, or of vegetable gluten, or of meat fibrin. 

 A reference to the Tables on p. 100 will show the striking difference 

 in composition between the various proteins of the food and the 

 proteins which have to be formed from them in the living tissues. 

 It is evident that to form serum albumin, for instance, out of 

 wheat gliadin, an entire reconstruction is necessary. This can only 

 be accomplished by taking the protein molecule to bits, and by 

 selecting certain of its constituent parts and building these up in 

 the proper proportions to form a new protein molecule. For the 

 purposes of nutrition therefore the changes in the protein molecule 

 must be greater the more variation there is in the composition of the 

 protein of the food from the composition of the proteins of the 

 tissues. 



In primitive alimentary canals every cell lining the canal may 

 be endowed with amoeboid properties and capable of devouring 

 the food particles, the subsequent changes in the latter to fit them 

 for their journey through the rest of the body being performed in 

 the body of the cell itself. In all the higher animals, however, 

 including ourselves, the greater part of the preparation of the food 

 is accomplished extracellularly in the lumen of the alimentary canal, 

 and the changes are effected by means of special digestive juices, 

 which are formed by the activity of masses of cells produced as 

 outgrowths from the wall of the canal. The digestive juices attack 

 the food-stuffs by means of ferments, and in every case the action 

 of these ferments is hydrolytic, the food-stuffs taking up one or 

 more molecules of water and undergoing dissociation into simpler 

 molecules. Since each class of food-stuff requires a different ferment, 

 a great variety of ferments is concerned in the processes of digestion. 



As the end-result of digestion the many kinds of food taken by 

 man are reduced to a fairly small number of simpler bodies. These 

 end-products are : 



(1) Carbohydrates. 



The monosaccharides : glucose, fructose or levulose, and galac- 

 tose. 



(2) Fats. Fatty acids, or (in alkaline medium) soaps, and glycerin. 



