THE PHYSIOLOGY OF DIGESTION 659 



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 Table on p. 91 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 en- 

 dowed 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 prepara- 

 tion 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 hydro- 

 lytic, the food-stuffs taking up one or more molecules of water and under- 

 going 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 galactose. 



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



(3) Proteins. Here we have a great variety of mono- and diamino-acids, 

 which may be enumerated as follows : 



MONO-AMINO-ACIDS 



Glycine (aminoacetic acid) .... 



Alanine (aininopropionic acid) 



Serine or oxyalanine (oxyaminopropionic acid) Monobasic acids 



Aminovalerianic acid . 



Leucine (aminoisobutylacetic acid) 



Isoleucine (aminocaproic acid) 



of fatty series 



