70 1 PHYSIOLOGY 



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 them- 

 selves, 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. 89 will show the striking difference in com- 

 position 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 mole- 

 cule to bits, and by selecting certain of its constituent parts and building these 

 up in the proper proportions to make 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 foodstuffs by 

 means of ferments, and in every case the action of these ferments is hydro- 

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

 going dissociation into simpler molecules. Since each class of foodstuff 

 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 laBvulose, 

 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 (aminopropionic acid) . 



Serine or oxyalanine (oxyaminopropionic acid) 



Aminovalerianic acid .... 



Leucine (aminoisobutylacetic acid) . 



Isoleucine (aminocaproic acid) . 



Monobasic acids of fatty 



series 



