NUTRITION PROPER. 2gI 



PREPARATION OF FOOD. 



Food must be prepared before it can be absorbed into 

 the blood, because it is nearly always solid and must 

 be reduced to a liquid condition before it can be taken 

 up — it must be able to soak* through membranes. The 

 food of plants is already dissolved, as gases in the air 

 bathing the leaves, or as liquids bathing the roots. It 

 is therefore absorbed at once without further prepara- 

 tion. It is already prepared by Nature. But in animals 

 there must be a reservoir in which the solid food is 

 stored and dissolved. This reservoir is the stomach, and 

 the process of solution is digestion. But if we compare 

 the lower with the higher animals, we find, in accordance 

 with the law of differentiation, a gradually increasing 

 complexity in the process. In the lowest protozoa — 

 amoeba — the living protoplasm flows around the prey, 

 dissolves, and appropriates it at once into the tissues. 

 The captured prey passes at otie step from crude food to 

 living tissue, but the tissue thus summarily made is a 

 poor article. In the highest animals, on the contrary, this 

 simple process is differentiated into many consecutive 

 processes, and the finished article of tissue is far more 

 perfect. In man and the higher animals the steps are : 





Mechanical process. 



Chemical process. 











Chymification. 

 Chylification. 



Peptonization. 

 Emulsification. 





By capillaries and lacteals. 



By liver and mesenteric glands. 













* We say soak, but it is necessary to remember that absorp- 

 tion is not a purely physical process, for it is selective. 



