ABSORPTION AND ASSIMILATION 



and again : 



(Ci7H35COO)3C3H5+3H20=3Ci7H35COOH+C3H5(OH) 



(the fat Stearin) 



(Stearic acid) 



(Glycerol) 



They are all initiated by organic substances called enzymes 

 (p. 444), which take part in the reaction but are restored at the 

 end. Enzymes are named by the addition of the suffix -ase to the 

 name of the substrate on which they act. 



Since proteins, carbohydrates, and fats are among the com- 

 pounds known as ' organic ', which, in nature, are found only in 

 the bodies of plants and animals and 

 in their remains, such bodies are a ^° 

 necessary part, and the chief part, 

 of the food of all animals. From the 

 same source animals must also 

 obtain (6) other organic substances 

 needed in small quantities. These 

 include the vitamins. These sub- 

 stances, originally manufactured by 

 plants, are transmitted to herbi- 

 vorous animals, and so to the 

 carnivores, and though needed in 

 very small quantities, are essential 

 to life. When, for instance, young 

 rats are fed upon an artificial liquid 30 

 containing the protein, sugar, and 

 fat of milk in the usual proportions, 

 they fail to grow, but the addition 



70 



50 



20 



^J 



Fig. 5. — Curves showing the effect 

 of vitamins on the growth of 

 rats. — From Hopkins. 



to their diet of a very small quantity Lower curve (white circles), rats fed on 



r r -I Mi/i-i J • 1 artificial milk alone. Upper curve (black 



01 iresn milk (Wnicn contains the circles), rats fed on artificial milk and 2 



• . , . , c.c. of cow's milk daily. Average weight 



vitamins) causes them to grow in a in grams, vertical. Time in days, hori- 



normal manner (Fig. 5). The 



structure and mode of action of many vitamins are now known. 



They are nearly all required to enable some particular reaction to 



go on ; thus both B^ and B2 (riboflavin) are concerned in cell 



oxidations. 



The digested materials undergo absorption into the substance 

 of the body, leaving the indigestible matter to be cast away as 

 the dung or faeces. Incorporation, however, is not brought about 

 simply by the absorption of digested matter. Neither before nor 

 after digestion is the food of the same composition as the substance 



