HOW PLANTS AND ANIMALS GROW. 511 



these upward and downward transformations of matter and 

 energy. 



In the higher animals the various functions are more highly 

 specialized, but we still find that protoplasm is the essential liv- 

 ing substance of every tissue and the dominant factor in nutri- 

 tion. It differs, however, from vegetable protoplasm in many of 

 its properties, and it can not, as formerly assumed, be formed by 

 plants, or built up from the simpler elements that plants feed on, 

 and from which they construct vegetable protoplasm. 



The proteids, fats, and carbohydrates which constitute the food 

 of animals are, as we have seen, products of the destructive me- 

 tabolism of the protoplasm of plants, and it was formerly sup- 

 posed that they are transformed into animal proteids and fats, 

 without any marked change in their chemical constitution. This 

 assumption is, in fact, the basis of the fallacious theory of nutri- 

 tive ratios, but it can not be reconciled with the known facts of 

 animal nutrition. There appears to be conclusive evidence that 

 the proteids, fats, and carbohydrates of the food can not be con- 

 verted into the proteids and fats of the animal body without un- 

 dergoing profound disruption and reconstruction through the 

 agency of animal protoplasm ; or, in other words, the products 

 of vegetable protoplasm can not be made available in the con- 

 struction of animal tissues without being resolved into sim- 

 pler compounds and formed anew in the laboratory of ani- 

 mal life. 



Without noticing the many details that would only tend to 

 divert the attention of the general reader from the significance 

 of the results produced, the essential or fundamental processes in 

 the nutrition of the higher animals may be broadly stated as fol- 

 lows : In the first place, the proteids, fats, and carbohydrates of 

 their food undergo a series of changes in the processes of diges- 

 tion (using this word in its widest sense) that reduces them to 

 simpler compounds and, in fact, almost to their elements, with 

 a liberation of energy which is made available in the reconstruc- 

 tion of the disintegrated food constituents. 



The activities of the various organs of nutrition are primarily 

 directed to the elaboration of a nutritive fluid, the blood, which 

 is distributed to all of the tissues through the circulatory appara- 

 tus provided for that purpose, funishing them the pabulum for 

 their nutrition, and receiving the excretory and other products 

 arising from their metabolism. " An average uniform composi- 

 tion of the blood " is maintained through the action of numerous 

 glandular organs, and the drafts made upon it in the constant re- 

 pair of the different tissues. The various ferments required in 

 the disintegration or digestion of the food elements that are being 

 transformed into blood are products of the destructive metabo- 



