32 HUMAN PHYSIOLOGY 



Plants synthesize proteins as well as starch. Little is 

 known of the process in detail, but experience of the most 

 practical kind shows that the most available supply of 

 nitrogen for use in protein formation is afforded by the 

 nitrates of the soil. These salts are received into the roots 

 of plants and are transported in the sap. The sulphur 

 required, a smaller quantity than the nitrogen, is secured 

 in a similar way as dissolved sulphates. So phosphorus 

 comes into the plant in the form of phosphates. These 

 various salts we aim to furnish when we fertilize a plot of 

 ground. 



Animals take their proteins ready-made from plants. 

 They do not, however, store the identical proteins which 

 they have eaten. They digest them a process of decom- 

 position and they synthesize a selected fraction of the 

 products. But this is a much less radical reconstruction 

 than that performed by the plants in manufacturing pro- 

 teins from the simplest materials. The earlier writers 

 used to make the sweeping assertion that animals are 

 altogether destructive and can never carry on any con- 

 structive work. This is now seen to be untrue; destruc- 

 tive changes predominate in the sum of their life activities, 

 but they often make complex compounds from simpler 

 ones in special cases. 



It may be asked how animals can synthesize chemical 

 compounds of high fuel-value when they cannot apply to 

 the task the energy of light. The answer is found in the 

 fact that where the prevailing reactions are in the line of 

 oxidation and attended by a release of abundant energy 

 a portion of this energy may be employed to advance 

 changes of the opposite order, those in which heat or other 

 energy is absorbed. A man on first observing the hydrau- 

 lic ram at work may be surprised to see water raised 

 from the bottom of a ravine to a house high above the 

 stream. But he can soon be convinced that the principle 

 of the conservation of energy is not violated by this de- 

 vice; much more water is falling than rising all the time. 

 The analogy is a close one; in the animal there is much 



