THE ANIMAL AS A PRIME MOVER. 305 



an effect upon our moods, our powers, and our ability to perform mental 

 even more than manual work is a daily observation with everyone; 

 and the suggestion has even been made that the selection of nutriments 

 may be made to produce effects of economic importance in both direc- 

 tions — in making the human as well as the lower order of machine 

 better as a motor, better as an intelligent worker, and even better as 

 a thinker and as a moral creature, which lines of improvement are all 

 essential to further progress in either direction. So far as both experi- 

 ment and general observation have gone, at present, it may safely be 

 stated there can be no question that the value, the power, the efficiency 

 and durability of the mechanical and of the mental side of the vital 

 apparatus are both influenced essentially by the nature of the material 

 selected as the reservoir of potential energy, to be rendered kinetic 

 and to be applied to useful purposes by it. 



As indicated by Dr. Carpenter, in the middle of the nineteenth cen- 

 tury, it would seem now certain that "motor force may be developed, 

 like heat, by the metamorphosis of constituents of food which are not 

 converted into living tissue; " while there is also no doubt that, in many 

 cases at least, the disintegration of tissue which has completed its 

 period of service in the organism may, precisely as does the digestion 

 of animal food, perform its part in the supply of potential energy to 

 the system for utilization in vital and other operations. As the same 

 great physicist and physiologist has stated it : 



The life of man or of any of the higher animals consists essentially 

 in the manifestation of forces of various kinds, of which the organism 

 is the instrument, and these forces are developed by the retrograde 

 metamorphosis of the organic compounds generated by the instrumen- 

 tality of plants. 



Thus, during the whole life of the animal, the organism is restoring 

 to the world around both the materials and the forces which it draws 

 from it; and after its death this restoration is completed, as in plants, 

 by the final decomposition of its substance. 



As was, perhax>s, first stated explicitly, by Liebig, we find that the 

 sources of all forces, powers, energies, in man and animals, are to be 

 found in the food constructed by the plants out of mineral substances 

 under the active energy of the sun's rays, which energy, becoming thus 

 latent, is reawakened by the vital apparatus and directed into useful 

 channels, constructing the whole animal machine, and doing all its 

 work — muscular, thermal, electrical, vital, mental. 



The quantity of energy imported into the vital machine, is, in any indi- 

 vidual case, readily measurable. It differs greatly with the species, 

 size, temperament and work, external and internal, of the animal, and 

 very greatly with the efficiency of its apparatus of digestion and assim- 

 ilation. While, for example, one man will live and enjoy life and do 

 his full share of work on 1 pound of good food per day, another will 

 often require 2 pounds or more; and, in some instances, in which dis- 

 ease had reduced the assimilative powers, as much as 7 pounds have 

 SM 96 20 



