14 FIRST PRINCIPLES. 



In the animal body, the oxygen which is required for the 

 combustion of food is obtained, as we have seen, from the air 

 breathed in by the lungs. 



The nitrogenous compounds formed in plants by the 

 agency of the rays of the sun, also possess energy in the form 

 of chemical affinity for oxygen, though to a much less extent 

 than fat or starch, as we may learn from their composition 

 (approximatively, 53.5 parts carbon, 7 parts hydrogen, 23.3 

 parts oxygen, and 16.5 parts nitrogen), and from the fact that 

 chemical affinity is stored up in urea and hippuric acid, in 

 which products almost all the nitrogen contained in the plant 

 food of the horse leaves his body. The life of plants which 

 are eaten by horses (non-parasitic plants) is a process of 

 deoxidation ; that of animals, one of oxidation. 



The absorption of carbonic acid gas in plants is effected by 

 chlorophyll, which is the green colouring matter of plants and 

 which is formed only in sunlight. Hence, plants which have 

 no chlorophyll, are unable to obtain their carbon from the car- 

 bonic acid of the air, and are consequently obliged to feed on 

 other plants or animals. Fungi (mushrooms, toadstools, etc.) 

 and bacteria (the disease germs of glanders and tuberculosis, 

 for instance) are varieties of such parasitic plants. 



THE ANIMAL BODY AS A MACHINE. 



The movements of the body do not appear to be produced 

 in a manner similar to those of a mechanical machine, as for 

 example, a steam engine, in which the chemical affinity of the 

 fuel for oxygen is converted into heat ; and the heat, into 

 motion to a greater or less extent. In the first place, the 

 energy required for the movements of the animal machine is 

 not obtained directly from the fuel (the food), but from the 

 machine itself (the tissues) ; the fuel being employed to build 

 up and repair the component parts of this living mechanism. 

 Secondly, the heat which is evolved does not seem capable 



