422 PHYSIOLOGICAL CHEMISTRY. 



quantity of oxygen sufficient to convert all carbon into carbon dioxide and all 

 hydrogen into water, which fact also explains the combustibility of all organic 

 substances. 



Why it is that the living plant has the power of forming organic substances 

 in the manner above indicated, we know not, and we know very little even in 

 regard to the means by which the living cell accomplishes this formation, but 

 we do know that sunlight is that agent the action of which is indispensable for 

 the plant in the formation of more complicated organic substances from simpler 

 ones. 



Decomposition of vegetable matter in the animal system. 

 It has been stated above that the process of chemical decomposition 

 taking place in the animal system is chiefly regressive or destructive, 

 that is to say, the substances formed in the plant are taken into the 

 animal system, where they are gradually oxidized by the inhaled 

 atmospheric oxygen, thereby being converted into simpler forms of 

 combination which are finally eliminated as waste products. 



It has been shown above how a molecule of glucose which is formed in the 

 plant requires not less than 6 molecules of carbon dioxide, and the same num- 

 ber of molecules of water for its formation, 6 molecules of oxygen being 

 eliminated. A molecule of glucose taken into the animal system undergoes the 

 reverse process ; by combining there with 6 molecules of oxygen it is converted 

 into 6 molecules of carbon dioxide and the same number of molecules of water, 

 thus: 



C 6 H 12 O 6 + 12O == 6CO 2 + 6H 2 0. 



Animal food. The food taken by animals is (beside water and a 

 few of its mineral constituents) all derived from vegetables, but it is 

 taken from them either directly or indirectly ; in the latter case it has 

 been taken previously into and assimilated by other animals, as in 

 case of food taken in the form of meat, milk, eggs, etc. While some 

 animals (herbivora) feed upon vegetable, and some (carnivora) upon 

 animal food exclusively, others are capable of taking and assimilating 

 either. 



The fact that animal food is derived from vegetable matter, renders 

 it superfluous to state that the elements taking an active part in the 

 formation of either vegetable or animal matter are identical. Of the 

 total number of 69 elements, only 14 are found as necessary con- 

 stituents of the animal body. These elements are carbon, hydrogen, 

 oxygen, nitrogen, sulphur, phosphorus, chlorine, fluorine, silicon, cal- 

 cium, magnesium, sodium, potassium, and iron. A few other elements, 

 such as aluminum, manganese, copper, etc., are sometimes found in 

 the animal system, but they cannot be looked upon as normal or 

 necessary constituents. 



