CHAPTER IV, 

 RESPIRATION. 



THE most constant phenomenon presented by living organisms, both 

 animal and vegetable, is the absorption of oxygen. This sub- 

 stance, either in the gaseous form as a constituent of the atmosphere, 

 or dissolved in water or other liquids, is indispensably requisite for the 

 manifestation of vital phenomena. Oxygen is diffused everywhere 

 over the surface of the earth, forming rather more than one-fifth part 

 of the volume of the atmosphere, and exists in solution in greater or 

 less abundance in the water of springs, rivers, lakes, and seas. Animals 

 and plants, accordingly, whether living in the air or in the water, are 

 surrounded by media in which this substance is present. Even para- 

 sitic organisms, inhabiting other living bodies, and the foetus during 

 intra-uterine life, though not immediately in contact with oxygen, are 

 supplied with nutritious fluids which have themselves been exposed to 

 its influence. Respiration consists in the process by which oxygen 

 penetrates the substance of living organisms, and the changes which 

 accompany or follow its introduction. 



Respiration in Vegetables. In regard to vegetables, a distinction is 

 to be made between respiration and the absorption of gaseous matter 

 for the production of organic material. All green plants, under the 

 influence of solar light, absorb carbonic acid and water ; partially deox- 

 idizing these substances, to form, with their remaining elements, starch, 

 cellulose, and fat. The oxygen thus separated is exhaled in a free form ; 

 while an accumulation of organic material takes place in the vegetable 

 fabric, which thus increases in substance, and may afterward serve for 

 the nutrition of animals. This process, therefore, is not one of respi- 

 ration, but of organic production. It is peculiar to vegetables, since 

 animals have no power to produce organic material, and depend upon 

 vegetables for their supply of food. 



Animals, on the other hand, consume the organic material thus pro- 

 duced, at the same time absorbing oxygen and exhaling carbonic acid 

 and water. In this respect animal and vegetable life stand in a com- 

 plementary relation to each other. Vegetables produce organic matter 

 by deoxidation ; animals consume it with the phenomena of oxidation. 



But this apparent opposition only exists because plants have the 

 special power of producing organic matter, by which they become a 

 source of nourishment for animals. The organic substances so pro- 

 duced do not immediately take part in the active phenomena even of 

 vegetable life. The"y are, on the contrary, deposited in a quiescent 



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