478 THE RESPIRATORY EXCHANGE 



The ventilation of the lungs is merely an accessory part of respira- 

 tion, and its mechanism will not be considered here. 



The necessity of oxygen for the vital activity of unicellular 

 organisms can be demonstrated by simple experiments. If an 

 amoeba or a rhizopod be observed in a moist chamber placed under 

 the objective of a microscope, the movements of its pseudopodia 

 will afford indication of its activity : the passage of a stream of 

 some inert gas, such as hydrogen or nitrogen, will displace the 

 oxygen from the chamber and liquid in which the organism is 

 confined, and the movements will become less and less until they 

 cease. A renewal of the supply of oxygen will restore the power 

 of movement. 



Plants breathe ; they absorb oxygen and discharge carbon 

 dioxide, and show no special method of respiration which will 

 enable the physiologist to draw a hard-and-fast line between 

 respiration in animal and vegetable forms of life. The phenomena 

 of assimilation, the absorption and breaking up of the carbon 

 dioxide of the atmosphere by the action of chlorophyll in the 

 presence of sunlight, the building up of carbon compounds and 

 the discharge of oxygen, are not processes of respiration. In 

 the daytime this assimilation may mask the respiratory exchange, 

 but at night it is easy to show that plants absorb oxygen and 

 discharge carbon dioxide. The difficulty is absent at all times 

 in fungi which contain no chlorophyll, and in the plasmodium 

 of some of the myxomycetes masses of unicellular protoplasm 

 can be obtained large enough for a direct determination of the 

 respiratory exchange. 



In the lower forms of life the exchange of gases may be a simple 

 process of diffusion, but there is no definite knowledge upon the 

 question, and it is possible that a ferment, an oxydase, is concerned 

 in the exchange. The problem of their respiration would indeed 

 appear to be the same as that of the internal respiration of the 

 higher animals, all of which pass through an unicellular stage during 

 their development. Increased complexity is introduced by the 

 growth and multiplication of the cells ; the outermost cells have 

 a more direct means of gaseous exchange with the surrounding 

 medium than the middle layer of cells ; compensation is, therefore, 

 necessary, and may take the form of a system of channels, which 

 communicate with the exterior and ramify through the different 

 tissues ; such a system is found in the tracheae, or air-tubes, of 



