COMPOSITION OF THE ATMOSPHERE 479 



insects. On the other hand, compensation may be effected by 

 a special differentiation of one part of the body for the absorption 

 of oxygen and the discharge of carbon dioxide ; gills and lungs 

 are such absorbing and discharging surfaces, and are able to supply 

 by means of the blood and lymph the respiratory needs 'of the 

 whole body. 



After this brief consideration of the comparative physiology 

 of respiration, the process of gaseous interchange must be dis- 

 cussed, and especially in relation to the higher animals, the 

 vertebrates. In these animals respiration can be divided ,into 

 (i) external respiration, the exchange of gases between the ex- 

 ternal medium and the blood ; and (ii) internal respiration, the 

 exchange between the internal media, the blood and the lymph, 

 and the tissues. 



External Respiration. The exchange of gases between the 

 external medium, the air, and the internal medium, the blood, 

 can be studied in two ways. Analyses of the air passing in and 

 out of the lungs furnish data which show that oxygen is absorbed 

 and carbon dioxide and water are discharged by the lungs. On 

 the other hand, comparative analyses of the gaseous contents of 

 the venous blood passing to, and the arterial blood leaving, the 

 lungs also prove that oxygen is absorbed and carbon dioxide dis- 

 charged by the blood during its passage through the lungs. It 

 must be remembered that these methods of observation do not 

 show how these changes are produced. It is necessary to know 

 by what means the oxygen is absorbed and the carbon dioxide 

 discharged. Is it simply by diffusion that the one gas passes in 

 and the other passes out ? Is it a more complex process, a secre- 

 tion by the living cells of the lung of oxygen into the blood, and 

 an excretion of carbon dioxide from the blood ? These questions 

 are in urgent need of solution, for the gaseous interchanges are 

 not such simple physico-chemical processes as some observers 

 have considered them to be ; the difficulties, indeed, are so great 

 that some physiologists, especially Bohr and Haldane, have con- 

 sidered them to be beyond explanation by our present know- 

 ledge of physics and chemistry. 



The gaseous interchange must now be discussed in detail, 

 and in the first place must be considered the composition of the 

 external medium, the air or water, as the case may be. 



Composition of the Atmosphere. The composition of pure 



