258 



A TEXTBOOK OF PHYSIOLOGY 



through very dilute solutions of inorganic colloids (uranic or ferric 

 oxide) in the presence of sunlight, the synthesis of formic aldehyde 

 and acid is obtained. It has been suggested that we have in this a 

 possible first stage in the evolution of organic material and life pro- 

 cesses from inorganic matter. * 

 The exchange of gases between the respiratory tissue and the 

 outer medium is known as "external respiration." The process 

 whereby the exchange of these gases takes place between the blood 

 and the different parts of the body is known as " internal," or " tissue 

 respiration." Cold-blooded animals can live for some hours in an 

 atmosphere of nitrogen, and hibernating cold-blooded animals e.g.,. 



Products 

 plant 

 Prate id. 

 Fa.t 



Plant 



Products of 



riaJ 



Decomposition 

 monia, etc. 



*;>- 



SOIL. 



WATER NiTKATES 

 A OTHCK SALTS. 



FlG. 130. TO ILLUSTRATE THE CYCLE OF PLANT, ANIMAL AND BACTERIAL LlFE. 



The arrows indicate the materials which each take up and give out to the world. 



snails which shut themselves up for the winter retain their existence 

 for months without respiration. On the other hand, a very active 

 respiration and rapid exchange of gases arc necessary in the warm- 

 blooded animal, because the rate of metabolism is very great, and 

 within the body there is no means afforded for laying in a store of 

 oxygen sufficient to last more than a minute. The carbon dioxide, 

 too, which in normal concentrations plays an important part in 

 regulating body processes, when present within the body in excess, 

 has a narcotic poisonous effect upon the organism. 



The blood is exposed to the air in the lungs over a very extensive 

 surface perhaps as much as 100 square metres in a film one corpuscle 

 thick, and the whole blood circulates through the lungs about once a* 





