REGULATORY MECHANISMS 409 



The other feature refers to the method of ossification of 

 certain cartilage-bones by means of a diaphysis and two 

 epiphyses, which is characteristic of the mammals. This 

 method enables the bones in question to function as supports 

 and hinges, and at the same time to grow and enlarge so long as 

 the diaphysis and the epiphyses remain separated by cartilage. 

 But once the diaphysis becomes firmly united by bone with the 

 epiphysis at each end of it, the growth of the bone as a wrfole 

 ceases. The maximum size of such bones is therefore limited, 

 as is that of the animal. In several respects, therefore, the higher 

 vertebrates differ from the lower. With the temperature, the 

 osmotic pressure and the acid-base relations of the blood 

 regulated and constant, the higher vertebrates are largely 

 independent of the environment. Indeed, they have a constant 

 internal climate and " environment " of their own, in which 

 they live sheltered from external agencies, with, in mammals, 

 a constant final adult size. 



The possession of this " internal environment " is not only 

 one of the chief means of survival of the higher vertebrates, 

 but it has also enabled them to become as specialised and 

 perfected as they are. 



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Haldane, J. S. Respiration. Yale University Press, 1922. 



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Krehl, L., und Soetbeer, F. Untersuchungen iiber die Warmeokonomie 

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