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would, without adaptation, exist is greatly diminished. This form 

 of adaptation is very clearly seen in persons living at great alti- 

 tudes, and will be discussed in detail in later chapters. In the 

 second place the tissues may adapt themselves to a lower partial 

 pressure of oxygen. About this second form of adaptation our 

 knowledge is at present very imperfect; but it seems to me 

 that clinical evidence points strongly to its existence. Perhaps the 

 clearest evidence is afforded by cases of congenital heart defect, 

 in which part of the venous blood passes direct to the left side of 

 the heart without first passing through the lungs. In these cases 

 of "Morbus coeruleus" the arterial blood is always more or less 

 blue, and becomes extremely blue on muscular exertion, so that 

 one can always recognize this condition in persons walking in the 

 street. The remarkable point, however, is that in spite of the an- 

 oxaemic condition of the arterial blood these persons may get on 

 quite well, and be able to walk at a good pace. On account of the 

 large increase in their haemoglobin percentage, they have plenty 

 of oxygen in their blood, but at a low partial pressure. It seems 

 hardly possible to doubt, therefore, that their tissues have become 

 adapted to the low partial pressure of oxygen; and the same 

 adaptation probably exists to a considerable extent in many 

 chronic cases of valvular heart disease, emphysema, etc. 



The fact that cyanosis may exist without harm in chronic cases 

 of disease has certainly contributed greatly to the general failure 

 to recognize the gravity of anoxaemia in persons not adapted. 

 Adaptation is a process which always requires time, and the time 

 factor must, therefore, be taken into account in judging of the 

 physiological effects of anoxaemia. 



