RESPIRATION 383 



After Harvey's discovery of the circulation and Lavoisier's 

 discoveries with regard to respiratory exchange and animal heat 

 many physiologists looked upon circulation and breathing as 

 processes which primarily determine and regulate tissue activity. 

 We can trace this, for instance, in the physiological ideas of 

 Descartes and Liebig, and in ideas still to some extent prevalent 

 as to the causes of respiratory exchange, secretion, and growth. 

 Closer examination has shown that breathing and circulation are 

 responses to tissue activity, and do not primarily determine it. 



Another tendency has been to regard the nervous system as the 

 primary autonomous regulator of breathing and circulation. The 

 evidence brought forward above has shown, however, that the 

 regulative influence of the nervous system is not autonomous, 

 but dependent on conditions of environment determined mainly 

 by varying tissue activity. 



In his **Le9ons sur les phenomenes de la vie" (p. 121) Claude 

 Bernard drew the conclusion that **all the vital mechanisms, 

 varied as they are, have only one object, that of preserving con- 

 stant the conditions of life in the internal environment" (the 

 blood). No more pregnant sentence was ever framed by a physi- 

 ologist, and the long series of investigations described in the 

 present book may be regarded as an attempt to follow out in 

 regard to blood reaction and oxygen supply the line which 

 Bernard indicated. Physiological activities can in one sense be 

 summed up in the ''preservation of the conditions of life in the 

 internal environment," with consequent maintenance of normal 

 structure. In another sense, however, physiological activity is 

 constantly disturbing the internal environment. What is actually 

 maintained is a dynamic balance between the disturbing and 

 restorative activities. The order displayed in this dynamic balance 

 is the order of biology. 



In view more particularly of Paul Bert's experimental demon- 

 stration that the physiological action of gases dissolved in the 

 blood depends on the pressures which they exert in the surround- 

 ing atmosphere — that is to say on their vapor pressures — we may 

 conclude that it is the diffusion pressures of substances dissolved 

 in the blood that correspond to Bernard's "conditions of life." This 

 definition includes temperature : for diffusion pressure, other 

 things being equal, varies as the absolute terAperature and indeed 

 gives us our measure of temperature, since the expansion of gases 

 or liquids, by which we measure temperature, depends on increase 

 of diffusion pressure. 



