REGULATION OF ENVIRONMENT 85 



the function of a part of a machine: for this part 

 does nothing else than fulfil its function, provided the 

 machine is assumed to be perfect and stable. In a 

 living organ however we are dealing with something 

 of which the functions, if we speak of functions, are 

 endless, since the activities are endless, constantly 

 seeming to grow in number as we investigate further. 

 Its true function, to the eye of a physiologist, is to 

 maintain these endless activities in balance with the 

 endless activities of other organs, and not merely to 

 perform one specified action. 



It is evident that the balancing of molecular activi- 

 ties on which the maintenance of the internal environ- 

 ment depends is centred in the bodies of the cells 

 which make up the living tissues. The composition 

 and volume of the blood are the outcome of their 

 joint active or passive influences. We are thus brought 

 back to the problems of cell-secretion, cell-respiration, 

 cell-nutrition, cell-movement, cell-heat-production — 

 problems which, as we have already seen, are only 

 different aspects of one problem — that of what may be 

 called cell-metabolism. Living cells are the nodal 

 points of the molecular and ionic streams of which 

 one outcome is the constant internal environment. 

 The living cells are the seat of the molecular or ionic 

 accelerations or retardations which manifest them- 

 selves in secretion, and of the main chemical changes 

 which express themselves as metabolism in its varied 

 outward forms. When we concentrate attention ex- 

 clusively on some one detail of cell-metabolism we 



