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 
