REGULATION OF ENVIRONMENT _ 67% 
extremely sensitive to the slightest changes in the 
concentrations of inorganic salts in their environment. 
Cell-secretion, cell-respiration, and cell-nutrition are 
clearly only different aspects of the same whirl of 
molecular activity. Where secretion or nutrition seems 
to be stationary, there is in reality only a balance 
between ingoing and outcoming molecular streams. 
Instances of this occur when the kidney is not secret- 
ing chlorides, or when no oxygen is passing into or 
out of the swim bladder, or when all external activity 
is latent, as in a dry seed. The apparent stand-still is 
similar to that in a blood corpuscle in a test tube of 
blood half saturated with oxygen, when the stream of 
oxygen molecules entering the corpuscle is balanced 
by the stream leaving it. The unstable oxyhaemo- 
globin molecules in the blood corpuscle are constantly 
losing oxygen molecules and as constantly regaining 
others, so that the half saturation of the blood cor- 
puscle with oxygen represents the average of the 
gains and losses of the haemoglobin molecules. This 
we can understand. But what conception can we form 
of the molecular streaming in the living cell and the 
strange co-ordination which the different molecular 
streams exhibit? I have tried to indicate how this 
problem, which will be followed up in the next lecture, 
rises directly out of the fact of oxygen secretion. But 
meanwhile we must follow further various other facts 
relating to respiration. 
The evidence existing at present is strongly against 
the theory of active secretion of CO, outwards by the 
lung epithelium. Krogh’s experiments gave very defi- 
