66 ORGANISM AND ENVIRONMENT 
sors are accelerated inwards from the blood into the 
cell. 
The step from secretion to the processes which we 
commonly designate as cell nutrition or cell respira- 
tion is only a short one. The microscopic study of 
secreting cells shows that the substances secreted, or 
their immediate precursors, are often stored up 
for some time until the moment for their discharge 
comes. This storage is comparable to ordinary 
growth. In his famous book on Secreting Glands, 
published in 1830, Johannes Miiller expressed the 
opinion that secretion and growth are merely different 
aspects of one kind of activity; the sole difference 
being that in secretion the product is removed, while 
in growth it remains. Miller was a vitalist, and his 
ideas on secretion were for the time swept away by 
the whirlwind of mechanistic speculation which passed 
over physiology about the middle of the last century ; 
but in the main he was right. We now know that even 
in ordinary nutrition nothing remains still and inactive. 
Living structure is really alive and full of molecular 
activity: it is the expression of the directions and 
velocities which this activity takes. Substances are 
constantly being taken up from and given off to the 
environment ; and even when these substances do not 
seem to be used up in adult nutrition, as for instance in 
the case of inorganic salts, there is a constant molec- 
ular interchange between the cell and its environ- 
ment. This is proved by the fact that, as was first 
shown in particular by Sidney Ringer, the tissues are 
