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. Muller 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 



