248 CORRELATION OF SECRETION 



to the liver; cellular processes, again, in which the excretory pro- 

 ducts are chemically modified; external processes by which the 

 products are carried to the excretory organ, such as the kidney; 

 and, lastly, in the special excretory organ itself, we have cellular 

 processes again in which the excreting cell provides energy for work 

 to be done against osmotic pressure with corresponding increase 

 in osmotic energy, at the expense of chemical energy obtained from 

 oxidation of nutrient matter. Throughout the processes, in addition 

 to external carriage in the blood- stream, there is also the play from 

 without upon the active cells of the external agencies (a) of the nervous 

 system directly, or indirectly through the vasomotors, and (6) the 

 stimulus of chemical substances in the circulation, which may also act 

 upon the cells, or intermediately through varying the blood- supply. 

 It is in the external parts of the processes that the chief differ- 

 ences in their mechanisms are to be found, and this statement 

 holds not only in contrasting the processes of respiration, secretion, 

 absorption, and excretion with one another, but in regard to the 

 variations between one type or case of secretion, etc., and another. 

 The variations in the external mechanisms are manifold between 

 one process and another, and from one animal species to another, 

 with regard to how the same fundamental process is carried out; 

 but in all cases the essential cellular processes are very much alike, 

 and the same types of phenomena are to be found. There is in 

 every case a living cell involved in the process, and by this living 

 cell the processes of diffusion and osmosis are profoundly modified. 

 Substances are separated often at higher concentrations than in 

 the bathing fluid, which can only take place on the condition that 

 energy is transformed by the cell and converted into osmotic 

 energy. New substances are produced in many cases which are 

 typical of the action of the cell involved, and can only be produced 

 as a result of energy transformations induced by the cell. Even 

 where the concentrations of every single instance in the secretion 

 may be less than in the bathing fluid, and no new substance is 

 produced in the passage through the cell, 1 the rate of secretion 

 or transmission is so much subject to variation apart from purely 

 physical factors, that the cell must be regarded as something more 

 than simply an inert membrane, because its permeability for 

 different dissolved substances, and for the solvent, varies from time 



1 It is improbable that this condition ever is completely realised in the 

 action of living cells. 



