142 CORRELATION OF SECRETION 



processes of absorption occur modified by the operation of the 

 nervous system, and new products are given out which are carried 

 by an extra cellular process to other cells. Similarly in excretion, 

 we have cellular activity in which the excretory products are 

 formed in the various cells ; external processes by which these 

 are carried to the liver ; cellular processes, again, in which the 

 excretory products 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 corre- 

 sponding 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 vaso-motors, and (b) 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, &c., 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 process is 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 



