246 CORRELATION OF SECRETION 



are as truly in aqueous solution as are the substances involved in 

 secretion, absorption, and excretion. It is clear, then, that the 

 purposes served in the processes of secretion, absorption, excretion, 

 and respiration differ, but we shall see that in so far as the intrinsic 

 nature of these processes and the mechanisms by which they are 

 carried out are concerned, they are closely similar or identical, and 

 are governed by the same laws. 



At the outset it may be pointed out that not only in respiration, 

 where the differentiation of the process into two parts an external 

 respiration and an internal or tissue respiration has been clearly 

 recognised, but also in the other processes of secretion, absorption, 

 and excretion, there are two parts to the process viz., (1) an 

 internal or cellular part, in which chemical changes, and processes 

 involving energy changes within the cells active in the process, occur, 

 and (2) an external or mechanical part in which the products acted 

 upon are brought to or carried away from the cell and transferred 

 to other parts of the organism, and by means of which, through the 

 activity of mechanisms external to the cells concerned in the active 

 process, the internal or cellular part is modified and regulated. 



Thus, in the case of secretion, we may point out as the internal 

 or cellular part of the process: (1) the formation and storage in the 

 cell of the intrinsic organic constituents of the secretion, as zymogens, 

 etc., in which process the cell acts as an energy transformer upon 

 the chemical energy supplied by the organic constituents of the 

 plasma, and builds up its own special products from these con- 

 stituents; (2) the formation from the inorganic constituents of the 

 plasma of the inorganic constituents of the secretion against the 

 laws of diffusion and osmosis, so that the osmotic energy is increased 

 by the separation of a secretion containing substances in greater 

 concentration than they possess in the plasma, the cell here again 

 acting as a transformer, and converting chemical energy derived from 

 its absorbed and oxidised food into osmotic energy. 



But we have also in secretion the external part of the process 

 in which agencies outside the secreting cell come into operation, 

 and either modify the action of the cell, or produce an effect apart 

 from the cell entirely. 



The agents which come into operation in the external part of 

 secretion may be classified as follows: 



(1) The alteration in the supply of fluid or solvent and of dis- 

 solved and nutrient matter to the cell, such as variations of the 



