140 CORRELATION OF SECRETION, 



into the body and after being removed from it, but in the process 

 of respiration itself the substances concerned, oxygen and carbon- 

 dioxide, 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, absorp- 

 tion, 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 respira- 

 tion, 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, &c., in which process the cell acts as an energy-trans- 

 former upon the chemical energy supplied by the organic con- 

 stituents of the plasma, and builds up its own special products 

 from these constituents ; (2) the formation from the inorganic con- 

 stituents 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 con- 

 taining 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. 



