CnAP. xvii.] OF SECRETION AND EXCRETION IN GENERAL. 263 



Neither the secreted humours nor the excreted humours are 

 living, but the first have a composition perceptibly stable, re- 

 actions nearly fixed. On the contrary, the composition of the 

 excreted liquids is variable, according to the greater or lesser 

 activity of the gland, the proportion and the nature of the 

 substances absorbed by digestion, the state of activity or of 

 inaction of such or such organic apparatus, and so on. 



We can range in graduated series the diverse modes of elimina- 

 tion, of excretion, and of secretion realised in the organisms. 



The mode the most simple, the only one existing in most 

 vegetals and inferior animals, is the direct passage of the expulsed 

 substances across the tissues, without the intervention of any 

 special organ ; it is simple transudation or exhalation, such as 

 continues to be in the superior animals on the pulmonary surface. 



Then comes the excretory mode, that is to say, a sort of trans- 

 udation through particular glands; such are the urinary and 

 sudorific excretions. 



At the degree, immediately superior, is found the secretion 

 which is called direct. In efEect, the gland drains, direct from 

 the general mass of the materials of the blood, substances which 

 it transforms and which have a physiological use. We may cite, 

 as example, the salivary and biliary secretions. 



The mode the least simple is that in which the open gland, 

 furnished with a canal, needs the co-operation of a closed gland 

 which p-epares materials for it. The only example well demon- 

 strated of this mode of secretion so complex, is that offered us 

 by the pancreas and the spleen. But it is very possible that 

 other correlations of the same kind exist in the superior 

 organisms, that the salivary secretion, for example, is connected 

 with the thyroidal body, and so on ; and that the most of the 

 closed and open glands are thus coupled together. 



