THE INNER TISSUES OF ANIMALS. 331 



may fairly infer that the excretion of it is the original 

 function of the liver. 



One further preliminary is requisite. We must for a 

 moment return to those physico-chemical data set down in 

 the first chapter of this work ( 78). We there saw that 

 the complex and large-atomed colloids which mainly compose 

 living organic matter, have extremely little molecular mo- 

 bility; and, consequently, extremely little power of diffusing 

 themselves. Whereas we saw not only that those absorbed 

 matters, gaseous and liquid, which further the decomposition 

 of living organic matter, have very high diffusibilities, but 

 also that the products of the decomposition are much more 

 diffusible than the components of living organic matter. 

 And we saw that, as a consequence of this, the tissues give 

 ready entrance to the substances which decompose them, and 

 ready exit to the substances into which they are decomposed. 

 Hence it follows that, under its initial form, uncomplicated 

 by nervous and other agencies, the escape of effete matters 

 from the organism, is a physical action parallel to that which 

 goes on among mixed colloids and crystalloids that are dead 

 or even inorganic. Excretion is a specialized form of this 

 spontaneous action ; and we have to inquire how the special- 

 ization arises. 



Two causes conspire to establish it. The first is that these 

 products of decomposition are diffusible in widely different 

 degrees. While the carbonic acid and water permeate the 

 tissues with ease in all directions, and escape more or less 

 from the exposed surfaces, urea, and other waste substances 

 incapable of being vaporized, cannot escape thus readily. 

 The second is that the different parts of the body, being 

 subject to different physical conditions, are from the outset 

 sure severally to favour the exit of these various products of 

 decomposition in various degrees. How these causes must 

 have co-operated in localizing the excretions, we shall see on 

 remembering how they now co-operate in localizing the sepa- 

 ration of morbid materials. The characteristic substances of 



