THE INNEli TISSUES OF ANIMALS. 319 



organism, is a physical action parallel to that which goes on 

 among mixed colloids and crystalloids that are dead or even 

 inorganic. Excretion is simply a specialized form of this 

 spontaneous action ; and what we have to inquire is, how the 

 specialization 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 all 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 organism, 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 

 gout and rheumatism have their habitual places of deposit. 

 Tuberculous matter, though it may be present in various 

 organs, gravitates towards some much more than towards 

 others. Certain products of disease are habitually got rid of 

 by the skin, instead of collecting internally. Mostly, these 

 have special parts of the skin which they affect rather than 

 the rest ; and there are those which, by breaking out sym- 

 metrically on the two sides of the body, show how definitely 

 the places of their excretion are determined by certain favour- 

 ing conditions, which corresponding parts may be presumed 

 to furnish in equal degrees. Further, it is to be observed 

 of these morbid substances circulating in the blood, that 

 having once commenced segregating at particular places, 

 they tend to continue segregating at those places. As- 

 suming, then, as we may fairly do, that this localization 

 of excretion, which we see continually commencing afresh 

 with morbid matters, has always gone on with the matters 

 produced by the waste of the tissues, let us take a further 

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