Chap. 30.] Sccretion t hov) performed. 367 



tributed in the gland, or whether it is poured out from 

 the open extremities of arteries into fmall receptacles* 

 and is thence received into the excretory duel, or in 

 what other mode the change wrought on the blood 

 conveyed to the gland is effected, we are entirely ig- 

 norant. So different, however, are the properties 

 of fecreted fluids from thofe of the blood, that it is 

 probable forriething more happens than a mere fe- 

 paration of principles, which previoufly exifted in that 

 fluid. 



By fome phyfiologifts it has been imagined, that 

 fecretion may be explained on the fimple fuppofition 

 of a difference of diameter in the veffels from which 

 the fecreted fluids are poured out. On this idea it 

 has been advanced, that the thinned fluids are formed 

 by the arceries of the fmalleft diameter, and the more 

 denfe by arteries of a larger fize , but it is evident, 

 that though the fmaller arteries would exclude the larger 

 particles, ftill the larger arteries would differ the fmaller 

 particles to pafs through them, and thus the fecretion 

 be in fome meafure confounded. 



Excretion, like fecretion, is performed in general 

 by arteries. The term fecretion is applied to the for- 

 mation of thofe fluids which are fubfervient to fome 

 purpofe in the animal machine $ that of excretion to 

 the formation of fuch as are apparently of no particular 

 life, and which fcem to be feparated for no other end 

 than to be difcharged from the body. It is difficult, 

 however, to apply thefe diftinctions to particular cafes, 

 fince there is hardly any one of thefe fluids, the pro- 

 duction of which is not in fome way ufeful, and but 

 very few which may not be confidered as in fome de- 

 gree excrementitious. 



Both fecretion and excretion are in many parts of 



the body performed by the minute ramifications of ar- 



X 2 teriea 



