ASSIMILATION. 653 



In short, it seems to hold, at least as far as experiments have 

 hitherto been made, that foreign substances may be incorporated 

 with those of the body, provided they be precisely of the same 

 kind with those to which they are added, whether fluid or solid. 

 Thus chyle may be mixed with chyle, blood with blood, muscle 

 with muscle, and bone with bone. The experiment has not been 

 extended to the other animal substances, the nerves, for instance ; 

 but it is extremely probable that it would hold with respect to 

 them also. 



On the other hand, when substances are introduced into any 

 part of the body which are not the same with that part, nor the 

 same with the substance upon which that part acts, provided they 

 cannot be thrown out readily, they destroy the part, and per- 

 haps even the animal. Thus foreign substances introduced into 

 the blood very soon prove fatal ; and introduced into wounds of 

 the flesh or bones, they prevent these parts from healing. 



Although the different assimilating organs have the power of 

 changing certain substances into others, and of throwing out the 

 useless ingredients, yet this power is not absolute, even when the 

 substances on which they act are proper for undergoing the 

 change which the organs produce. Thus the stomach converts 

 food into chyme, the intestines chyme into chyle, and the sub- 

 stances which have not been converted into chyle are thrown out 

 of the body. If there happen to be present in the stomach and 

 intestines any substance which, though incapable of undergoing 

 these changes, at least by the action of the stomach and intes- 

 tines, yet has a strong affinity, either for the whole chyme and 

 chyle, or for some particular part of it, and no affinity for the 

 substances which are thrown out, that substance passes along 

 with the chyle, and in many cases continues to remain chemically 

 combined with the substance to which it is united in the stomach, 

 even after that substance has been completely assimilated, and 

 made a part of the body of the animal. Thus there is a strong 

 affinity between the colouring matter of madder and phosphate 

 of lime. . Accordingly, when madder is taken into the stomach, 

 it combines with the phosphate of lime of the food, passes with it 

 through the lacteals and blood-vessels, and is deposited with it 

 in the bones, as was proved by the experiments of Bechier* and 



Phil. Trans. 1736, p. 287. 



