ASSIMILATION. 655 



stances ; yet it is well known that the stomach is not affected by 

 digestion while the animal retains life ; though, as Mr Hunter 

 ascertained, the very gastric juice which the living stomach se- 

 cretes, often dissolves the stomach itself after death. * Now what 

 is the power which prevents the gastric juice from acting on the 

 stomach during life ? Certainly neither a chemical nor mecha- 

 nical agent, for these agents must still retain the same power af- 

 ter death. We must, then, of necessity conclude, that there ex- 

 ists in the animal an agent very different from chemical and me- 

 chanical powers, since it controls these powers according to its 

 pleasure. These powers, therefore, in the living body, are merely 

 the servants of this superior agent, which directs them so as to 

 accomplish always one particular end. This agent seems to re- 

 gulate the chemical powers, chiefly by bringing only certain sub- 

 stances together which are to be decomposed, and by keeping at 

 a distance those substances which would interfere with, or dimi- 

 nish, or spoil the product, or injure the organ ; and we see that 

 this separation is always attended to even when the substances 

 are apparently mixed together ; for the very same products are 

 not obtained, which would be obtained by mixing the same sub- 

 stances together out of the body, that are produced by mixing 

 them in the body ; consequently all the substances are not left 

 at full liberty to obey the laws of their mutual affinities. The 

 superior agent, how r ever, is not able to exercise an unlimited 

 authority over the chemical powers ; sometimes they are too strong 

 for it : some substances, accordingly, as madder, make their way 

 into the system ; while others, as arsenic, decompose and destroy 

 the organs of the body themselves, 



But it is not in digestion alone that this superior agent makes 

 the most wonderful display of its power ; it is in the last part of 

 assimilation that our admiration is most powerfully excited. How 

 comes it that the precise substances wanted are always carried to 

 every organ of the body ? How comes it that fibrin is always 

 regularly deposited in the muscles, and phosphate of lime in the 

 bones ? And, what is still more unaccountable, how comes it that 

 prodigious quantities of some one particular substance are formed 

 and carried to a particular place,in order to supply new wants which 

 did not before exist ? A bone, for example, becomes diseased 



Phil. Trans. 1 772, p. 447. 



