ASSIMILATION, in animal economy, is that process, by which the different in gredients of the blood are made parts of the various organs of the body. Over the nature of assimilation, says Dr. Thom son, the thickest darkness hangS ; there is no key to explain it, nothing to lead us to the knowledge of the instruments employed. Facts, however, put the ex istence of the process beyond the reach of doubt. The healing of every bone, and of every wound of the body, is a proof of its existence, and an instance of its action. Every organ employed in assimilation has a peculiar office, and it always performs this office whenever it has materials to act upon, even when the performance of it is contrary to the in terest of the animal. Thus the stomach always converts the food into chyme, even when the food is of such a nature that the process of digestion is retarded, rather than promoted, by the change. If warm milk be taken into the stomach, it is decomposed by that organ, and con verted into chyme, yet the milk was more nearly assimilated to the animal be fore the action of the stomach than after it. The same thing occurs when we eat animal food. If a substance be in troduced into an organ employed in as similation, that has already undergone the change which that organ is to produce, it is not acted upon by that or gan, but passes on unalterated to the next assimilating organ. Thus it is the office of the intestines to convert chyme into chyle ; and whenever chyme is introduced into the intestines, they perform their office, and produce the usual change ; but if chyle itself be introduced, it is ab sorbed by the lacteals without alteration. Again, the business of the blood-vessels, as assimilating organs, is to convert chyle into blood; chyle therefore cannot be in troduced into the arteries without under going that change ; but blood may be in troduced from another animal without any injury, and consequently without un dergoing any change. Though the dif
ferent assimilating organs have the power of changing certain substances into others, and of throwing out the useless ingredients, yet this power is not abso lute, even when the substances on which they act are proper for undergoing the change which the organs produce. The stomach converts food into chyme, and the intestines change chyme into chyle ; and the substances that have not been converted into chyle, are thrown out of the body. If there should 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 intestines, 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 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 the substance has been completely assimilated, and made a part of the body of the animal. Thus there is an affinity between the co louring matter of madder and phosphate of lime ; and when madder is taken into the stomach, it combines with the phos phate of lime of the food, passes with it through the lacteals and blood-vessels, and is deposited with it in the bones. In the same way musk, indigo, &c. when taken into the stomach, make their way into many of the secretions. These facts prove that assimilation is a chemical pro cess; that all the changes are produced according to the laws of chemistry ; and Dr. Thompson adds, that we can derange the regularity of the process, by intro clueing substances whose mutual affinities are too strong for the organs to overcome.
See PHYSIOLOGY.