The moment chyle is formed digestion proper may be considered as accomplished, though on this function also depends the absorption of the chyle, which must still be perfected in its course through the smaller intestine and the veins before it mingles with the blood.
The mechanism of absorption is still unknown. It has been explained as taking place by endosmosis—a phenomenon discovered by Dutrochet, which results from the property which tissues possess, under certain conditions, of permitting fluid or gaseous bodies to pass through their capillary canals. If, for example, two fluids which may be mixed, though they may be of different natures and different densities, are separated by a membrane, two currents are established through this membrane in opposite directions, and of unequal force, tending to mix the two fluids, the stronger current is generally produced by the fluid the least dense; and this is called endosmosis—the feebler current exosmosis. In this experiment the substances mingle without changing their nature; but it is not so in absorption. The substances absorbed by the organic tissues change incessantly during their progress, borrowing or lending elements to each one of the molecules through which they pass. And farther, the different tissues absorb more or less of the same substance by virtue of properties which are unknown. In this way a poison which remains inert on the mucous membrane of the stomach is rapidly absorbed by the lungs.