Absorption. See the different kinds under Absorbentia vasa.

Thgugh we are convinced by the most undoubted evidence that fluids are absorbed, we have little knowledge of the power by which this is effected. The absorption of capillary tubes is the only analogous fact, which very imperfectly assists us in the explanation. The power, however, by whatever means it is exercised, is very general: it takes place in every part of the body, and is not influenced by the weight of the atmosphere or any other evident cause: it takes place under the earth, for plants absorb their nutriment in the same way. The foetus in utero is nourished by absorption, nor is it clear that the blood from the arteries is not conveyed to the veins by a similar process.

If the power of attraction of the sides of a vessel of a minute diameter is greater than the attraction between its particles, the latter will arrange themselves round its internal surface, leaving a space in the middle which the subjacent fluids will fill. In other words, the fluid will rise in the vessel. Yet the extent of this power is limited: if in the absorbent it extends beyond the first valve, the problem is solved; for every pressure will urge the fluid onward since the valve impedes its return. In this view we dare not deny that the red veins sometimes absorb, since it is probable that they do not form continuous vessels with the arteries; that their extremities are open in cavities; that these are minute, and abundantly provided with valves. This idea is, however, on the whole, improbable; since, as the absorbent vessels are so generally diffused in the whole body, and probably through the whole of animated nature, it would appear that no other organ is designed to supply their place.

If the opinion of capillar}- attraction, as just now explained, be well founded, it must follow that liquids only can be absorbed: yet we find solid bones taken up and conveyed into the circulating mass. It is indeed probable that these are dissolved; but it will be obvious, that, if reduced to such a minuteness as to be suspended in a fluid; in other words, if their surfaces be increased in a ratio more than equivalent to their densities, the effects will not greatly differ. We know that in this way flint passes through the minute vessels of some plants, particularly some of the arun-dinaceae, and is deposited on the epidermis, or in the cavities.