As organized living beings, the soundest philosophy and best ordered experiments lead us to infer that there is little if anything me chanical in the mode in which either vegetables or animals absorb nutriment. The absorption of their aliment by vegetables is influenced by the seasons, their state of health or disease, their age, and external circumstances gene rally,—the temperature, state of dryness or moisture, &c. of the air with which they are surrounded ; the cause of the absorption of their food by vegetables is, therefore, some thing different from what is called capillary attraction, or the law by which fluids ascend in tubes of small calibre.
The proper passage of the nutriment into the bodies of animals occurs from their interiors, and in a very large proportion (probably in every somewhat perfect member) of the class, by means of a special set of vessels denomi nated lacteals or lymphatics, no system cor responding to which exists among vegetables.
The very lowest tribes of the animal king dom, the entozoa, acalephw, polypi, &c. having no proper vessels of any kind, the cellular membrane of which they consist absorbs, and by virtue of a peculiar vital process, distributes the nutritive juices extracted from the matters received into the stomach and alimentary canal to all parts of their bodies. Those tribes of animals which have naked skins have the faculty of absorbing by their exterior also.
Still less than in vegetables, can we suppose that the process by which in animals nutriment is ultimately absorbed into the body, whether from the exterior or the interior, is akin to mechanical or capillary attraction. The tissues of which animal bodies consist are, indeed, permeable to fluids, but this does not explain the collection of these fluids in so many tribes into particular canals, and still less does it solve the problem of the continued motion onwards in determinate directions within these channels.
Absorption of alimentary and other matters, therefore, in both of the grand divisions of the organized world, must be held as a vital act,— as one of the particular laws superadded in organized beings to the general system of phy sicochemical ordinances that rule the universe and its parts. This quality is common to vegetables and animals.
By far the greater number of animals have one or more special openings,— a mouth or mouths, by which they take in such sub stances as are fitted for their nourishment. Even the greater number of animals as low in the scale as the infusoria, have been recently demonstrated (by Ehrenberg) to be provided with an opening of this kind. Several, how ever, seem to receive aliment by the way of absorption alone. The mouth is a cavity of
extremely varied character and construction adapted universally to the circumstances in which animals exist. Nothing analogous to a mouth is met with in any vegetable.
The food having been selected and seized is next transferred to the cavity in which it un dergoes an elaboration that fits it to be received into the proper system of the animal and con verted into its own substance. We do not find anything like the pouch denominated a stomach in any member of the vegetable king dom. The matter fitted for its nourishment, absorbed by the root, is transmitted to the stem, and from thence makes its way into the leaves of the vegetable. It does not pass un changed, however, from the earth into the root, or at least it has advanced but a very short way on its course to the leaves, before it is found to have undergone certain changes, which are also known to be greater in amount as it is examined at a greater height or distance from the root. Although growing from the same soil too, the sap of vegetables, i. e. the fluid which is passing upwards through the woody fibres, is found to be universally different. Whether the peculiar qualities thus acquired by the simple moisture holding certain salts, &c. in solution, which is the food all vege tables derive through their roots, be the effect of vital elaboration within the cells of the woody fibre, or result from an admixture of the cambium or fluid which has already undergone assimilation in the leaves, is still uncertain. We are inclined to believe that a process ana logous to digestion does actually take place within the woody conduits of the sap of vege tables ;—why should it not, or why should any new properties acquired by matters sub jected to the influence of the peculiar laws of vitality be held as resulting from mere ad mixture ? The very same thing, in fact, happens among the lowest tribes of animals which takes place in all vegetables : the substances fitted for their nourishment penetrate or are absorbed into their systems, and are there assimilated without the intermedium of any special apparatus. We mount but a very short way in the scale of the animal creation, however, before we meet with a peculiar pouch, destined for the reception of the aliment, and accomplishment of the first steps in the processes by which, in the more perfect animals, it is finally assimi lated. This pouch is the stomach, and with the rest of the digestive apparatus with which it is connected, is in intimate and uniform re lationship with the kind of food upon which animals are led by their instincts to live.