one hand, and of the blood which is formed out of it on the other. But with all this insight into the matter, and knowledge of the intimate principles, we cannot from food make chyle, nor can we from the chyle make blood; nor do we find any process similar to it out of the living body. One man called it mechanical; for he supposed the stomach and duodenum rubbed their sides together, and ground down their contents as we would in a mill, or as we knead bread between the hands: others said that this was a fermentation, a concoction, a mere chemical decomposition and combination. Yet, after all their investigations and theories, they have not demonstrated that it is like any operation, chemical or mechanical, out of the living system. No; it is an operation peculiar to life; and some, and we think with the best possible reason, have called it a vital operation;—not pretending by this that they have explained the matter, but only to set at rest all speculation; for all admit that the specific qualities of vitality are altogether unknown, and probably must be forever beyond the vision of mortality. Whatever may be the cause, we find that soon after the chyme is received into the duodenum, it is separated into two portions; the nutritious, or chyle, and the innutritious, which is to be thrown off as useless.

This chyle is nearly the same in all animals; and the chemical analysis shows but little difference between that which is formed out of animal food and that which is made of vegetable food in the same animal; and even this small difference is almost entirely lost when the chyle goes through another operation, and is transformed to blood.

This finishes the third stage of digestion; and so far the animal exhibits a series of organs, at least, though not of operations, which find no counterpart in the vegetable: for the tree has no mouth like ours, nor gullet, nor stomach, nor alimentary canal, as the quadruped has, but it has a set of absorbents, like the lacteals of the animal, which we shall next explain.

LACTEALS.

The inner coat of the intestines is filled with innumerable absorbent mouths, which are situated in the villous

hair-like projections, which cover this lining like the nap of velvet cloth: each one of these hairs has a mouth, and a tube leading from this mouth into other and larger tubes behind. These tubes are at first inconceivably small; but they unite together, and form larger and larger trunks, like the roots of trees, and at last all unite into one, called the thoracic duct, which begins in the abdomen, passes up along the back bone to the upper part of the chest, and opens into the subclavian vein, just before this reaches the heart. As these lacteals are spread over all the inner surface of the intestinal canal, and as this surface is equal nearly to the skin of the whole body, every portion of our aliment must come in contact with some of them, and its chyle be absorbed by them. We cannot particularly explain their action: many have tried, and thought they succeeded: many have guessed at it, but still, in all their experiments and theories which they built upon them, 'they know but little, presume a good deal, and jump at their conclusion,' and would have us follow after them.*

It is sufficiently certain that these lacteals select only the nutritious chyle, while the rest passes onward in the canal. Rabbits and other animals have been killed soon after eating, while the work of digestion was in busy operation, and then these little mouths were found full and swollen, and the surface of the intestines covered with knobs. These contract and force their contents into the tubes behind them, and these carry the chyle onward through the successive small and large branches to the great thoracic duct, which delivers the chyle into the blood vessels near the heart.

While the food is digesting and the chyle taken up by

* Boerhaave supposed the peristaltic motion of the intestines aided the absorption of the chyle. Dumas thought each lacteal possessed an elective sensibility. Young ascribes this power to electricity. Richerand calls it all the effect of sensibility. Bell speaks of the appetency of the capillaries, and the discriminative power of the small vessels. Most physiologists of modern times admit of a certain species of elective attraction. Bichat conceives there is a vital action in which there is a relation between the vessel and the fluid of an elective nature. Tiedemann and Gmelin maintain that the absorbents and blood vessels possess a vital power, by which they propel their contents, but which is different from irritability. This seems the nearest to the truth, or at least to contain the least error; for it assumes to know nothing of the matter, but leaves it, as they found it, and as it really is, in the dark; for all the theories only load the subject with obscurities, which are neither clear of themselves, nor do they give any light on these secret operations of nature in the living system.

the lacteals, the innutritious portion is carried forward in the intestinal canal. This has a muscular coat, the fibres of which pass in belts around it. These fibres successively contract and diminish the calibre of the tube. While this happens, the next band relaxes to admit the contents, which are forced out of the portion next above. This alternate and successive contraction and enlargement of the tube carries the waste of the food through its whole course. It is probably by similar means that the sap is carried onward in the tree.

This constitutes the whole process of digestion; and we have now the chyle in the heart, mixed with the blood, but not yet forming a part of it; for, in order to be perfect blood, ready for the support of the body, it must be submitted to the influence of the air in the lungs. But previous to entering on the explanation of this, which involves the whole description of respiration, let us see what we can discover in the vegetable analogous to the digestive process, which we have just described in the animal.

VEGETABLE DIGESTION.

The roots of the tree are covered with an infinite number of very minute fibres, which project from every part of the surface, like the lacteals in the alimentary canal: and like those lacteals within the animal body, these little roots without the vegetable body are absorbents, having each a mouth capable of living action, and a tube leading from this mouth into the larger root, and communicating with other and larger tubes, which extend from the root through the trunk and the branches to the leaf. This is the vegetable digestive tube, very simple, compared with the more complicated one in the animals, yet performing operations on its contents very similar to those heretofore described in the animal.

Dr.

We perceive here a broad difference between the first part of the digestive apparatus of these two races. Waterhouse says that he knows no other distinguishing mark, which divides the animal from the vegetable, than that the one has a stomach for receiving and digesting food, and

And another physiologist tells us

*

the other has none.' that the animal may be considered as a being, which lives by internal absorption of food which has already been eaten and digested, while the tree lives by external absorption of the nutriment which is accidentally about its roots. The animal selects its own food by a voluntary effort, and prepares it for digestion; then, by the absorbents, it is taken up here the anatomical analogy commences. The food of the tree is decayed vegetable and animal matter mixed in the soil and dissolved in the waters of the earth.* This absorption of food through the water is constantly going on. Animals take their food at various periods, and when the appetite is satisfied, they eat no more till this again calls for another supply. But the absorption through the lacteals, in the bowels, is almost continually going on, like that of the absorption from the earth by the roots. So we see, however wide the difference may be, so far as the mouth and stomach are concerned, yet a very great similarity commences immediately after the food has passed these. Both act by the almost infinitely small absorbents, which are not under the control of the will, and almost never cease their work.

If the lacteals of the perfect animal opened on its external surface, as is the case with some of the imperfect kinds, its operation would be like the tree. Or, if the tree had a sack about its roots, into which it placed, of its own accord, the food in some way prepared for the action of the little absorbing mouths of the fibrils on the roots, we should have a perfect analogy; but as neither of these conditions happen, we find the want of the stomach in the one case, and the presence of it in the other, affords a tolerable ground of distinction. Yet this organ seems not to be necessary in the tree, in order to accomplish what is performed by it in the animal.

If we could examine the sap at the moment it was absorbed from the earth, we should, without doubt, find it the same simple decayed vegetable substance dissolved in water, as is found in the earth. If we tap a root at a little

*Thomson's Chemistry.

distance from these absorbent mouths, we find the same sap has changed a little, and presents some of the characters of the tree. If, again, we draw it from the trunk, we find that it is still farther altered, and assumes still more of the peculiar properties of the tree: and the higher we open the tree, the more distinctly do we find the change produced, showing a gradual alteration in all its progress from the root to the leaf; from the nearly tasteless water, with scarcely any sensible properties, and similar in all vegetables, to the strong and pungent sap, with properties differing in almost every tree of the earth. How is this change brought about? By what means is this water, with few particles of vegetable matter dissolved in it, made into sap of such different sensible properties? We know in what organs it is effected; for it is done while the sap is on its way from the root to the leaf, and consequently it must be by the action of these sap-vessels themselves. If it be allowed that the change of the food into chyle, which renders crude and dead matter fit to enter into and support the living system, is done by means of the stomach and alimentary canal, in which the process is carried on, we may as confidently say that the change of the simple aliment of the plant into the living nutritive sap is effected by the agency of the sap-vessels in the root, trunk and branches, out of which it has not passed. In the animal this is called digestion, and it will not be denied that the corresponding process producing corresponding results should be called by the same name, although effected in organs anatomically different: and we must admit an analogy of function of digestion, however much the digestive organs may differ in structure.

The glands of the mouth secrete the saliva, those of the stomach secrete gastric juice, and the liver throws out bile; all these secretions enter into and aid the process of digestion. So we believe that glands in the tree afford peculiar substances which enter the new sap, and aid its transformation from its simple to its more complicated na

ture.

Vegetable digestion, like that of the animal, produces changes on the nutrient matter entirely different