cases of a similar kind, referred the whole to the extreme difficulty with which external air had obtained any communication with the inhumed bodies, in consequence of the close adaptation of coffin to coffin, and the compactness with which every pit had been filled up. Difficult, however, as this communication must have been, he conceived that, from the natural elasticity of atmospheric air, some small portion of it had still entered, conveying, perhaps, just oxygene enough to excite the new action of decomposition. This having commenced, the constituent oxygene of the dead animal organs would itself be progressively disengaged, and rapaciously laid hold of by all the other constituent principles, from their strong and general affinity to it. During this gradual evolution, there can be little doubt that the greater part of it would be seized by the predominant azote, a very considerable part by the carbone, and the rest by the hydrogene; and the result would be, upon the total but very slow escape of the constituent and disengaged oxygene, that the whole or nearly the whole of the azote, a considerable portion of the carbone, and a certain quantity of the hydrogene, would escape also-leaving behind the remainder of the carbone and the hydrogene, now incapable of escape from the want of oxygene to give wings to their flight, together with the residual earth of the animal machine.

But hydrogen and carbone, though in this case incapable of sublimation for want of oxygene, would still, from their mutual attraction and juxta-position, enter into a new union and produce a new result, and this result must necessarily be fat; for fat is nothing else than a combination, in given proportions,

of carbone and hydrogene. And hence, whatever the respective animal organs of the bodies deposited in these burial caverns may have antecedently consisted of, whether muscle, ligament, tendon, skin, or cellular substance, when thus deprived of their oxygene and azote, the whole must of necessity be converted into fat. Pure and genuine fat it would have been, provided there had been nothing left behind but mere carbone and hydrogene, and in their respective proportions for the formation of fat; but as we can scarcely conceive such proportions could take place, or that every corpuscle of the azote could be carried off before the total escape of the oxygene, many parts of it must necessarily have assumed a flaky, soapy, or waxy appearance, from the union of the azote left behind with some portion of the hydrogene, and the consequent production of ammonia or volatile alkali; since, by an intermixture of alkali with fat, every one knows that soap, or a saponaceous substance, is uniformly produced.

in

But, excepting in situations of this kind, in reality,

every situation in which dead animal matter, destitute of its living principle, is exposed to the usual auxiliaries of putrefaction, putrefaction will necessarily ensue, and the balance will be fairly maintained : — the common elements of vital organization will be set at liberty to commence a new career, and the animal world will restore to the vegetable the whole which it has antecedently derived from it.

In this manner is it, then, that nature, or rather that the GOD of nature, is for ever unfolding that simple but beautiful round of action, that circle of eternal motion, in which every link maintains its relative importance, and the happiness of every part

flows from the harmony of the whole. Can we, then, do better than conclude with the correct and spirited apostrophe of one of our most celebrated poets?

Look round the world! behold the chain of love
Combining all below and all above.

See plastic nature working to this end;

All served, all serving, nothing stands alone,

The chain holds on, and where it ends unknown.

* This line is altered to answer the present purpose in a better manner.

340

LECTURE XIV.

ON THE PROCESSES OF

ASSIMILATION

AND NUTRITION; AND THE CURIOUS EFFECTS TO WHICH THEY LEAD.

We have traced out in the preceding lectures something of the means by which form, and magnitude, and motion, are produced in the inorganized world: -how the various substances that surround us combine and separate, vanish from us and re-appear, and, in the multifarious processes they undergo, give rise to new products by new and perpetually shifting involutions. We have further traced an outline of the means by which organized matter is capable of building up the curious structures of plants and animals; how the chief functions they possess are carried on, and by what means they respectively acquire maturity and perfection.

But it is not only necessary that the system should in this manner be matured and perfected by a fresh application of materials, but that the old materials which constitute every organ should be progressively removed from the system, in consequence of their being worn out by use, and their place supplied from definite stores. Let us, then, devote the present hour to an enquiry how this latter change occurs in vascular and living matter, in the vegetable and animal system: by what means the dead or exhausted and worn-out elements of the different organs are carried off, and replaced by new reformative

materials, and what are the principal phænomena that result from such a series of operations.

The blood, then, in animals, and the sap, which may be regarded as a species of blood, in plants, of both which we have already treated, are the vital currents from which every organ of the individual frame derives the nourishment it stands in need of, and into which it pours ultimately a considerable portion of its waste and eliminated fragments; for the provident frugality of nature suffers nothing to be lost, and, as far as possible, works up the old materials, time after time, into fresh food for the subsistence of the entire system.

To produce this double purpose two distinct sets of vessels are necessary: one for that of separating from the common mass of the blood, and re-combining into new associations, those particular parts of it which the formation of the fresh matter demands; and the other for that of carrying back the rejected materials into the general current. And hence these two sets of vessels bear the same relation to each other as the veins and arteries of the animal frame accompany every part of the frame to its farthest extremities, and, indeed, constitute the general mass of the frame itself. From the respective offices they perform, they are denominated SECERNENT and ABSORBENT systems: in their utmost ramifications they are too minute to be traced by the keenest eye, or the nicest experiment of the anatomist; but, where they are not quite so minute, they are sufficiently discoverable, and their course is sufficiently capable of being followed up, from the delicate apertures or mouths by which, in infinite