VI. We may next enquire, what is the most probable original source of the matters which are thrown out of the body in the way of ex cretion. As it is generally believed, and on strong grounds, that the solid textures, as well as prepared fluids of the body, are liable to continual decay and renovation, it has long been the general belief, that the materials for the ex cretions are supplied chiefly from those sub stances which have formed part of the textures, and, after fulfilling their office there, have been taken back into the circulation with a view to their discharge from the body. And it has been conjectured, certainly with much probabi lity, by Berzelius and by Autenrieth, that the animal matters thus mixed with the blood on their way to the exeretories, are distinguishable from the albuminous or nutritious parts of the blood, by their solubility both in hot and cold water, and constitute the animal matter of the serosity, or uncoagulable animal matter of the blood. This is supported by the observation, that, when the kidneys are extirpated, this part of the blood is first observed to increase in amount, and afterwards it is here that the urea is detected.* And the connexion of the excretions with absorption from all parts of the body seems farther illustrated by the pheno mena of diabetes, which may be held to be the disease in which there is the strongest evidence of increased absorption in all parts of the body, from the rapid digestion, the rapid recurrence of thirst after drinking, the dryness of the sur face, and the progressive emaciation notwith standing the excessive amount of ingesta ; and in which the quantity of the urine is often ten times, and the solid contents of the urine often twenty times, the average quantity in health.f But it should not be too hastily concluded, that all the solid constituents of the animal body are liable to continual absorption and renova tion. The permanence of coloured marks on the skin, noticed by Magendie, is sufficient evidence, that, in some of the textures, any such change must go on very slowly ; and some of the best observers doubt whether any such pro cess of alternate deposition and absorption takes place in vegetables, in which, nevertheless, as we have seen, excretion is a necessary process.
Dr. Prout has lately stated strong reasons fur thinking, that great part of the contents of the lymphatic vessels are not excrementitious, but destined for useful purposes in the animal eco nomy; remarking particularly on the way in which hybernating animals appear to be nou rished by absorption of their own fat.* And it is obviously possible, that the excre tions may be required to purify the blood of matters taken in from without, or evolved in the course uf the circulation and its abundant changes, as well as to purify it of what has been absorbed from the system itself Now that we know, that great part of the ingesta into the stomach are taken up by the veins, and pass through the liver on their way to the heart ; and, likewise, that the venous blood is the chief source of the excretions of bile, it seems pro bable, that one important use of this excretion is, to subject a part of the ingesta to a second filtration, or rejection of part of their ingre dients, subsidiary to that which they undergo in the primre vim. This may also be probably one principal reason why the great mass of the chyle, and other products of absorption in the body, should be mixed with the blood just before its concentration at the heart, and subsequent dif fusion through the lungs; and thus participate in a purification, by the rejection of water and carbonic acid, before they are applied to the purposes of nutrition. We know, that in birds,
reptiles, and fishes, there is a venous circulation similar to that of the vena ports:, through the substance of the kidneys, of most of the blood coming from the lower half of the body ; a part of the ingredients of that blood will, there tore, be evolved with the urine; and, in the case of the reptiles, it has been lately ascertained, that this venous blood receives, before entering the kidneys, the contents of numerous and large lymphatics.} At all events, if we are right in supposing, that, in the higher animals, all the great chemi cal changes which are wrought on the blood, even the formation of the excretions, are effected during its circulation in the bloodvessels them selves, we can thereby acquire a general notion of the intention of several contrivances, the use of which is otherwise very obscure. We can understand, that the object of the concentration of the blood at the heart may be not merely mechanical, but, partly, also chemical ; and we can see the intention of the heart being so ad mirably adapted, by the articulated structure of its internal surfaces, not only to receive and propel, but also most effectually to intermix, all the component particles of the blood, both be fore and after its exposure to the air ; the most perfect illustration of which power of the heart is afforded by the effect it produces on any com pressible and elastic fluid which is received in a mass of any considerable volume into its cavi ties, and which is necessarily subdivided into so many minute globules, and compressed in so many directions, that it cannot escape from the heart, and so stops the circulation.
Again, when we attend to the manner in which substances foreign to the circulation are absorbed into it, whether from the system itself, or from without, we see a great deal of contri vance, evidently adapted, and probably intended, to secure the most gradual introduction, and the most perfect intermixture possible, and to allow the escape of certain parts of the compound fluid formed. Thus of the contents of the prima; vim, part are absorbed into the veins, and sent through the capillaries of the liver and those of the lungs, (both admitting of excretion,) before they are admitted into the arteries. What is taken up by the lacteals has already undergone much elaboration by living fluids ; this portion passes through the mesenteric glands, and is, probably, so far intermixed with the blood there, and partly received into the veins passing from them to the liver;' and the rest is mixed with much matter flowing from other parts of the system by the lymphatics; and, according to the views of Dr. Proud- as to the nature of absorption, is so far assimilated by this mixture also, before it is poured into the great veins in the state of chyle, to undergo the thorough agitation at both sides of the heart, and to participate in the changes at the lungs. What is absorbed from other parts of the body seems to be partly taken up by the veins, partly also by lymphatics which immediately convey it into adjacent veins; the remainder passes through lymphatic glands, and is there pretty certainly subjected to an intermixture and an interchange of particles with blood; after which it has necessarily much further admix ture, and two thorough agitations at the heart, as well as the exposure at the lungs, to undergo, before arriving at the left side of the heart.