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ON THE PROCESS OF ASSIMILATION AND NUTRITION; AND THE CURIOUS EFFECTS TO WHICH THEY LEAD.
We have traced out in our preceding studies 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 inquiry 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 numbers, they open on all animal surfaces, or hollows whatever, to their incipient sources.
The SECERNENTS, or that set of vessels whose office it is to separate particular parts from the blood for particular purposes, are evidently continuations of some of those very subtile ramifications of the arteries which,
on account of their fineness, are called capillary; and the ABSORBENTS, or that set of vessels whose office it is to imbibe or drink up the waste and exhausted materials, are as evidently distinct and attenuate tubes, progressively uniting, and ultimately emptying themselves into the venous system, the common trunk in which they concentre, and in which also concentre the lacteals of the alimentary canal, named the thoracic duct, being a tough membranous channel situate upon the interior part of the spine, of about the diameter of a crow-quill in man, and running in a serpentine direction through the diaphragm or midriff to an angle formed by an union of the jugular and subclavian veins, into which it opens, and where of course it terminates, leaving the waste and the new food, now intimately intermixed, to be still farther elaborated and refitted for use by those subsequent and specific operations of the heart and the lungs which we have already described.*
The simplest action, perhaps, that is evinced by the mouths of the secretory or secernent vessels, consists in separating and throwing forth a fine lymph from the surface of all membranes and organs whatever, for the purpose of lubricating them, as we grease the axle-tree of our carriage wheels; and thus preventing one membrane or organ from being injured by the friction of another. Of this every one who has been present on the cutting up of slaughtered oxen must have seen an abundant and striking instance, in the vapour that ascends from every part of the warm carcass, which vapour, when condensed by cold or any other cause, is found to be little more than the serum or watery part of the blood. And one of the simplest actions evinced by the mouths of the absorbent vessels, consists in their drinking up, as with a sponge, this attenuate or lymphatic fluid, when it has answered its purpose, so as to make room for a fresh and perpetual effusion; whence these vessels are often called LYMPHATIC, as well as absorbent, in consequence of their being so frequently found loaded with this fine and colourless material.
And here, perhaps, the first remark that must occur to every one is the necessity there seems to exist, that these correspondent systems of vessels should maintain the nicest harmony or balance in their respective functions; since, if the one operate either with a less or a larger power than the other, disease must inevitably follow; the nature of the malady being determined by the nature of the cause that produces it.
We have all of us heard, and most of us have seen instances of the disorder called dropsy; and many of us have surveyed it both in a local and a general form, as dropsy of the head, dropsy of the chest, dropsy of the abdomen, and dropsy of the cellular membrane or system at large. This disease may take place from two causes; as, for example, from a too great excitement of the secernent system, or a too little excitement of the absorbent. If, from a morbid irritability in the secernent vessels of any one of the cavities I have just adverted to, an undue proportion of
*This double action by a double set of vessels was little, if at all, known to the ancients, who referred the economy of both secretion and absorption to the powers of peculiar arteries and veins; and hence, the porosity of these vessels was a doctrine in common belief, till the time of Hewson, Hunter, and Cruickshank. M. Magendie and M, Flandrin, of Paris, have of late been very active in establishing a view of the subject in many respects not essentially different from that of the old school, and in teaching that the only general absorbents are the veins; that the lacteals absorb food, but nothing else; and that the lymphatics have no absorbent power whatever. Their experiments are plausible and striking, but by no means decisive enough to subvert the system explained above. The argument on both sides may be found in the author's Study of Medicine, Vol. v. p. 278. edit. 28. 1825.
lubricating lymph be secreted and steam forth, the natural tone and action of the correspondent absorbent vessels will not be sufficient to carry off the surplus; and hence that surplus will accumulate, and dropsy ensue, although the absorbent vessels of the part affected be in a state of usual health and vigour the disease depending altogether on the morbid and predominant excitement of the secernents.
But suppose the absorbent vessels of a particular cavity, in consequence of cold, exhaustion from great previous exercise, or any other cause, to be rendered torpid and inert, and consequently incapable of continuing their accustomed measure of action: in this case dropsy will also ensue, notwithstanding the corresponding secernent vessels are in a state of natural health, and no larger portion of lymph is secreted than a state of natural health demands; for the fluid will now accumulate, from the morbid torpitude of the absorbent system, and its inability to fulfil its function. It is hence, as every one must perceive, a point of the utmost consequence to determine the nature of the cause in dropsy, as, in truth, it is in every other disease, before we attempt a remedy; since an error upon this subject may be productive of the most serious, and indeed fatal consequences. For it is obvious that we may stimulate where we ought to diminish action, or we may diminish action where we ought to stimulate.
Occasionally, however, the action is equally increased in both sets of vessels; as, for example, in inflammation of the leg or arm; and in this case there is great heat and dryness, and at the same time considerable intumescence or swelling. For under this affection the mouths of the secernent vessels, being more distended than in a natural state, pour forth the coagulable lymph in a grosser and less attenuate form, and not unfrequently, perhaps, intermixed with some particles of red blood; while the mouths of the absorbents, though they as eagerly drink up the finer parts of what is thus rapidly strained off, are incapable of carrying away with equal ease those of a grosser texture; in consequence of which these last remain behind, and produce tumefaction by their accumulation.
At times, also, we meet with an equal degree of diminished instead of increased action in both these sets of vessels; as on exposure to cold and damp temperatures; in cases of spare and coarse diet; or of old age. And the result of this double decrease of energy is dryness, as in the former instance, but combined with leanness and corrugation of the organs that are thus affected. It is hence the bones of old people are more easily broken, and the skin is harsher and more wrinkled than in the middle of life; hence the shrivelled and squalid appearance of gipsies and beggars; and hence, in a considerable degree, the low and stinted stature of the Esquimaux, Laplanders, and Tongooses.
For all the usual purposes of health and organic nutrition, the common action and common degree of action evinced by these respondent systems of vessels are perfectly sufficient, though not more than sufficient. It may happen, however, that in consequence of severe violence from external injury or internal disease, a considerable portion of an organ, as a part of some of the muscles that belong to an arm or a leg, may be totally destroyed or killed, and consequently rendered incapable of performing its proper function. How is nature, or, which is the same thing, the remedial principle of life, to act in such circumstances? If the dead part remain, it is manifest that it must impede the living parts that surround it in the execution of their appropriate office; independently of which they
want the space which the dead part occupies, and the aid which it formerly contributed. It is obvious that two processes are here necessary: the dead part must be carried off, and its post must be filled up by a substitute of new matter possessing the precise properties of the old. And here we meet with a clear and striking instance of that wonderful instinctive power which pervades every portion of the vital systems, both of the animal and vegetable world, and which is perpetually prompting them to a repair of whatever evils they may encounter, by the most skilful and definite methods.
In order to comply with this double demand of carrying off the dead matter, and of providing a substitute of new, each of the systems before us commences, in the living substance that immediately surrounds that which requires removal, a new mode and a new degree of action. A boundary line is first instinctively drawn between the dead and useless, and the living and active parts; and the latter retract and separate themselves from the former, as though the two had been skilfully divided by a knife. This process being completed, the mouths of the surrounding absorbent vessels set to work with new and increased power, and drink up and carry off whatever the material may be of which the dead part consists, whether fat, muscle, ligament, cartilage, or bone; the whole is equally imbibed and taken away, and the hollow is produced, where the dead part existed. At the same time the mouths of the corresponding secernent vessels commence a similar increase and newness of action, and instead of the usual lymph, pour forth into the hollow a soft, bland, creamy, and inodorous fluid which is denominated pus; that progressively fills up the cavity, presses gradually against the superincumbent skin, in the gentlest manner possible distends and attenuates it, and at length bursts it open, and exposes the whole of the interior to the action of the gases of the atmosphere.
It was at one time conceived, and by writers of considerable eminence and judgment, and of as late a date as the time of Mr. Hewson, that the injured and dead parts were themselves dissolved and converted into pus; but this opinion has been disproved in the most satisfactory manner by the minute and accurate experiments of Mr. John Hunter, Sir Everard Home, and Mr. Cruickshank; and the process has been completely established as I have now related it.
In what immediate way the gases of the atmosphere operate so as to assist the secernent mouths of what is now the clean and exposed surface of a wound, in producing incarnation, or the formation of new matter of the very same kind and power as that which has been carried off, and enable them to fill up the cavity with such new matter, and perfect the cure, we do not exactly know. Various theories have been offered upon this very curious subject; but at present they are theories, and nothing more; and I shall not, therefore, detain you with a relation of them. Thus much, however, we do know, that the co-operation of the atmosphere with the action of the mouths of the secernent system engaged in the work of restoration is, in some way or other, peculiarly beneficial; and that, generally speaking, the wider the opening, and the freer the access of atmospheric air of a due temperature to the surface of the wound, or, which is the same thing, the freer it comes in contact with the mouths of the secernent vessels, the more rapidly and auspiciously the work of impletion and assimilation proceeds. Neither do we know, precisely, why pus, rather than any other kind of fluid, should in the first instance be
poured forth, for the purpose of filling up the hollow, and producing a rupture of the skin; but we know to a certainty that some such general process is in most cases absolutely necessary; we know that such a rupture must take place in the natural mode of cure; that the atmosphere must come into close contact with the mouths of the restorative secernents; that a milder or softer fluid could not possibly be secreted for such a purpose; and that the entire process exhibits proofs of most admirable skill and sagacity. It is at times possible for us to assist the process by the lancet, which accelerates the opening. Yet even in this case we do no more than assist it, and are only, as we ought ever to be in all similar cases, bumble coadjutors and imitators of nature, and admirers of that all-perfect and ever-present wisdom which we are so often called upon to witness, but are never capable of rivalling.
A process closely similar to this is perpetually unfolding in vegetable life. And it was merely by taking advantage of this process that Mr. Forsythe was able to make old, but well-rooted, stumps of fruit-trees throw forth, far more rapidly than he could saplings, a thrifty family of vigorous and well-bearing shoots: for the compost for which he was so celebrated does nothing more than merely increase the secernent and absorbent action of the vegetable frame by its stimulating property, and defend the wounded part to which it is applied from being injured by the inclemency of the weather.
From what has thus far been observed it appears obvious that all the different parts of the living body are assimilating organs, or, in other words, are capable of converting the common nutriment of the blood into their own respective natures, and for their own respective uses. And it has also appeared, that under particular circumstances every part is capable, moreover, of secreting a material different from that of its own nature, as, for example, the material of pus, whenever such a substance is necessary.
This view of the subject will lead us to understand with facility how it is possible for various organs of the system to maintain two distinct secretions at the same time: one of a matter similar to its own substance, and exclusively for its own use; and another of a matter distinct from its own substance, and in many instances subservient to the system in general. Of this last kind are the stomach, the liver, the respiratory organ, and the brain: each of which secretes, independently of the matter for its own nourishment, a matter absolutely necessary to the health and perfection of the general machine; as the gastric juice, the curious and wonderful properties of which I described on a former occasion; the oxygenous principle of the inspired air, and, as some suppose, those of light or caloric; the bile; and the nervous fluid, or material of sensation.
There are various other organs of a smaller kind, and simpler texture, which also perform the same double office, and secrete materials of a much more local use, or which are intended to be altogether thrown away from the system, as waste or noxious bodies. And to the one or the other of these classes belong the kidneys, the intestinal tube, the minute and very simple perspiratory follicles of the skin, the delicate organs that separate the saliva and mucus that serve to lubricate the mouth and nostrils, and those that elaborate the tears, the wax of the inner ear, and the fat. The organs, of whatever size or texture, that perform this double function, are called secretory glands; and they are distinguished into different sets, either from their peculiar office or peculiar structure; as salivary,