5. Again, it cannot be deemed improbable that the changes which the crude alinient un dergoes, from the time of its first reception into the absorbents and blood-vessels, to that of its conversion into organised tissues and into the materials of secretions eliminated for some special purpose in the economy, involve the liberation of many products, of which the elements are superfluous, and therefore injuri ous to the system if retained within it. The condition of organic chemistry, however, is not at present such as to admit of anything being advanced with certainty under this head.
From these various sources, then, a large aniount of effete matter is being continually received back from the tissues into the current of the circulation, or is generated in the blood by the changes to which it is itself subject ; and it is one great object of the secreting ap 'paratus, to fi.ee that fluid of the products which would rapidly accumulate in it, but for the provision which is thus made for their re moval.
The first product of the decay of all or ganised structures is carbonic acid ; and this is the one which is most constantly and rapidly -accumulatiw, in the system, and the retention of which, trierefore, within the body, is the most injurious. Accordingly, we find two organs, the lungs and the skin, specially des tined to remove it ; and their action is so contrived, that whilst eliminating a noxious product, they shall be subservient to the in troduction into the system of the vivifying element, oxygen, without a continued supply of which the animal functions cannot be long kept in activity, nor the heat of the body sus tained.
The skin, again, is one of the organs for the removal of the superfluous water from the body ; and the exhalation from its surface, the amount of' which varies with the degree of external heat, serves the additional purpose of keeping down the temperature to its nor mal standard. The lungs also regularly throw off a considerable quantity of water, of which the amount is but little subject to vari ation, and of which some portion may have been actually generated in the blood by the union of hydrogen and oxygen. And the kidneys, the structure of which is beautifully adapted to eliminate the superfluous fluid by simple mechanical transudation, draw off the residue ; the amount of water which they re move being the complement of that exhaled by the skin.
All azotised substances have a tendency, during their decomposition, to throw off ni trogen ; and in the animal body this element is for the most part eliminated by the kidneys, entering largely into the composition of the urinary secretion. Thus we find urea to con tain a larger proportion of nitrogen than exists in any other organic compound ; and uric acid, hippuric acid, kreatine and kreatinine, and other compounds, which are charac teristic elements of this secretion in different animals, are all rich in nitrogen. But it is not only by the kidneys that azotised substances are thrown off, for the solid matter exuded from the skin closely corresponds in composi tion with that of the urinary secretion ; and urea has been detected in it.
The biliary secretion is peculiarly rich in hydrocarbon, and may probably be regarded as the complement of that of the kidneys ; it having been shown by that if the em pirical formulie for the bile and urine be added together, the result comes very near to the empirical formula for blood. Of this secre tion, a part is certainly destined to be imme diately carried off through the intestinal canal ; but another part seems to be, re-absorbed, in combination with the fatty matters of the food, and to be subsequently thrown offby the respiratory process. What proportion is ap plied to each purpose cannot be definitely stated, and probably varies much with circum stances.
But besides the metamorphos's of the or ganised tissues, and of the organic elements of the blood, into the definite (generally crys talline) compounds, which are the character istic elements of the secretions already men tioned, it would seem that a portion of the effete matters take on a putrescent state ; and that for the elimination of these a special and most appropriate apparatus is provided, namely', the extensive system of glandulm in the wall of the intestinal canal. As this point
has been much less attended to than its im portance deserves, it seems desirable to dwell upon it here in some detail. It has been too much the custom to regard the fiecal evacua tions as little else than the indigestible re sidue of the food, mingled with portions of the biliary and pancreatic secretions ; where as vve think that a little consideration will show, that the peeidiarly focal matter is a real excretion, which must have been eliminated from the blood by the intestinal glandulm. The undigested residue of the food may form a greater or a smaller proportion of the bulk of the evacuation, according to the nature of the ingesta and the completeness of the digestive pro3ess. When the alimentary canal is in an irritable state, and the aliment is hurried through it without time being allowed for the proper action of the gastric secretions, a con siderable part of it may be recovered from the fmces in an almost unchanged condition. It has been found that even starch vesicles, if not ruptured by the masticating process, or by the heat employed in the preparation of the food, resist the digestive process so com pletely as not to give up their contents ; being readily detectible in the fmces, in an en. tire state, by the assistance of the microscope. Further, there is no evidence whatever, that the undigested residue of the food could ac quire the fmcal character, during thg short period which suffices in the state of health for its transmission along the alimentary canal ; and there is every reason to believe the contrary, since the substances which re sist the action of the gastric solvent are pre cisely those which have the least tendency to this kind of decomposition. Moreover, in purely Carnivorous animals, and in Man when he adopts, the same diet, fmeal matter is still voided, though in smaller quantity than in Herbivora. The case is still stronger in re gard to sucking animals ; since the milk by which they are supported is pure nutriment, of which no part can be supposed to pass di rectly into the fmces. The continued evacu ation of fmcal matter when little or no food has been taken in, the large quantity brought off' by purgative medicines after the bowels have been completely emptied of their solid contents, and the colliquative diarrhcea which so frequently occurs at the close of exhaust ing diseases and previously to death by starvation, are so many obvious confirmations of the same view. And Dr. Williams* has pointed out many pathological phenomena, which indicate that the inflammation and ul ceration of the intestinal glandulm, which is so frequent a complication of fevers and of other diseases induced by the presence of a morbid poison in the blood, results from the continued operation,upon their own structure, of the noxious matter which these glandulm are endeavouring to eliminate from the system. This view has derived important confirma tion from experirnents recently made by Prof. Liebig ; these having indicated that the sub stances to which the fxces owe their peculiar fcetor may be artificially produced by the im perfect oxidation of albuminous compounds.* The immense relief frequently given by an attack of diarrhcea, which spontaneously eli minates morbific rnatters that were operating prejudicially on the system, and the corre sponding effects of mild purgatives, which excite the secreting action of these glandulx, furnish additional evidence, if such be re quired, to the same eiTect. It is obviously important in a therapeutic point of view, that definite ideas should be entertained on this subject ; and although it may be difficult to obtain positive proof of the position here ad vanced, — that it is the special function of the glandulm of the lower part of the small intes tines, and at the upper part of the large, to eliminate from the blood the putrescent matter which results from the disintegration of the tissues,—it will scarely be denied that a strong probability has been established by the foregoing evidence, in favour of such a view.