106. I shall conclude these remarks by stating that one of the objects of the urinary secretion appears to be the removal of a portion of nitrogen, and perhaps also of oxygen, from the blood, as that of respiration is the abstraction of carbon. This hypothesis is supported by the ultimate analysis of lithic acid and urea. It is, at least, evident, that some principle is withdrawn from the circulation, whose presence would act upon the system as a destructive poison; total suppression of urine is followed in a few hours by insensibility; but this consequence is prevented by the discharge of a few drops only of the secretion.

107. The function of the skin may be considered as the last link in the chain of the digestive process. It removes from the blood a considerable portion of water, with some saline matter; and in this respect, it may be compared to the office of the kidneys, although, unlike them, its function may be suspended without immediately fatal consequences. Experiments have also proved that an acid is discharged by the cutaneous emunctories; and if this be suppressed, it appears to pass off by the kidneys, and to give origin to a deposite to be hereafter noticed.

1 The vulgar have ever considered the urine as a glass, in which the physician may behold everything that passes within the body; and the quacks have, for obvious reasons, been willing to indulge the credulity. It has been stated, that the origin of the imposition of urine-casting is to be looked for in the ignorance and barbarism of the middle ages, when the greater part of physicians were ecclesiastics, who either saw their patients in their churches, or were satisfied with inspecting their urine.

108. From a review of all the transformations which the aliment undergoes, from its conversion into chyme to that into blood, can we arrange the different changes under distinct heads, so as to bring each under the operation of any general law? I fear that our present state of chemical knowledge will not afford a satisfactory solution of this problem. The production of chyle in the duodenum certainly differs in its nature from that of any of the secreted fluids, since the mass from which it is evolved is acted upon by foreign reagents, as the bile and pancreatic liquor; whereas a secreted fluid is at once separated from the blood without the aid of any precipitant. In this point of view, the operation bears a closer analogy to a chemical process; but here the comparison must end, for we are unable to account for the presence of any of the new compounds by an analysis of the ingredients from which they are produced. If we admit the operation of a galvanic power, generated by the nerves, and directed through their means to the decomposition of the alimentary materials, and to their recombination into other forms, we shall obtain, if not a satisfactory solution of the problem, at least some clue for its investigation.

We shall thus perceive, that elements having a strong chemical affinity for each other, may be separated and re-united in different proportions; and we shall be able to explain why substances, distinguished by their strong attractions for each other, do not combine in the alimentary organs, as they would in the vessels of our laboratory. But it will be asked, what reason there is for believing in the existence of any power analogous in its operation to electricity? This question cannot be better answered than by a relation of the ingenious experiments of Dr. Wilson Philip, who has shown that when a nerve is divided, so as entirely to intercept the transmission of its energy, its function may, to a certain degree, be supplied by an electrical stream from the galvanic battery. The eighth pair of nerves were divided in the necks of three recently-fed rabbits, and every precaution was taken to keep their divided ends asunder. One of these animals, when subjected to the galvanic influence, remained singularly quiet, breathing freely, and with no more apparent distress than the twitches usually produced by electric action, which was in this case kept up without interruption. The other rabbits laboured strongly in their respiration.

They were all three killed at the same period, and their stomachs successively opened. In the two non-galvanised animals chymification had scarcely made any progress; but in that which had been galvanised, the process appeared to have been completed.

109. It is not my intention, in this work, to enter 10 into any speculations with respect to the more minute changes which may be supposed to take place under this galvanic influence of the nerves; such details would be wholly inconsistent with the practical objects of my present publication. I shall therefore conclude this part of my subject by observing, that most of the digestive products are acid: the chyme is uniformly distinguished by this character; and, if the experiments of Dr. Prout be correct, muriatic acid is always present in the stomach: we may, therefore, suppose that the nerves of this organ have the power of decomposing the muriatic salts, and transferring its alkali to some distant reservoir, perhaps the liver. The intestinal juices are also acid; the faeces, unless they have undergone a degree of putrefactive decomposition, redden litmus; the urine, as well as the perspirable matter, are likewise acid; and it is scarcely necessary to observe, that the only product of the respiratory function is carbonic acid.

110. If the nerves act like galvanic conductors, may we not explain the transference of matter from one organ to another, without the necessity of supposing the existence of tubes or hollow vessels?

111. There still remains for our consideration a phenomenon, intimately connected with the digestive process and with the derangements to which it is liable, - the formation of fat. This substance may be regarded as the least animalized product of the animal body, since it does not contain any nitrogen, an element which, it will be remembered, is peculiarly characteristic of animalization; nor does it contain any considerable proportion of oxygen, but consists principally of bydrogen and carbon, the latter element being far less in its proportion than that which occurs in any of the constituents of the blood, from which fat is secreted. It is true that we have no certain data for enabling us to judge from what part of the blood the fat is immediately produced, nor does there appear to be any specific organ for its secretion; but, as its deposition is evidently connected with the state of the digestive functions, and bears a relation to the quantity of chyle produced, and as fat is deposited in so many parts of the body, or, at least, is connected with textures of such various descriptions, it is fair to conclude that the blood must be the source from which it proceeds, and the medium through which it is distributed; and, moreover, that the capillary vessels must constitute the organ for its secretion.

Dr. Bostock observes that there are two modes in which we may suppose the fat to be produced - while in the vessels themselves, or not until it is just upon the point of being excreted from them. In the first case the operation must consist in the abstraction, from the albumen or fibrin, of its nitrogen and oxygen, and a part of its carbon; in the other, of the hydrogen and a part of the carbon, while the remainder of the elements is left in the vessels.

112. Since an excess of nutritive matter received into the blood, while the various secretions and excretions remain unchanged in quantity, is the circumstance which favours the deposition of fat, we may conclude that, in such a case, nature has ordained a provision for the discharge of the superfluous matter, and that this is effected by various means, and by different organs. The principal part of the hydrogen appears to be disposed of in the formation of fat; the carbon is also carried off partly by these means, and partly by the lungs; while the nitrogen is, perhaps, principally removed by the kidneys. But it will be observed that the elements eliminated by these latter organs are at once excluded beyond the sphere of the system, whilst those removed from the circulation by the former are deposited as it were in a storehouse, to be again used whenever the waste of the body should call for supplementary nutriment, whence, in cases of extreme hunger, or of abstinence from food, fat is re-absorbed and carried into the blood-vessels.