V. One principle may already be laid down, almost with certainty, as to the exercise of these powers in the present instance, viz. that the peculiar matters characterizing the excretions are not actually formed from the blood at the parts where they appear, but only separated from the blood at these parts,—their formation, if not actually completed, having been at least considerably advanced, in the blood itself which reaches these parts. Of this we are well assured, chiefly by the following facts.
1. The experiments already mentioned, first made by Prevost and Dumas, have proved that within a short time after the extirpation of the kidneys in animals, urea may be detected in the blood, showing clearly that the existence of these glands is not necessary to the forma tion of this very peculiar excrementitious matter, and giving us reason to conjecture that the office of the kidneys is, not to form the urea, but to attract it out of the blood as fast as it is formed there. The same existence of urea in the blood has been ascertained in the human body, both in cases of diseased kidneys, when the excretion there was much impeded, and in cases of malignant cholera, when the excretion was suppressed. The cases of rapidly fatal jaun dice already mentioned, where the bi I e-d ucts were pervious and empty, would seem to have been eases where the peculiar matter of the bile has been in like manner formed in the blood, without finding the usual vent at the liver. And it will appear under the head of Respira tion, particularly from the experiments of Dr. Edwards, and of Collard de Martigny, that there is good reason to believe the carbonic acid of expired air to be formed in the course of the circulation, and only exchanged for oxygen at the lungs.
2. There are various instances in disease, of substances generally found in the secretions of certain glands only, being deposited in situa tions quite unusual, and where no texture similar to these glands exists; e. g. cholesterine, which in the natural state is found only in the bile, has been found deposited in diseased structures in the brain, kidneys, pelvis, scro tum, &c.; and litliic acid, naturally existing only in the urine, is deposited in cases of chalk stone in the textures immediately surrounding the joints of the fingers and toes. It seems to be nearly in like manner that purulent matter, when mixed in unusual quantity with the blood, as by inflammation of a vein, is fre quently deposited in individual parts of the body, with little or none of the usual sym ptoms, or of the other accompaniments, of in flammation at these parts.
3. There are a considerable number of cases recorded on unexceptionable evidence, where excretions have passed off per aliena cola, i. e.
by organs which in the natural state yield no such products, and the structure of which is widely different from that of the glands where they are usually secreted. This has been most frequently observed of the milk and of the urine, and of the latter, both in cases where the secretion at the kidneys had been sup pressed, and in cases where its discharge by the urinary passages has been obstructed, so as to occasion its re-absorption. In both cases it is obvious that the peculiar matter of this excretion must have been first mixed generally with the blood, and then deposited in indivi dual parts of the system, widely different as well as distant from those where it usually appears.
In cases of this kind collected by Haller,* the vicarious discharge of urine is stated to have occurred from the skin, from the stomach, from the intestines, and from the nipples; and in cases recorded by Dr. Arnold and Dr. Sen ter in America, it is stated to have been passed by vomiting, by stool, from the nose and from the mamma, as well as other parts.i. Both in cases given by Haller, and in one recorded in Magendie's Journal de Physiologie, (vol. vii.) milk is stated to have been evacuated in quan tity from pustules that formed on the thigh ; and among the former are instances of its hav ing passed off from the salivary glands, the kidneys, and the uterus. Such statements were formerly considered as fabulous, but since the facts already mentioned (and particularly the appearance of urea in the blood after ex tirpation of the kidneys) have been ascertained, this scepticism seems no longer reasonable.
It must be here observed, that the healthy blood is easily shown to contain in itself mat ters more nearly akin to all the solid textures and to the other secreted fluids of the body, than to the bile and the urine ; and hence, if we are satisfied that the elaboration of these latter fluids is effected in the blood itself, and does not essentially require any special action of the organs in which they usually appear, there can be little hesitation about extending this inference to other acts of secretion and to nutrition. It appears, therefore, at least highly probable, that the whole processes of as similation and elaboration of the fluids in the living body are carried on, as other chemical changes on fluids are, in the interior of these fluids themselves, and that the solids of the body are concerned in these changes only in two ways : first, by securing the complete sub division and intimate intermixture of the fluids necessary to their chemical changes ; and second ly, by determining the parts of the body where peculiar matters, already existing in the blood, shall be deposited from it, or attracted out of it.