The foregoing facts, taken together, afford a very strong presumption that the epithelial particles of the lobules are the agent assimilating the secretion from the blood. It would be still more satisfactory if particles could be found undergoing decay. Meanwhile it seems im possible to assign to them any other office, if it be granted that the sole function of the liver is to secrete bile. For in the case of other glands, the only other use that can with any degree of plausibility be attributed to the epi thelium is that of its serving to defend the secreting membrane from the contact of the secretion, and to prevent the latter from re entering the blood. And it cannot exist for that purpose in the liver, because it is itself the only structure besides the bloodvessels, and does not constitute a lining membrane.
,The peculiarity in the minute structure of the kidneys, which bears on the present ques tion, is of a kind entirely different from any presented by the liver, and yet tends to establish a similar conclusion. It consists of a special apparatus at the extremity of each secreting tube, apparently designed to furnish a flow of water down the canal.* A large quantity of water is evidently required in this secretion as a menstruum for the salts and proximate prin ciples it contains; and there is no doubt, from the analogy of other glands, that the walls of the tubes are the membrane secreting these substances. Now the epithelium constitutes at least iSths of their thickness, and is the only part of them with which the water can come into contact. It therefore seems highly pro bable that this fluid is provided in the manner described, in order to dissolve, out of the epithelial particles, the peculiar principles which they have previously assimilated from the blood.
In support of this general position it may be observed, further, I. That the epithelium, which constitutes so large a portion of the true glands, is solid and bulky, usually character ized by its finely granular texture, and in this respect contrasts strongly with that lining the vascular system, which is of extreme delicacy and transparence. The exceptions to this re mark confirm its importance. In the air-cells of the lungs, the secretions of which are ga seous and not solid, the epithelium is of great tenuity, and in the Malpighian capsules of the kidney, which appear to serve principally as receptacles for the aqueous fluid that escapes from the bare capillaries within them, this structure is either wanting or consists of per fectly transparent particles. In many inter
mediate varieties, too, there appears traceable a correspondence between the bulk of the nu cleated particles and the activity of the secreting function ; of which the scaly form in general may be mentioned as an instance. 2. That many peculiar substances are secreted into the interior of nucleated cells, although prevented, by the position of those cells, from escaping from the body. Such are various fats and fixed oils, colouring matters, &c. 3. That this func tion of abstracting somewhat from the blood, and elaborating it, seems the most probable one that can be assigned to the thymus and thyroid bodies, the spleen, and supra-renal capsules, and specially to the nucleated par ticles forming so large a portion of these several structures. On the whole there seems much weight of evidence in favour of the proposi tion " that secretion is a function very nearly allied to ordinary growth and nutrition : that whereas these are a combination of two func tions, assimilation of new particles and rejection of old, the old being reconveyed into the blood, secretion consists in a corresponding assimila tion and rejection, but the old particles are at once thrown off from the system without re entering the blood. According to this view, all effete material received into the blood, from the old substance of the various organs, must be reassimilated by an organized tissue, specially designed for the purpose, viz. the epithelium, before it can be eliminated : and all substances thrown off from the system, but designed for an ulterior purpose, must in like manner be assimilated in order to their separation." It places in a strong light a principle of great im portance in physiology, the subordination of the bloodvessels and their contents to the tis sues among which they are distributed.
The function of secretion may therefore be considered to be universal over the mucous system, and its different activity in various si tuations to be dependent on, as it certainly is closely associated with, differences in the ar rangement and structure of the epithelial ele ment. The basement membrane, from being absent from the lobules of the liver, seems a tissue of inferior (perhaps of no) importance in respect of this function, and probably is chiefly subservient, wherever it exists, to the mechanical support of the epithelium.