Qf the separation of material from the body. This function appears to be carried on in every part of the mucous system. One great division, that of the glands, is specially des tined to it, as are likewise those portions of the compound mucous membranes, which have been already described as coming properly under the designation of glands. If, however, the essential nature of the function of secretion be adequately considered, it will scarcely be doubted that even the simplest parts of the mucous membranes, and the whole cutaneous surface (as distinguished from its sebaceous and perspiratory glandular offsets) share largely in this important office. It is true that in the skin this function holds a subordinate place to that of defence and protection, but its existence is only an example of what an attentive survey of nature everywhere discovers; the accom plishment of various ends by means of the same simple instruments.
The notion that a secreted product must be fluid, is one that has arisen out of a partial and imperfect insight into the nature of the secreting process. Those matters which are eliminated in the largest quantities and by the largest glands are for the most part so, in the shape under which they meet the eye, that is, after their separation from the organ in which they are secerned. But in the case of the lungs the secretion is gaseous as well as fluid, and in numerous instances, which have been recently brought to light, chiefly by the labours o lIenle, it is found, when minutely scruti nized, to consist of organic forms entitled to be styled solid.
The problem which physiologists have now to resolve, is how far these organic forms, which are more or less altered epithelial par ticles, are necessarily concerned in the per formance of the function, for epithelium is all but universal in the mucous system. It would be foreign to the province of this article to enter at length on the general question of secre tion, and I shall confine myself to a few re marks tending to show in what direction recent researches point.* When the secretion of a sebaceous follicle o the skin is minutely examined, it is found to consist entirely of epithelial particles contain ing the sebaceous matter, and more or less broken and compressed. These are similar to the particles lining the follicle, and are main festly the same structures, detached and matted together. The secretion found in the tubules of the testis is chiefly composed of epithelial particles resembling those attached to the base ment membrane of the tubules. Some of these are very perfect, others have undergone changes. It has been already stated that the seminal ani malcules are most probably a developement of some of these particles, not altogether different in its nature from that of the cilia found upon them in other situations. The secretion of an
ordinary mucous follicle is likewise made up of epithelial particles resembling those still attached to the membrane. The thick, semi fluid mucus found in the stomach has been shown by Wasmann. to consist of rounded nucleated particles, which both in size and shape correspond with those of the stomach tubules. This mucus may be even seen pro jecting from the cells into which these tubules discharge themselves, and no doubt can exist that the proper secretion of this organ is chiefly composed of the bulky epithelium thrown off by the tubules; a view corroborated by the fact,t that this mucous membrane, consisting almost solely of epithelium, when mixed with certain acids naturally existing in the gastric juice, evinces the same powers of dissolving alimentary substances as that wonderful men struum itself. The same thing may be ob served in the intestinal canal, where the adhe sive mucus is little else than the aggregated epithelial caps of the villi, together with that which has escaped from the vertical tubes of the membrane. These facts may be always verified in a healthy animal just killed, and may thus be shewn to be independent of any morbid action. The legitimate conclusion from them seems to be this : that the peculiar prin ciples of these respective secretions are lodged in the epithelial particles ; having been depo sited there from the blood, in the natural course of developement. In other words, the process of secretion in these cases consists in an assimilation of the material from the blood by an organized tissue, which, when fully de veloped, is loosened and shed.
This view, so captivating by its simplicity, has certainly much satisfactory evidence in its favour, and it may, at least, be regarded as sufficieutly established to constitute a strong presumption in favour of the general position, that all secretion is primarily assimilation.
That the epithelial particles, when their growth is completed, should detach themselves in a more or less entire state, in all cases, from the membrane to which they have adhered, cannot be supposed essential to this general position, and even the total absence of any vestiges of these particles from any particular secretion would scarcely form a valid argument against it. For at present we know of no reason why the assimilated material should not be gradually given up by a slow disintegration or deliquescence of the particles, or even by a continual separation of it without a concomitant destruction of the particles themselves.