There are probably three ways in which the secretions are finally separated from the body : and these three ways appear to have a reference to the chemical qualities of the product, and to their effete or non-effete character. I. The par ticles assimilated into the nucleated cell may be thrown off by virtue of minute chemical changes occurring in it, without the cell itself being altered in form. In this case the nu cleated cells will be permanent, or only very slowly renewed, and the secretion will be formed, or at least perfected, by the passage of its elements through the cells. 2. The nu cleated cells, as they arrive at their full size, may undergo a slow change in the arrangement of their elements, and gradually disappear by a kind of solution or deliquescence, thus form ing the secretion. 3. The nucleated cells, when mature, may be cast off at once, and entire, with their contents. The two last modes are attended with a continual formation of new cells.
It would appear that, in general, where the secretion is formed by the rejected chemical elements of the cells (1), or by the destructive solution of the cells (2), it is effete; but that, when formed chiefly by the separation of cells that are mature and contain much organic matter (3), it is destined for ulterior purposes in the economy. Of the first the kidney seems to be an example, of the second the liver, of the third the lining membrane of the stomach.
The varieties in the qualities of the products secerned by different portions of the mucous system are only referrible to varieties in the elective powers of the tracts which respectively furnish them, and admit of being most readily explained by the view of the nature of secre tion already advanced. It is unnecessary, in this place, to enter on a particular description of the boundaries of these several tracts, and I shall only offer a few observations on the nature and extent of that secretion which has given its name to the structures here treated of.
The term mucus, like so many others trans mitted from an early period, was originally employed to denote an exaggerated and partial condition, was subsequently applied more loosely and widely in a generic sense, and now requires to be reduced to a more definite application in accordance with that necessity for precision of thought and expression which characterizes modern science. The exposition contained in the article Mucus will render it superfluous for me to define its present accept ation. It is denied by Dr. (iruby that the viscid form of mucus is a normal secretion from any membrane whatever, and he considers its existence as a certain mark of diseased ac tion. This view, if less absolute, would be in a great measure correct, since there is no doubt that in a state of perfect health most mucous surfaces are wholly unprovided with any pro tection of this kind. If the nasal cavities, the
trachea or bronchia, the intestinal or urino genital tracts, be examined in a healthy animal killed for the purpose, we may search in vain for any slimy covering, such as they are com monly imagined to possess. But in a state of disease, each of these surfaces will secrete great quantities ; and it is not a little remarkable, that, even when healthy, if moistened and allowed to undergo slight putrefaction, they will become coated with a viscid fluid, having the physical characters of mucus. Yet the slimy fluid of the mouth cannot with propriety be considered abnormal. The true saliva is not viscid, as it escapes from the ducts of the glands into the cavity of the mouth : it probably becomes so by dissolving the substance derived from the scales of epithelium lining the mouth, as they advance to the surface and flatten. The fluid of ranula is not merely the accumulation of a natural secretion, but seems gradually to ac quire its great viscidity by receiving the debris of the epithelium lining the excreting channels, and by the partial reabsorption of its aqueous portion.
In the intestinal canal, however, although there is no viscid mucus naturally present, yet there is a large amount of" inspissated mucus" being continually separated from the villi and follicles of Lieberkuhn. This mucus, as already mentioned, is nothing more than the debris of epithelial particles.
But chemists have detected, in most of the secretions, a small proportion of a substance nearly allied to mucus, and probably a form of it. There is good reason to believe this to be the product of the membrane lining the ex cretory passages, and to represent the old epi thelium of that membrane. Where the secre tion of the gland is fluid and in considerable quantity, it seems to be sufficient to convey away this debris from the surface which it tra verses on its way out of the system, as in the salivary and allied glands, the liver, kidney, &c. But where, from the absence of this means of carrying off the debris of the epithe lium, it might be supposed to be liable to accumulate and clog the surface, cilia are de veloped ; of which the best example is fur nished by the respiratory tract, the nasal ca vities, and the tympana. That this is the great office of these wonderful organs upon these extensive surfaces appears to be proved by the fact that the currents they produce are uniformly towards an outlet. Ilenle has ob served this in several parts, and I have.ascer tained it by experiment in the case of the tra cheal and bronchial membrane.