The production of the simple structureless membranes which exist in various parts of the body must be attributed, we think, to the con solidation of a thin layer of blastema, rather than to any metamorphosis of cells. The basement or primary membrane which lies beneath the epithelium of the mucous and serous membranes, and of the glandular pro longations of the former, as well as the mem brane lining the bloodvessels, and bearing epithelium upon its inner surface, must pro bably be regarded in this light. It may be questioned, however, whether this is not to be regarded, in most ca.ses at least, a.s a transitional form, rather than as a permanent structure. We have reason to believe that in many situa tions (as the lining of the alimentary canal and of its glandular prolongations,) the nuclei con tained in this membrane must be continually developing themselves into epithelium-cells ; and in some other instances it would seem, that a fibrous structure developes itself from it by a metamorphosis of a different kind. It is not difficult to imagine, that these variations may have their origin in the degree of plasticity of the element, of which the membrane was origin ally coniposed, and in the number of cell-germs which it includes. Considerable differences in the appearance of this primary membrane may be seen, in examining the residua left after dis solving away the calcareous matter of shells by dilute acid. Putting aside the cellular tissue which certain shells exhibit,* the most general animal basis of each layer is a very delicate inembrane, which sometimes appears com pletely homogeneous, even when viewed with the highest powers of the microscope; but which in other instances presents a distinctly granular aspect, as if it consisted of a layer of molecules consolidated together by a structure less cement. These membranous films are in cluded between strata of calcareous matter, poured out from the surface of the mantle, and thus undergo no change subsequent to their first production.
We have next to consider the mode in which the tissues, whose form is distinctly cellular, or which can be clearly proved to originate in cells, derive their nutriment from the blood. In the early stage of embryonic developement, as already stated, the svhole fabric is composed of cells which present no recognizable differ ences amongst themselves, and which yet, by a process of histological transformation, become the elements of the different orpns,—some of them still retainims the form of cells,—whilst others undergo ch''anges which remove them altogether from that category. To the former class belong adipose tissue, pigment-cells, the various kinds of epithelium and epidermis, cartilage-cells, &c. Of the latter, the capillary bloodvessels, and the muscular and nervous tissues, are characteristic examples. Now there would seem much reason to believe, that in the regular process of nutrition each of these tissues draws from the bloOd the materials necessary for its reparation and growth, as it does in the earlier stages for its first develope ment ; and that the function of the blood is confined to the supply of these materials,—the germs of the new tissue being supplied by that previously existing. At any rate it may be safely affirmed that no evidence has been ad duced which renders any other view probable. The self-nutrient power of the tissues is evinced by this fact among others,—that in no instance are their ultimate elements penetrated by the capillary bloodvessels. Thus although adipose tissue is traversed by a minute capillary net work, the fat-cells lie in the meshes of thts net work, and are as independent of it, except as regards the supply of nutrient materials which they derive from it, as if they adhered closely r to each other. The muscular fibres and nerve
tubes again, are not penetrated by capillary vessels, but are only surrounded by them. The connection of the cartilage-cells with the ves sels is still more remote; for the true cellular cartilages are not penetrated by bloodvessels at all (in the healthy state at least), but are nourished by the imbibition of fluids from a plexus of dilated vessels that comes into rela tion with their external surface. We may infer, therefore, that the bloodvessels are subservient to the act of nutrition only by conveying the nutrient fluid into the neighbourhood where i is required,—just as, in the irrigation of a dow, the water is carried in channels over general surface, but has to find its way percolation into tbespaces between these; that it is by the materials which they de from it that the several tissues are enabled maintain their integrity, by reproducing tl structure as fast as it is disintegrated. An may not be unre-asonable to infer that, in very act of the death and disintegration of parent structure, the germs of the new st tures destined to replace it are set free, as pens in the reproduction" of the simple c lar plants.
It may be doubted, however, svhether same holds good in regard to newly-for parts, or with respect to the epithelium which are formed on the free surface of basement membrane, and which are cast without reproducing themselves. These la. seem to originate in germs contained in flit subjacent membrane, and a continual supply ot such germs must therefore be required. It rat scarcely be doubted, therefore, that these are supplied directly from the blood. Dr. Barry and Mr. Addison have spoken with much con fidence of the metamorphosis of the white cor puscles of the blood into epithelium-cells ; but that this idea is totally inadmissible is proved by the existence of a continuous stratum of basement-membrane, between the capillary net work and the epidermic or epithelial layer. It is not impossible, however, and perhaps may be considered probable, that the cell-germs contained in this basement-membrane, from which the cells on its external surface appear to take tbeir origin, may be the offspring of the white corpuscles of the blood, which thus sup plies both the plastic materials and the germs or the constantly-formiug new crops of epithe lial cells. There is no other tissue in the body, after all its organs had attained their full deve lopement, which can be reg,arded as taking its origin from the blood in the same degree ; but it may be questioned whether in the formation of new parts, either during the developement of the embryo, or in the reparation of injuries, the office of the blood is not of a similar du plex character. Thus when plastic lymph is thrown out, between the two surfaces of a wound, the first process, as already mentioned, is its fibrillation ; but at the same time a deve lopement of cells takes place in it, which cells may possibly undergo a subsequent metamor phosis into the various forms of tissue which the newly-formed part afterwards contains, precisely as in the first developement of the em bryonic structure. Such a view, at least, would seem probable in regard to the capillary vessels, which seem to be formed at least as much by the inherent powers of the coagulum, as by the extension of the vessels from the subjacent I surface.