But the best-known example of this form is presented by the cuticle, which from its ex posed position is thicker and denser than any internal epithelium. This variety, then; is the one which offers the most convincing proof of the homology of the mucous membranes with the skin. The term scales' is only applicable to these particles in the last of the stages through which they pass. They first appear on the surface of the basement membrane as gra nular dots, each of which soon becomes in vested with a cell membrane. Both nucleus and cell increase in size up to a certain point, the cell being then more or less globular, and con taining a material that appears transparent and almost entirely fluid. By this circumstance, chiefly, it is distinguished from the spheroidal form of particle, presently to be noticed. The cell now begins to flatten, loses its fluid con tents, and is at the same time the seat of certain changes by which its chemical properties are modified. At length its opposite surfaces unite, except where the nucleus intervenes, and a lamella of extreme tenuity results, which being now arrived at the surface is loosened and shed. It appears to be by the continual pres sure arising from the growth of newly-formed particles that the peculiar characters of this variety result. Accordingly, the scales are only found constituting the superficial layers of a series (fig. 278, ad). It is met with in those parts only where foreign pressure, or more pro_ perly friction, has to be encountered. In such parts a thick coating of epitheliumis evidently desirable, and the hard and almost horny qualities which these particles at length assume where most exposed to violence, admirably adapt them for their object. On such parts, moreover, cilia would not be needed, and it would even seem that this variety of epithelium when converted into true scales possesses neither sufficient substance nor vital power to develope and support these exquisite organs.
The scaly epithelium is remarkable for the tenacity with which itsparticles adhere to one another, and to the surface on which they rest. This adhesion is manifest at all the stages through which the particles pass. It is stronger between particles at the same stage than be tween those at different stages of growth, so that there is always a tendency to a separation into successive lamina: on maceration or other wise. Hence have resulted the divisions of the epidermis into two, three, or more layers, and especially that remarkable fallacy of re garding the rete murosum as a distinct structure. How far this adhesion is owing to the presence of an intercellular substance in all instances it is difficult to decide ; but it seems highly probable that, in the deepest layers, where the particles are small and rounded, such a su bstance must exist in considerable abundance, filling up the interstices, and serving as a kind of blastenue, in which the nuclei (or cytoblasts) of fresh parti cles originate. I have lately (Jan. 1842) ascer tained a very curious fact, giving evidence of this adhesion. This is, that the delicate threads drawn out of the cubs when the cuticle is stripped from a piece of macerated skin, con sist entirely of the epithelium of the sweat ducts, the particles of which are so intimately united with one another, and with those of the deeper layers of the epidermis, as to allow of being thus dragged out of their tube of base ment membrane, often for a length of an eighth of an inch.
The scaly epithelium is subdivisible into two forms, the regular and the irregular. In the former, the scales are united edge to edge in a regular manner, as in the skin of the Frog and other reptiles, and on many internal surfaces, especially in the lower animals. In this form, the particles do not become so thin as in the other, and the superficial scales are cast off in lamina consisting of a single series and of uniform thickness. In the latter form, they overlap one another without order, and present no regular figure. This is the ordinary form, and is that presented in the skin and other parts of Mammalia and Birds.
Of the prismatic varietg.'—In this the par ticles have the shape of small rods, disposed endwise on the basement membrane, in a single layer, the thickness of which depends on their length. These rods are united to one another by their sides, which are flattened for that pur pose. They are, therefore, prisms and not cylinders, as Ilenle terms them. They are also almost invariably of very unequal thickness in different parts, being bulged somewhere near the middle by their nucleus, which is oval, with its long axis parallel to that of the particle. Their deep or attached extremity, also, usually tapers to a point, in order, probably, to allow room for new particles to spring up in the in tervals. This is more decidedly the case where they clothe a convex surface, (as that of the intestinal villi,) and their sides tend to assume the direction of radii from a common centre. Hence they are sometimes even triangular in outline. Their opposite or free extremity is much thicker, often as thick as the part bulged by the nucleus, and near this extremity neigh bouring particles are generally very intimately attached to one another, having often the ap pearance of being blended into a single mass. The best example of this is on the villi of the small intestine (fig. 280). The contiguous par ticles, however, are fitted closely together in the greater portion of their length, and to effect this the bulging nuclei vary in the height at which they are placed. There can be no doubt, that, in certain situations at least, as will be afterwards shown, these particles are being con tinually shed, and consequently are being per petually renovated. But it is very difficult to ascertain their early condition and changes, and I am not aware of any satisfactory observations having been made for this end. From the great facility with which they become detached from the surface they invest, it is next to im possible to examine them in situ on thin verti cal sections. But there is no reason for sup posing their mode of growth to be originally different from that of the scaly variety. Their nuclei probably appear first on the surface of the basement membrane, and around these a cell is developed (fig. 280, a). But this cell from its earliest period seems to contain an amorphous substance, which under high micro scopic powers looks finely mottled, but not so definitely so as to allow of being called granu lar. As the particle advances towards its full size, it loses its cell-membrane, and when com plete is to be regarded rather as a solid mass of organic substance, surrounding a nucleus, than as a cell. Here, then, is a striking difference between the scale and the prism: maturity being marked in the one by the disappearance of the substance of the cell ; in the other, by that of the cell-membrane.