After this account of the properties of membranous matter in general, we shall proceed to make some obser vations upon the different species of it reserving, how ever, for their appropriate places, descriptions of those parts that derive their distinguishing character from some other substance superadded to the membrane, such as muscle and bone, or those that, in consequence of their peculiar organization, serve for the performance of some specific function, as the blood-vessels and glands.
All animals, except those of the simplest structure, possess an outward covering, which connects their parts together, protects them from injury, and prevents the too powerful impression of the various external agents to which the body is exposed. In the human species, and those of the most perfect organization, it is called the cutis, or skin. It has been divided by anatomists into three layers, or rather into three distinct organs, which possess peculiar and distinct functions. These are, the epidermis, the rete mucosum, and the cutis. The epi dermis, or cuticle, is the external part ; it is thin, and semitransparent; it seems to possess no sensation, and is not furnished with blood-vessels that are visible to the eye. It is frequently destroyed, and is easily re-produced, with out causing any material derangement in the functions of the subjacent parts.
As the cutaneous perspiration is supposed to issue from the whole of the surface of the body, it has been inferred that the cuticle must be furnished with pores for its trans mission, yet we have no satisfactory evidence of these pores having been ever actually detected. We have in deed little knowledge respecting the minute structure of the epidermis; and it would appear, from the best obser vations which we possess, that it consists merely in a thin expansion, in which no specific texture can be perceived. Most of the older anatomists, and even Haller and Bichat among the moderns, were induced to regard it simply as a crust or film, spread over the surface, and supposed it to be formed by an exudation from the cutaneous vessels, merely hardened by exposure to the air. But such an opi nion seems scarcely compatible with our ideas respecting the nature of any organ which forms a part of the animal body, and appears inconsistent with that extreme minute ness with which the cuticle is spread over the whole sur face, and applied with perfect accuracy to all its inequali ties. In some of its morbid states the cuticle is obviously
connected with the vascular parts of the system ; and the analogy of the inferior animals would lead us to the same conclusion; for the scales of fish, the thick folds with which the elephant is covered, and other similar sub stances, are properly productions of the cuticle.
There is a remarkable fact respecting the epidermis, that, independent of any morbid state, it becomes increas ed in bulk under certain circumstances. It is always found to be naturally thicker in some parts than in others, as, for example, in the soles of the feet; and we likewise find that it may be still farther thickened or increased by pressure. This affords one instance among many others, of that admirable adaptation of the organs to their ap propriate uses, by which they are not only fitted for per forming certain actions, but possess the property of.ac commodating themselves to incidental circumstances. The physical cause by which this change is effected, may perhaps be referred to the increased action of the cutis, which we may conceive is excited in these cases, but it must be confessed that this 'explanation is, in some mea sure, conjectural, and at best is only supported by a loose analogy.
There has been much controversy respecting the next layer of the skin, the rete or corpus mucosum. Its ex istence was first announced by Malpighi; he described it as a layer of soft matter, disposed in the form of fibres, crossing each other in various directions. Some of the later anatomists have conceived it to be merely a thin stratum of pulpy matter, without any distinct reticulated structure, while Bichat seems altogether to doubt its existence, as a proper layer or membrane, and supposes that the net-work which Malpighi described, is merely an extremely delicate congeries of vessels, which, after hav ing passed through the cutis, ramify on its surface in all directions, and produce the appearance of a number of reticulated fibres.