The valuable art of making leather depends upon the property which the solid texture of the skin possesses of uniting with the tan, and forming a new chemical com pound, without having its mechanical texture destroyed, so.that while it retains its original form, its nature is so far changed as to be no longer soluble in water, and but little susceptible of the action of moisture. The art of forming leather from skins is one of very ancient date, and there are few nations which do not possess some kind of process of this description ; but it was not until of late years that the theory of tanning was understood, and for this knowledge we are much indebted to the French che mists, and especially to M. Seguin. Considerable light has also been thrown upon the subject by Mr. Hatchett, and some improvements have consequently taken place in our manufacture of leather ; but it is, we believe, generally admitted, that by expediting the operation, as was propos ed by Seguin, we materially injure the nature of the article that is produced.
Nearly allied to the external skin, both in their texture and their uses, are the membranous coats or tunics, which line the internal parts of the body, and cover the different N iscera. In consequence of their being less exposed to in jury, and of the necessity of a greater delicacy of structure, the proper membranes are much thinner than the skin of the external surface ; they are also, for the most part, ho mogeneous in their texture, and are not divisible into dif ferent layers. They are generally thin, transparent, and of considerable tenacity ; they possess but little sensibility, and are scantily supplied with blood-vessels. It was to these bodies that the name of membrane was originally ap plied, from which it has been extended to the whole class of substances that exhibit a general similarity in proper ties and composition.
The proper membranes have been made the subject of a very elaborate treatise by Bichat, in which he has arrang ed them into different classes, has minutely examined the structure and functions of each, and pointed out their re lation to the other parts of the system. The three princi pal divisions which lie lays down are the mucous, serous, and fibrous ; of each of these we shall give a short descrip tion. The mucous membranes are characterized by the peculiar semifluid substance with which their surface is covered ; they are always found lining those cavities that are disposed in the form of irregular passages or canals, that open externally, and are connected at their termina tion with the cutis. Of these the principal are the mouth, the nostrils, the oesophagus, the urinary passages, and the whole of the digestive organs. Their external surface is soft and pulpy, and is diversified by various projections, w hich serve different purposes, according to the functions of the part where they are found. The mucous membranes are the immediate seat of some very important operations in the animal economy ; they constitute the organs of taste and smell, of digestion, of assimilation, and of various se cretions. On this account they differ from most membra
nous bodies in being plentifully supplied with blood-vessels and nerves, as well as in possessing an extensive apparatus of glands and absorbents.
The serous membranes differ materially, in almost all respects, from the mucous. They are always found in close cavities that do not communicate with the atmos phere, as, for example, in the thorax and abdomen, and they form coats for many of the most important organs, as the heart, the lungs, and the abdominal viscera. In their texture the serous membranes are smooth, compact, and thin, but of considerable strength in proportion to their bulk; they have their surface always moistened by a fluid which exhales from them, but as no glands have been de tected, and as it differs very little in its chemical nature from the serum of the blood, it is supposed to be produced rather by a kind of infiltration through very minute pores, than by what can properly be regarded as secretion. The fluid that is thus produced is always in health absorbed as speedily as it is formed, but in certain states of disease it is liable to accumulate, when it gives rise to the different species of dropsy-. The serous membranes have scarcely any vessels of sufficient size to convey red blood, and have very few, if any, nerves ; they are therefore without sensi bility, and possess only in a small degree the general powers of vitality. They have a considerable share of elasticity, and are capable of great extension, but they are not pro perly contractile, nor do they possess any powers except those that are common to many other parts of the body.
The fibrous membranes are named from their obvious texture, as consisting of a visible assemblage of united into a continuous extended surface. They differ from the mucous and serous in not being moistened by any fluid, but they bear a considerable resemblance to the lat ter in their general aspect, being dense, thin, and smooth, although, according to their situation, and the uses which they serve, they are more varied in their form and con sistence. Among the most important of the fibrous mem branes are the periosteum, the Jura mater, the aponeuro ses, which are found in different parts of the body, the cap sules of the joints, and the sheaths of the tendons. The texture of these membranes is obviously fibrous, without blood-vessels, nerves, glands, or any specific apparatus. Their use in the animal economy is purely mechanical ; to enclose the soft parts and preserve them in their proper form, to separate them from each other, and to keep them in their relative position. The chemical composition of membrane appears to differ in the different species ; but they all of them consist of a basis of albumen, united to certain proportions of jelly ana mucus.