The framework of the cutis may be said to consist entirely of a modified form of the areolar tissue. Both elements are enormously deve loped, but especially the yellow fibrous one. The fibrillae of this are thicker than elsewhere, I and branch and inosculate with great freedom, enclosing interstices open on all sides, and giving passage to the wavy bands of the white fibrous element as well as to vessels, nerves, the ducts of the sweat-glands, the sebaceous glands, and the roots of the hairs. These in terstices are in general very close, but they vary with the size of the parts which occupy them. On the deep surface of the cutis the yellow fibrous element changes gradually into that of the subcutaneous fascia, or that of ordinary areolar tissue. It cannot be doubted that the skin chiefly owes its elasticity and toughness to this remarkable developement of the yellow I fibrous element.
Topographical view of the mucous system in man.—lieferring the reader to the article SKIN for a detailed description of that part of the mucous system, and its immediate dependen cies, I shall now proceed to point out some of the more remarkable varieties of the internal tracts. These tracts have been usually com prehended under two general divisions, the gastro-pulmonary, and the genito-urinary. The former is continuous with the skin at six points, the two eyelids, the two nostrils, the mouth, and the anus; the latter at a single one, the 1 orifice of the urethra in the male, and the labia pudendi in the female. Besides these, there , are two smaller tracts, the mammary, each of which is subdivided into several, which open separately on the skin.
The description of the gastro-pulmonary tract may be commenced at the lips. It covers their inner surface, the cheeks, gums, tongue, and palate, and extends into the labial, buccal, and larger salivary glands, of which it consti tutes the chief mass. It passes over the arches of the palate, (where its involutions form the tonsils,)and lines the pharynx, Eustachian tubes, and the cavities of the tympana. Penetrating into the nose by the posterior pares, it lines all the passages and chambers of that organ, and advances along the nasal duct to the lachry mal sac. Thence it may be traced along the canaliculi to the front of the eye, where it takes the name of tunics conjunctiva; covers the posterior surface of the eyelids, a certain por tion of the sclerotic, and the cornea, and forms the caruncula, the Meibomian and lachrymal glands. In these complicated portions of its course, the membrane shares more or less in the construction of the five organs of special sense, and is the essential seat of two of them, taste and smell. From the pharynx it spreads in two directions ; first, into the larynx, trachea, tracheal glands, and bronchial ramifications, until it terminates by forming the air-cells of the lungs ; secondly, into the alimentary canal. Here it lines the oesophagus, stomach, and intestinal tube, as far as the anus, and it penetrates along the excreting ducts of the liver and pancreas, into the inmost recesses of those glands, to form their secreting surface.
The genito-urinary tract may be traced along the urethra into the bladder, ureters, and pelvis of the kidneys ; and thence into the substance of those organs as far as the Malpighian bodies, the extremities of the uriniferous tubules.. In connection with the urethra, processes pass to the glands of Cowper; and, in the male, into the interior of the prostate, the vesiculm seminales, vasa deferentia, and tubules of the testes. In the female, the vagina, uterus, and Fallopian tubes receive a lining from it, which, at the fimbriated extremity of those canals, be comes continuous with the serous membrane of the abdomen.* The very remarkable differences presented to the eye by different parts of this system have been a source of great difficulty to anatomists, who, on other grounds, believed them to be nearly allied ; and it would appear that hitherto no satisfactory explanation has been given of the anatomical conditions on which this variety depends. This deficiency I shall now endea vour in some degree to supply. From the ex aminations I have made, I have been led to consider in a distinct and separate manner the several elementary tissues already mentioned, composing the simple mucous membrane, and lying underneath it; and am come to the con clusion that the must complicated diversities that are met with, admit, when studied in this manner, of being explained and reconciled to a common type of structure.
Peculiarities of the skin, mucous membranes, and glands.
Of the skin.—This is chiefly peculiar in its epithelial element and its submucous areolar tissue. The epidermis is composed of a vast number of superimposed lamina of scales, which, in the earlier stages of their develope ment, and especially in certain races of man kind, contain minute pigment granules in their interior. The pigment disappears more or less completely as the particles attain the surface. It is continued for some distance down the hair follicles and sweat-ducts, and thus serves to mark the continuity of these parts with the general surface. Hairs, nails, hoofs, and other simi lar appendages are all composed of modified epithelial particles, and are nearly peculiar to the skin. The sebaceous and perspiratory glands, and the spiral ducts of the latter travers ing the epidermis, are also among the most characteristic features of this part of the mucous system. The papilla' of the skin have their counterpart in the villi of the mucous mem branes ; the cutis vera, as it is called, has also its analogue in the submucous areolar tissue, but it is so enormously developed that the re semblance has escaped the notice of anatomists. Its characters have been already briefly de scribed. It is a striking fact that the cutis, like the submucous areolar tissue, contains no fat, even in the most corpulent subjects. I have repeatedly made this remark. The cutis differs in this respect from the subcutaneous fascia, which is therefore, perhaps, to be regarded as less allied to the submucous areolar tissue.