(Stratum subcutaneum; Hypoderm)
Most authorities agree that the division of the subcutaneous tissue (1, Fig. 1) from the corium is purely arbitrary with no sharp boundary line. Unna states a patent truth when he says that it owes its recognition as a distinct layer “only to the circumstance that, in consequence of the macroscopically appreciable deposit of fat in it, a distinct border-line is visible even to the naked eye.” The subcutaneous tissue immediately underlines the reticular layer of the corium and, like the latter, is composed of a network of interlacing and anastomosing fibrous connective tissue. These bundles are less densely arranged than in the corium and are attached more or less firmly to the periosteum or superficial fascia, and pass in oblique directions upward and outward to merge in the substance of the corium. The interspaces between these bundles are filled with fat and constitute the panniculus adiposus.
The fat globules are spherical vesicles composed of an elastic capsule with an oval nucleus at one point and a drop of oil filling the remainder of the cavity. They are found clustered in masses forming lobules of varying sizes, separated from one another by delicate fibrous connective tissue and well supplied with blood-vessels, having an afferent artery, one or two efferent veins and a capillary plexus. In some of the thicker portions of the skin, columns of fat (columnae adiposae; fat-columns of Warren) pass obliquely into the lower half of the corium and probably give increased support and elasticity to the skin, assist in the passage of blood-vessels and lymphatics and aid in the nourishment of glands. Subsequent to birth, the fat is gradually absorbed over certain surfaces where mobility or firm attachment are essential such as on the eyelids, in the auricles, on the penis, scrotum and labia minora. The subcutaneous tissue contains sweat-gland coils, the deep hair-follicles, blood and lymph-vessels, nerves, nerve-corpuscles. Under normal conditions this layer gives form and plumpness to the body and serves as a double cushion; first to the parts beneath which are liable to pressure, and second to the more delicate corium externally, as well as the appendages seated within and passing through both layers. In starvation and wasting diseases, the contents of the oil globules disappear, leaving the cell-wall intact, but these rapidly refill with the return of nutrition. An abnormal production and deposit of fat leads to obesity in which the subcutaneous tissue may become an inch or more thick.
CORIUM (Derma; Cutis; Cutis vera; True skin)
The corium (2, Fig. 1) is the most important portion of the skin and, embryonically speaking, it represents the original foundation, in that it is derived from the superficial layer of the mesoderm called the “skin-plate.” At the beginning of the second month of fetal life the cutis is composed of embryonic corpuscles which develop between the second and third months into spindle-shaped bodies of a fibromyxomatous nature. A collagenous substance replaces the tissue just mentioned about the fifth month of embryonic life. Blood-vessels are first noted about the third month.
The corium constitutes the fibroelastic envelope of the body and contains blood-and lymph-vessels, oil-and sweat-glands, hairs, muscles, fat cells and nerves with their various terminal organs. The inter-fascicular spaces are smaller than in the subcutaneous tissue and as they approach the surface, lessen in size. The thickness of the corium varies in different individuals, at different ages and on different parts of the same individual. It is thickest over the palms, soles, back and nates, and thinnest on the prepuce and eyelids. For the sake of convenience, an arbitrary division of the corium may be made into two parts or layers, the reticular and the papillary.