GENERAL CONSIDERATIONS [General Considerations]
DISEASES OF THE SKIN INCLUDING THE EXANTHEMATA
FOR THE USE OF GENERAL PRACTITIONERS AND ADVANCED STUDENTS BY FREDRICK M. DEARBORN, A.B., M.D.
The skin is a fibroelastic membrane which may be truly called the living envelope of the human body. It is complex in structure and endowed with active and passive functions so that it affords covering and protection to the deeper tissues, and receives impressions from the external world to which it is continually exposed. It is a continuous covering merging into the mucous membranes at all mucocutaneous orifices. It is closely related to the structures beneath through its connective tissue, blood-vessels, nerves and lymphatics.
While the surface is approximately smooth, careful inspection reveals numerous ridges, furrows and pores, variously sized hairs and at the end of the terminal phalanges, hard nail formations. The ridges, just referred to, are caused by the row-like arrangement of the cutaneous papillae and may assume straight, wavy or crescentric lines. The minute depressions known as pores represent the orifices of the follicles and glands of the skin. For the purpose of giving roundness to the angles and curves of the body, and for other more practical reasons, the skin varies greatly in thickness indifferent regions. It is thickest on the buttocks, the palms of the hands and soles of the feet, and thinnest on the eyelids and prepuce. Generally where the areolar tissue is dense and considerable adipose deposit normally exists, the attachment of the skin to the parts beneath is close and firm and its mobility is limited. Where mobility is essential, as around the joints, there is little or no fat and the areolar tissue is loose. Other differences besides that of thickness may be noted as influenced by age, sex, race, health and climate. In fact, there are certain variations in different locations upon the same individual. Thus, the color may range from the fair skin of the blond to the black skin of the negro, from the rosy pink of an infant’s skin to the dark hue of the genitocrural regions of the aged. The patient, which cause the varying color in different races and individuals and under varying circumstances, is located in the deeper protoplasmic areas of the epidermis. The external epidermal layers are composed of transparent flattened epithelial cells which shrivel up into polymorphous scales as they approach the surface and are constantly, although invisibly, thrown off and as constantly renewed from below. It is by this process of continual repair that the epithelial covering of the skin is preserved in a healthy condition by the activity of its own protoplasm. The natural secretion of the skin which continually anoints its surface, making it comparatively impermeable, acts as a passive barrier to a too rapid loss of fluids from within and protects from irritants, variations in temperature and from invasions of bacteria from without.
1. Subcutaneous tissue. 2. Corium. 3. Epidermis. R. Reticular layer of the corium. P (on cut), Papillae of the corium. M, Mucous layer of the epidermis. G. Grandular layer of the epidermis. C. Corneous layer of the epidermis. S. Sebaceous Gland. T (on cut), Sweat-glands. H, Hair. N, Tactile corpuscle. A, B, F, Capillaries supplying fat lobule, sweat-gland, papillae of the hair and papillae of the corium. V, Veins from the papillae of the corium. E, Arrector pili muscles (diagrammatic).
A knowledge of the fundamentals of the anatomy and physiology of the skin is essential in order that the nature of pathological changes in the skin and the curative action of remedies may be understood. Microscopically, the skin is found to be composed of two constant organic parts, the corium and the epidermis. But for practical purposes, a third subdivision, the subcutaneous tissue, is usually described. Several less constant parts such as the sebaceous glands, sweat-glands, hair, nails, pigment, blood-vessels, lymphatics, nerves and muscles must be considered in order to appreciate the complex nature of the skin. For purposes of study, it seems wise to consider the deeper portions before describing those more superficially placed because, by so doing, their mutual relations can be made clearer.
(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.
Pars Reticularis (R, Fig.1).- The reticular layer constitutes the bulk of the corium and is composed of white fibrous tissue which, in the deepest portion, separates in bundles without division, forming a distinct net. As the bundles proceed upward, they divide and subdivide, until they reach the upper layer and there form a network by the interlacing of single fibrillae. The larger spaces in the reticular layer are filled with fatty tissue, blood-vessels, nerves, lymphatics, hairs and outwardly opening sweat-ducts. The smaller spaces contain connective tissue corpuscles and wandering cells.
Pars Papillaris. The papillary layer of the corium touches the lowest layer of the epidermis above and extends to just below the basal portion of the papillae. It may be distinguished from the lower or reticular layer by its more intricate structure, the interfasicular spaces being so minute in places that the general aspect is decidedly homogeneous. The innumerable projections or prolongations of the corium into the rete, so-called papillae (P, Fig. 1), give the upper surface of the papillary layer a wavy or zigzag appearance. The gaps in this surface are filled by downward prolongations of the rete which fill up the intervening spaces. A hyalin substance is interposed between the rete and papillae and is supposed to be identical with the cement-like material surrounding and separating the fibrillae of the corium. The papillae may spring each from a single or several from a common base and their apices may be conical, club- shaped or square. They vary greatly in number and size in different regions. There is also much differences as to their disposition, some being arranged in linear series, others in concentric or crescentric shapes causing furrows which apparently cross, often visible to the naked eye as markings upon the surface of the epidermis. They are most highly developed and numerous upon the tips of the fingers, palms, soles, nipples, clitoris, glans penis, and labia minora. It has been estimated that nearly a hundred are developed within each square millimeter of the body surface, and that the whole skin contains from 150 to 200 millions. The importance of the papillae is due to their being the residence of the terminal expansions of the cutaneous nerves and vessels. The vascular papillae are supplied with terminal loops of a minute artery and vein. The nervous papillae contain medullated nerve fibres and one or more tactile corpuscles. Occasionally a papilla is provided with both blood-vessels and nerves.
EPIDERMIS (Cuticle; Cuticula; Scarf skin; Epithelial layer)
The epidermis (3, Fig. 1) is the external layer of the skin. It is a distinct formation, quite independent of the corium, having its origin in the ectoderm, and is not regenerated after complete loss of this matrix. It is entirely cellular in structure, contains no blood-vessels and only a few nerves in the innermost part. The junction between the corium and epidermis is nearly a straight line at about the middle period of fetal life. During the latter part of fetal period, the epidermis grows downward by linear processes into the corium and, as the capillaries in the papillae develop, centers of nutrition are established for the epidermis. The full development and interrelations of the papillae and epidermis, however, are not attained until after birth. Despite the intimate union and close relationship existing between the epidermis and the corium, their behaviour both in health and disease is widely different.
The minute furrows on the surface of the skin, especially noticeable on the back of the hand, are due to the depressions of the epidermis between the papillae. The coarser furrows, as seen upon the back of the neck, extensor surfaces of the joints, forehead and other parts of the face, are due to repeated tensions or muscular contractions of the skin. The epidermis is conveniently divided into four layers from within outward, the stratum mucosum, the stratum granulosum, the stratum lucidum and the stratum corneum.
The stratum mucosum (the rete; rete mucosum; mucous layer; germ layer; rete Malpighii) (M, Fig. 1) is an important layer of the epidermis, being concerned in most of the morbid processes of the skin. During embryonic life, the appendages of the skin are formed by the nutritive activity of the vitalized protoplasm of this portion of the epidermis, and after birth, it is the source of the organic substances of all physiological or pathological secretions. It lies immediately above the papillary layer of the corium and directly beneath the grandular layer of the epidermis, hence it is the deepest and most active stratum of the epidermis. This layer is composed of several layers or strata of nucleated cells, polyhedral in shape, rich in protoplasm and arranged in parallel rows. The cells of the lowest layer are columnar or cylindric in form with oval nuclei which are surrounded with granular protoplasm and often largely provided with pigment. Their long axis is perpendicular to the surface of the corium; they have no cell-wall and sometimes appear as a mass of protoplasm with scattered nuclei. The cells of the next few rows are larger, cuboid in form, with well-defined nuclei and a distinct cell-wall, and contain granular and pigmentary matter. The more superficial rows of the mucous layer are made up of still larger cells, more granular and flattened and their axes generally assume a horizontal position to the cutaneous surface. All the cells of the mucous layer have characteristic protoplasmic processes, called prickles, which unite the cells to each other. Hence the name, prickle-cells or prickle layer, given by some authorities to this part of the epidermis. These processes unite the cells firmly, but at the same time the body of epithelia are kept separated from each other by the so-called cement substance of the skin. This substance also permits the free passage of nutritive material from the papillae of the corium, the ingrowth of nerve-threads, the immigration of white blood corpuscles, and the counterflow of lymph inward to the interpapillary depressions, thence to the lymph-vessels of the corium. The epidermis contains no lymph-vessels proper and no blood-vessels.
The stratum granulosum (granular layer) (G, Fig. 1) is made up of one or two, rarely more, rows of coarsely granulated nucleated epithelia arranged directly above the prickle cells of the mucous layer. In disease, the cells of this layer may be increased to four or five rows. The cells are attached to each other by short threads, hence the intercellular spaces are much narrower than in the mucous layer and the nutrition of these epithelia is correspondingly poor.
This layer is often considered a part of the rete and its granular material, composed of some chemical substance, is generally believed to have an important place in the process of cornification. Waldever named the granular material keratohvalin; some authorities believe that there are two distinct substances; while still others believe that it is a nitrogenous substance (chitin). But whatever the true nature of this granular substance, it first appears as isolated granules near the nuclei of some of the cells of the mucous layer. It is greatly increased in and is characteristic of the granular layer. Unna believes that to this layer the white race owes the color of its skin and supports this belief by the fact that, before the appearance of the granular layer during fetal life, the outer portion of the skin is transparent, so the blood-vessels of the corium can be seen through it, and also, that the color at the border of the lips and the nail beds is due to the absence of the granular layer in these parts throughout life. The changes which occur in the cells of the granular layer, as they are progressively forced upward and outward from the mucous layer, are necessary to further changes in the external layer of the epidermis, known as cornification.
The stratum lucidum, so-called by Oehl, is a thin, ribbon-like, translucent or shining layer, situated immediately above the stratum granulosum and below the stratum corneum of which it is considered a part by most authorities. There is no particular reason why these two or three rows of cells which only represent a stage in cornification should be designated as a separate layer, except that the subject can be more clearly presented by so doing. Kaposi believes this layer to be due to some chemicobiologic change that must take place in the cells of the granular and mucous layers before they become horny cells.
The stratum corneum (horny layer; corneus layer) (C, Fig. 1) is the external layer of the epidermis and hence of the entire skin and extends from the stratum lucidum below to the outside atmosphere. It is essentially the dead layer or surface division of the epidermis and consists of several layers of flattened imbricated cells whose exact shape depends upon the degree of keratinization which they have undergone. By rapid changes, the upper cells of the granular layer are transformed by the apparent melting of their granules into the surrounding cell protoplasm, and clear transparent epithelia are produced. The lower cells of the corneous layer are arranged in polygonal plates with shrivelled nuclei and rudimentary threads, which still serve to connect the frame-work of cells with each other. Still nearer the surface, the cells become thin, flattened, dry scales, lying parallel to the cutaneous surface, the outer rows wrinkled and curled up, preparatory to being shed. According to Unna, cornification of the epidermis is not a complete process but consists of an alteration of the periphery and connecting threads of the cells into horny tissue which, after digestion of the central part of the cells, presents a honeycomb-like structure. No traces of the intercellular canals are found in the corneous layer. The hard, dry character of the cells of this layer is due to the presence of a very hard and resistant substance, known as keratin.
The epidermis has no vascular supply, but the corium and the subcutaneous tissue are liberally supplied with truncal and capillary vessels. Tomsa has shown that the arterial supply is derived from subcutaneous branches which pass through the fascia and, by division and subdivision, form three separate vascular districts. The deepest variety supply the subcutaneous fat with numerous capillaries in a net-like arrangement inside and between the fat lobules (A, Fig. 1). The middle district sends off arterioles to form a capillary plexus for the sweat glands (B, Fig. 1). These capillaries end in small veins, some of which pass upward with the ducts of the sweat-glands and anastomoses with the veins of the papillary region. The third or upper district is supplied from an ascending artery (F, Fig. 1), which sends off branches to form capillaries for the hair-follicles, the sebaceous glands and the papillae. Each papilla is furnished with one or two capillary loops. The papilla of the hair has its own arteriole and capillary similar to those of the papillae of the corium. The capillaries of the papillary layer anastomose freely with those of the upper part of the hair-follicles from which loops pass to supply the sebaceous glands.
Venous plexuses accompany the arterial in all portions of the skin and in general, it may be said that all the venous vessels are somewhat larger than the arterial. The most superficial veins of the skin are derived from the capillaries of the papillae (V, Fig. 1) and form narrow meshes which, together with the deeper and circular veins, form venous branches which anastomose with branches from the hair-follicles and sebaceous glands. These unite into larger vessels and, with the veins from the sweat-glands and fat lobules, merge into the venous sinuses which end in the subcutaneous veins.
The capillaries of the skin are supplied with vasomotor nerves. Through their control, the capillary circulation largely influences the physiological functions of the skin as well as the pathological changes. A sudden dilatation of these vessels produces the common phenomenon of blushing and a sudden contraction, the equally marked blanching of the surface.
All portions of the skin are provided with a system of lymphatic channels which aid in the important processes of absorption, and the currents of which are eventually directed to the larger vessels beneath the skin. Lymph is so abundantly supplied to the skin that Darier has aptly described the deram as a “true lymphatic sponge.” In all parts of the skin may be found juice spaces or lymphatic channels which usually lack independent walls and fail to show an absolutely free flow of lymph into the lymphatic vessels. These lymph-channels are uniformly present in the papillae of the corium and converge near the middle of the base where a lymphatic vessel usually begins. From the apices of the papillae, lymph flows into the mucous layer of the epidermis in all directions through the interepithelial spaces and between the prickle threads which unite the epithelia.
Lymph slowly returns to the corium by way of the interpapillary depressions of the epidermis through minute openings, or through the ducts of the sweat-glands which emerge at these points. Juice spaces similar to those of the epidermis exist in the hair-follicles, in the sebaceous glands, and in the ducts of the sweat-glands, and form a sheathlike covering about the connective tissue bundles, the oblique muscles and the fat-cells. The course of the lymph in the corium is slowly downward to the lymphatic vessels. The passage of lymph from the spaces of the sweat-glands and fat tissue is by slow filtration into the neighboring veins or lymphatic vessels. This anatomical peculiarity facilitates the formation of subcutaneous fat. No lymphatic vessels proper are found either in fat tissue or in subcutaneous tissue free from fat; in fact, lymphatic vessels proper are relatively few and are commonly mere appendages of the blood- vessels.
The skin is well endowed with medullated and non-medullated sensory nerve- fibers, each in certain instances being substituted for the other, or they may be found in combination. These elements from the cerebral and spinal nerve- systems enter the skin and form horizontal bundles of nerve twigs in the subcutaneous tissue. From thence, branches pass upward with the blood-vessels through the corium, and divide into numerous ramifications in the papillary layer, some assuming a horizontal position to the surface and are disposed about the subpapillary vessels and capillaries of the papillae. Other short nerves break up near the epidermis into non-medullated fibers, which send off numerous branches, of which a smaller number end with free extremities in the connective tissue or on the endothelia of capillaries, while a large number penetrate into the epidermis between the basal cells. The nerves of the epidermis, after many divisions and, possibly, reunions to form plexuses (Unna) in the intercapillary spaces, finally send off from different points fine threads for each prickle cell, as far as the granular layer. These nerve-threads penetrate the cell protoplasm and terminate in minute bulbs on or about the nucleus. Some branches end in bulbs between the epithelia, but without any regularity of distribution. Non-medullated nerve-fibers are also supplied to the cells of the hair-sheath and the ducts of the sweat-glands. According to Krause, all sensitive nerve- filaments ultimately terminate without medullary substance and in minute enlargements. As these nerve termination are found largely and uniformly in the cells, of the epidermis up to the corneous layer, they would appear to be the transmitters of general sensation.