The description of the anatomical structure of the skin is from Hogg's book called A Parasitic or Germ Theory of Disease, published in 1876.
The skin is composed of three layers, the epidermis, or cuticle, the dermis or true skin, and the subcutaneous cellular tissue. It is largely supplied with blood vessels, nerves, glands, and adipose tissue. Muscular fibres are found in the superficial layer of the dermis in close connection with the hairs of the body. In some of the lower animals, as the cat tribe, mouse, &c. these muscles are arranged in bands about the nose, and they serve to erect the hairs and convert them into sensitive tactile organs.
Commencing with the deepest layer, and the most important, the dermis or corium as it is sometimes called, is not a plain surface, it exhibits elevations, with corresponding depressions almost throughout. The uppermost layer assumes the appearance of numerous conical eminences or vascular loops termed papillae. The fat, adipose, and connective tissues, are situated in this layer; but where the skin is loose, or can be thrown into folds, as about the eyelids and over the joints, no fat is stored. No papillae exist where the skin is fur rowed, as in the palm of the hands, &c. ; but they are very freely distributed to semi-moist parts, as the lips, gums, mouth, &c. ; and to such parts they give a soft, smooth appearance. The sudoriferous glands are situated in this layer ; they are small oval bodies with spiral tubes running up through the several layers of skin to the surface. The sweat-glands are very important, and it is computed that there are 3,528 in every square inch of the hand. The secretion from these glands constitutes the sensible and insensible perspiration ; the former is that which appears on the surface, where it rests for a time and then drops off; the latter is that which passes off insensibly, or without notice, in the form of watery vapour. The average quantity excreted from the body in the twenty-four hours is estimated at from two and a half to five pounds : this of course varies with the temperature, time of year, &c. It is one of the most watery fluids of the body, leaving only about one per cent, of solid matter after evaporation. The perspiration contains sodium chloride, ammonium phosphates, and other organic matters, largely mixed with epithelium scales and fatty matters, which impart at times a remarkably sour smell to the body.
The epidermis or cuticle, the outermost layer of the skin, is composed of several layers of flattened- out scales. It was thought that no well-defined limit existed between the dermis and epidermis, but this is only true during the earliest stages of existence. In very young children, the two structures are invariably continuous with each other ; the most superficial portion of the derinis consisting of a homogeneous layer of plastic material, protoplasm, and fine granular matter. The mucous layer, rete Malpighii, is composed of granules that are enlarged or enlarging by the absorption of a definite proportion of the protoplasmic mass. These scales or cells, as they come nearer the surface layer, assume a flattened-out form, and are somewhat horny in character. This change appears to commence with a shrinking of the internal body, nucleus, which divides into spaces, vacuoles. Besides these cells there are others in the mucous layer that resemble the connective tissue cells of the dermis ; these are small in size, and of a fusiform shape, each having in its interior one or more nuclei. This layer of cells no doubt plays an important part in cutaneous affections. The scales of the epidermis are incessantly thrown off from the surface of the body ; and, should the process be in any way impeded or arrested, derangement of health or disease will assuredly follow. The fat tissue of the skin is necessarily an important structure ; and although each fat-cell contains but a single globule of oily matter, it requires a complete network of capillary blood vessels to maintain and renew the supply. The fat-glands, although present in almost all parts of the body, are more numerous in those covered with hair ; they are, however, entirely absent in the palms of the hand and soles of the feet.
The papillae, or vascular loops, have medullary nerve fibres running into them, and which afterwards pass to the tactile corpuscles. The lymphatics of the skin are accompanied by one or two blood vessels throughout their whole course. I may here observe of the glandular structure situated in the eyelids, that it is more complex than the rest ; the gland ducts there ramify about in all directions, for the purpose of pre serving the healthy nature of the secretion that nourishes the hairs about the eyelids. Should these glands become inflamed, the ducts are obstructed, and a disease is produced, tinea tarsi, common enough among a certain class of people. This disease of the lids is accompanied by the formation of small pustules and scabs, and leads to destruction, falling out of the eyelashes.
In the Malpighian layer of the epidermis, the first series of cells is described as columnar in form, while the immediately superimposed layer is cubical in form, of a larger size, with a granular nucleus, in which nucleoli are visible. The superficial or horny layer of the epidermis is composed of flat polygonal tesselated scales, the nuclei of which have nearly disappeared. The colour of the skin, noticed in certain persons, and the deep brown of the negro, depends on the presence of coloured granules contained in the cells of the Malpighian layer. In addition to the nervous supply already mentioned, preparations of the skin, made by staining with a solution of the chloride of gold, demonstrate the presence of an abundant supply of non-medullary nerves, ending in free extremities, and which run between the cells of the mucous layer. The subcutaneous nerve trunks are made up of both medullary and non-medullary nerve fibres; these dip down into the deeper parts of the dermis, and there divide and sub-divide into several branches, for the purpose of accompanying the bloodvessels. It is in this part of the skin that the Pacinian bodies, tactile corpuscles, are found ; the finer nerves seem to terminate in these bodies; this doubtless explains their extreme sensitiveness. The nervous structure is one of great interest and importance to the Dermatologist. The sweat gland, as we have already seen, is scarcely a less important appendage of the skin. It usually presents a contorted knot-like body, with a spiral-shaped tube making its way through the dermal, the mucous, and epidermal layers. According to Krause, there are 2,736 sweat glands in every square inch in the palm of the hand, 2,685 on the sole of the foot, 1,490 on the back of the hand, 1,303 on the neck and forehead, 417 on the back and buttock. Those situated under the armpits cannot be exactly compared, as regards their number, with those of other parts of the body ; but their size is very remarkable. Besides the various structures enumerated, there are the muscles of the skin, composed of both smooth and serrated fibres ; the latter reach the skin from the deeper lying parts in the face, the beard, and nose, while the former, or smooth, are in immediate connection with the hair follicles. Many hairs possess two muscles ; these split up and pass over the nearest sebaceous gland, partly encircling it. As the hairs are inserted obliquely in the skin, forming a moderately acute angle with the surface, and the muscular bundles lie in a plane of a corresponding obtuse angle, the muscular contraction must obviously cause the hair to become almost erect. The well known effect of goose skin is in the same way produced by muscular contraction. Beside the several layers enumerated, there is a basement membrane which has been always described as structureless, but by the aid of reagents this is seen to be cellular.
The hairs are next in importance of the cutaneous structures ; these may be briefly described as cylindrical modifications of the epidermis, peculiar outgrowths of the epithelial scales, having a hardened, vitreous or horny exterior. The hair scales overlap each other, and are compactly held together by a layer of connective tissue, that becomes firmer and more solidified when brought into immediate contact with the outer air. A hair consists of a root, a shaft, and a point. The portion of the shaft situated beneath the skin presents a bulbous enlargement, and here it is surrounded by a network of bloodvessels, nerves, fat, and sebaceous matters, all of which are necessary to maintain it in a healthy condition. The central portion of each hair is filled with a medulla, and this, together with the pigment granules contained in the outer cells, imparts the variety of colouring observed in animals provided with a thick hairy covering.
To be somewhat more precise in describing a hair, we observe that it is composed of several layers of connective tissue and epithelial cells ; this is better seen on making a horizontal section of a small bundle of hairs, and submitting them to a magnifying power of about two hundred diameters. It is then noticed that the external layer of each hair appears to be serrated; this arises from the overlapping of the scales, one lying over the other as tiles on the roof of a house. That portion of the hair concealed beneath the skin, the root, is fixed in a peculiar fold or sac, the hair-bulb. It is scarcely right to call this part of the hair shaft bulbous, as the hair often terminates in a blunt point. The hair follicles vary a good deal in length ; those in connection with the longest hairs as of the head and beard pass into the subcutaneous tissue, while in other parts of the body they extend only a very short distance. In every case a small conical network of bloodvessels surrounds the hair-bulb, and it is from this point that the growth of the hair proceeds; and here the elements of the cortical substance changes into horny scales, the more recently formed epithelium being the deepest. The hair is formed, like the teeth, from a pulp enclosed in a follicle; when a hair is plucked from its follicle, it is found that the root-sheath adheres closely to it. The stem of the hair possesses a certain power of growth; indeed so highly organised is it, that it would be strange if it were not so ; and this view receives confirmation in a disease called Plica Polonicd. The hairs in this affection are split up into fibres, and at a considerable distance from their bulbs a glutinous substance exudes which mats them together ; nevertheless, they continue to grow.
The colour of the hair is sometimes suddenly changed, by some strong mental emotion, from black to white. One of the most remarkable cases of the kind on record is that of the Pere Lefevre, narrated in a recent trial. The loss of pigment or colouring matter in the cortex and medullary substance produces grey hair ; but this does not destroy its vitality. The permanent hairs increase to a certain length ; and when the limit of growth is attained, the papillae are no longer able to support their weight, and they then fall out ; new hairs are, however, developed in their place. In human beings, a succession of new hair is continually taking place ; but in the lower animals there is a periodical shedding and renewal of what forms an outer warm coat. The falling off of the hair arises from the circumstance that no new epithelial scales are formed, or the last formed receive no nervous supply, and are consequently drawn out into condensed elongated hair cells ; while those shed during some deranged state of the health are either not replaced, or, when replaced, the hairs are stunted and colourless. Baldness frequently depends upon a double cause a diseased condition of the bulbs and surrounding connective tissue. In advanced age, loss of hair is no doubt the result of a deficient blood supply, and is more frequently constitutional than local. It has been observed again and again that persons whose circulation is languid, slow, or defective, become bald at a very early period of life. The arrest of development, diseased condition, here referred to, does not originate in structures beneath the surface of the skin, and apparently far removed from the attacks of vegetable parasitic growths. The fungi, in exceptional instances, do appear to penetrate the dermic structure. It has been discovered that a rare form of fungoid disease occurs among the native population of certain parts of India; but this is always confined to a foot or a hand, and never attacks any other part of the body.
I shall have occasion to revert to the fungus footdisease more in detail further on. I proceed to consider the structural character of the nails. It is well known that the nails form a horny and somewhat transparent termination to the fingers and toes; they differ from hairs, inasmuch as, although very dense outgrowths of the epidermis, the epithelial cells spread out and array themselves in an imbricated manner, one layer leaving only a portion of the next uncovered, the last lying embedded in a fold of the skin, and forming a matrix beneath the surface. The nails, like the hairs, are composed of epithelial scales, connective tissue, dermis, and a mucous layer of the bed-matrix. The moundlike elevations of the matrix form a series of from fifty to ninety ridges, which are chiefly made up of parallel fibres of connective tissue and fusiform cells. The deepest layer of cells is columnar, and the blood-vessels form a plexus, from which numerous vascular loops are sent off to supply the papillae. The nerves lie embedded in the subcutaneous tissue of the nailbed. Sections of nail, soaked in a potash solution for a short time, easily break up into numerous fragments well suited for an examination under a magnifying power of 300 diameters. The dense mass is then seen to be composed of numberless nucleated epidermic cells cemented together by connective tissue and albumen. When the nail-wall is deficient in structure, as frequently occurs with the little toes, the nail does not grow out, but, having increased in thickness and attained to a certain height, it breaks off. Irritation of the matrix, produced by pressure or other cause, is attended by inflammation and suppuration, and loss of the nail. New nail is formed by the matrix, so that this is a very important part of the structure ; and even suppurative inflammation occurring in the nail-bed will not stop the growth of new nail. This fact constitutes another instance of the almost entire dependence of the dermal appendages upon the deeper-seated internal structures, and quite independent of external causes. Late researches prove that, in the earliest embryonic stage, the formative processes and changes begin with a division and splitting up of the same cells as those seen in the adult. The small round cells increase and multiply, and become transformed into various definite tissues, recognisable as skin, hair, glands, &c. up to the wonderfully perfect optical crystalline structure of the eye. These, together with the epithelial membranes of the nose, ear, &c. are all developed from one primary series of cells, and the order of development in the normal state never seems in any way to vary. This indicates an intimate relationship between the organs of sight, hearing, and the skin ; hence the constant liability of these several parts to become simultaneously affected.
If this be so, how necessary it is to possess a perfect knowledge of the microscopical anatomy of the skin, and of the pathological processes which determine and maintain the human frame in a healthy state!
Who can rise from a study of the outer covering of the body, and say that when abnormal changes are observed in any part of it they belong to a distinct and separate nosological order? No one doubts that disease, whether of the external or internal parts of the body, must be considerably modified by the anatomical or structural arrangements in its own immediate locality. An eruption taking place on the surface of the body is but a manifestation of a pathological change in the organization generally, modified by the structure in which it makes itself apparent to the eye. It has been observed that although the quantative and qualitative errors of the chemical composition of the different structures of the body attract and demand much attention, nevertheless little is known of the causes on which these errors depend. "The excess or deficiency in the supply of nutritive material the wrong quality and quantity of the matter supplied the wrong chemistry, in the act of assimilation in the different textures the excess or deficiency of chemical action in the removal by the used organs these constitute a multitude of diseases ; some of which are known as hypertrophy, atrophy, and degeneration ; and most of which are only guessed at even at the present day."
As regards the pathological anatomy of the skin, it is believed, and with a good show of truth, that the varied forms of cutaneous affections are essentially of an inflammatory type, differing among themselves in intensity, and changing with the inflammation present and the part affected. Take eczema as an example. In this disease the papilla are the chief seat of the affection ; and in the earliest stage of the disease these bodies increase in size, and contribute towards the exudation of a clear serous fluid. The connective tissue corpuscles become stimulated in their growth by the abundance of pabulum ; these enlarge and multiply, and the resultant free cells migrate from the papilla and insinuate themselves between those of the mucous layer. Such cells present an elongated form, branch out towards the corneal layer of the epidermis,and there unite to form an irregular network with those of the mucous layer. Some anatomists look upon these especial cells as constituting a system of nutritive canals for the mucous-layer ; and by their agency a larger quantity of serous fluid infiltrates the papilla, and is brought to the tissue, thus causing it to swell out even to bursting. The epidermis, in a somewhat similar manner, from irritation becomes separated as a minute blister, bursts, and ends in a running sore. It is further supposed that even pus cells, or pustules, filled with a serous fluid, do not proceed from the multiplication of the cells of the rete mucosum, or mucous layer, but are rather offsets from the spindle-shaped cells of the connective tissue and papilla, which, multiplying very rapidly, are forced to the surface.
Although it may appear that I have entered somewhat minutely into the anatomical and pathological relations of the skin, I believe it will be conceded, by those acquainted with the subject, that no one can cope satisfactorily with its morbid lesions unless he possess a perfect knowledge of the several parts with which he has to deal ; neither is it possible to distinguish one disease from another without an acquaintance with secondary results. For this reason, it is often necessary to proceed by another and very generally adopted method ; namely, by comparing the special characters of one form of disease with that of another, and by a process of elimination, arrive ultimately at a correct diagnosis.
If, for instance, the epidermis or cuticle presents a simple elevation with a clear transparent fluid, it may be safely inferred that the disease is neither an exanthera nor a variola. By another method we obtain a knowledge of the secondary products which form the basis of Willan's classification; and this may enable us to distinguish varieties as an eczema from a herpes.
A simple view of the causes and morbific influences at work in the production of skin diseases, in almost every form of eruption, is exhibited in an intelligible tabular form by Mr. Hunt. The only diseases mentioned by Willan which are not included in Hunt's table are pompholyx, ephelis, spilus, and aphthae. But as the first is only another name for pemphigus ; ephelis a form of pityriasis ; spilus a kind of nevus; and aphtha a parasitic growth occurring in the mouth during childhood ; these may be eliminated.
Parasitic Skin Diseases in General
Common Skin Diseases of Children