LESSON XXIII.

THE SKIN.

1. SECTIONS of skin from the palmar surface of the fingers. The sections are to be made vertical to the surface, and should extend down as far as the subcutaneous tissue. They may be stained with logwood or picro-carmine and mounted in Canada balsam. In these sections notice the layers of the epidermis and their different behaviour to the staining fluid. Notice also the papillæ projecting from the corium into the epidermis, and look for tactile corpuscles within them. In very thin parts of the sections the fine intercellular channels in the deeper parts of the epithelium (see Lesson VI. p. 22) may be seen with a high power. The convoluted tubes of the sweat-glands will be seen here and there in the deeper parts of the corium, and in thick sections the corkscrew-like channels by which the sweat is conducted through the epidermis may also be observed. Make a sketch showing the general structure under a low power, and other sketches to exhibit the most important details under a high power. Measure the thickness of the epidermis and the length of the papillæ.

2. Sections of the skin of the scalp, vertical to the surface and parallel to the slope of the hair-follicles, and others parallel to the surface, and therefore across the hair-follicles. Stain and mount in the same way as in the last preparation. Examine also the structure of the hairs.

In these preparations the details of structure of the hairs and hair-follicles together with the sebaceous glands and the little muscles of the hair-follicles are to be made out.

3. Vertical sections across the nail and nail-bed, cut with a strong scalpel or razor. The sections are stained with hæmatoxylin or picro-carmine. Notice the ridges (not papillæ) of the corium projecting into the epidermis. Observe also the distinction of the epidermis into Malpighian layer and nail proper.

4. Mount in Canada balsam a section from a portion of skin of which the blood-vessels have been injected, and notice the distribution of the capillaries to the sweat-glands, to the hair-follicles, and to the papillary surface of the corium.

The skin is composed of two parts, epidermis and cutis vera.

The epidermis, or scarf skin, is a stratified epithelium (fig. 129). It is composed of a number of layers of cells, the deeper of which are soft and protoplasmic, and form the rete mucosum of Malpighi, whilst the superficial layers are hard and horny; this horny portion sometimes constituting the greater part of the thickness of the epidermis. The deepest cells of the rete mucosum, which are set on the surface of the cutis vera, are columnar (fig. 129, c) in shape. In the coloured

races of mankind these cells contain pigment-granules. In the layers immediately above them the cells are polyhedral (fig. 129, p). Between all these cells of the rete mucosum there are fine intercellular clefts which separate the cells from one another, but are bridged across by fine fibres, which pass from cell to cell. The intercellular channels serve for the passage of lymph, and within them occasional lymphcorpuscles may be found, often having a stellate figure from compression. The most superficial layer of the rete mucosum is formed of somewhat flattened granular cells (stratum granulosum, s.gr). Im

H, horny layer, consisting of s, superficial horny scales; sw, swollen-out horny cells; s.l. stratum lucidum; M, rete mucosum or Malpighian layer, consisting of p, prickle-cells, several rows deep; c, elongated cells forming a single stratum near the corium; and s.gr. stratum granulosum of Langerhans, just below the stratum lucidum; n, part of a plexus of nerve-fibres in the superficial layer of the cutis vera. From this plexus fine varicose nerve-fibrils may be traced passing up between the epithelium-cells of the Malpighian layer.

mediately above this layer, the horny part of the epidermis commences, as a layer of clear compressed cells several deep (stratum lucidum, s.l.). Above this comes the main part of the horny layer. It is composed of a number of layers of somewhat swollen cells (sw.), the nuclei of which are no longer visible. These cells become flatter as they approach the surface, where they eventually become detached in the form of thin horny scales (s).

The growth of the epidermis takes place by a multiplication of the

cells of the deeper layers. The newly formed cells, as they grow, push towards the surface those which were previously formed, and in their progress the latter undergo a chemical transformation, which converts their protoplasm into horny material. This change seems to occur at the stratum granulosum (see fig. 130); the granules which occupy the cells of that layer being composed of a substance termed eleidin, which is transformed into keratin.

No blood-vessels pass into the epidermis, but it receives nerves which ramify between the cells of the rete mucosum in the form of fine varicose fibrils (fig. 129).

FIG. 130.-PORTION OF EPIDERMIS FROM A SECTION OF THE SKIN OF THE
FINGER, COLOURED WITH PICROCARMINATE OF AMMONIA. (Ranvier.)

a, stratum corneum; b, stratum lucidum with diffused flakes of eleidin; c, stratum granulosum, the cells filled with drops of eleidin; d, prickle-cells; e, dentate projections by which the deepest cells of the epidermis are fixed to the cutis vera.

The cutis vera or corium is composed of dense connective tissue, which becomes more open and reticular in its texture in its deeper part, where it merges into the subcutaneous tissue. The superficial or vascular layer of the corium bears minute papilla, which project up into the epidermis, which is moulded over them. These papillæ for the most part contain looped capillary vessels (fig. 137), but some, especially those of the palmar surface of the hand and fingers, and the corresponding part of the foot, contain tactile corpuscles, to which medullated nerve-fibres pass (fig. 97, b).

In some parts of the body (scrotum, penis, nipple, and areola), involuntary muscular tissue occurs in the deeper portions of the cutis vera, and in addition, wherever hairs occur, small bundles of this tissue are attached to the hair-follicles.

The blood-vessels of the skin are distributed almost entirely to the surface, where they form a close capillary network, sending up loops.

into the papillæ. Special branches are also distributed to the various appendages of the skin, viz. the sweat-glands and hair-follicles, with their sebaceous glands and little muscles, as well as to the little masses of adipose tissue which may be found in the deeper parts of the cutis.

The lymphatics originate near the surface in a network of vessels, which is placed a little deeper than the blood-capillary network. They receive branches from the papillæ, and pass into larger vessels, which are valved, and which run in the deeper or reticular part of the corium. From these the lymph is carried away by still larger vessels, which course in the subcutaneous tissue.

The appendages of the skin are the nails, the hairs, with their sebaceous glands and the sweat-glands. They are all developed as thickenings and downgrowths of the Malpighian layer of the epidermis.

(Heitzmann.)

P, ridges with blood-vessels; B, rete mucosum; N, nail.

The nails are thickenings of the stratum lucidum of the epidermis, which are developed over a specially modified portion of the corium, which is known as the bed of the nail, the depression at the posterior part of the nail-bed from which the root of the nail grows being known as the nail-groove. The distal part of the nail forms the free border, and is the thickest part of the body of the nail. The horny substance of the nail (fig. 131, N) is composed of clear horny cells, each containing the remains of a nucleus; it rests immediately upon a Malpighian layer (B) similar to that which is found in the epidermis

generally. The corium of the nail-bed is beset with longitudinal ridges instead of the papillæ which are present over the rest of the skin; these, like the rest of the superficial part of the corium, are extremely vascular. The nails are developed in the fœtus at about the third month, the groove being formed at this time in the corium, and the nail-rudiment appearing in it as a thickening of the stratum lucidum, which extends forward over the bed. It becomes free in the sixth month, its free end being at first thin, but as it grows forward over the bed it appears to receive additions on its under surface, so that after a time the distal part becomes the thicker. The superficial layers of the cuticle which originally covered the developing nail become detached, and, after birth, only remain as the narrow border of cuticle which overlies the lunula.

The hairs are growths of the epidermis, which are developed in little pits the hair-follicles-which extend downwards into the deeper part of the corium, or even into the subcutaneous tissue. The hair grows from the bottom of the follicle, the part which thus lies within the follicle being known as the root.

The substance of a hair is mainly composed of a pigmented, horny, fibrous material (fig. 132, f), which can be separated by the action of sulphuric acid into long tapering cells, the nuclei of which are still visible. This fibrous substance of the hair is covered by a layer of delicate imbricated scales termed the hair-cuticle (c). In many hairs, but not in all, the centre is occupied by a dark-looking axial substance (medulla, m), formed of angular cells which contain granules of eleidin, particles of dark pigment, and frequently minute air-bubbles. The latter may also occur in interstices in the fibrous substance. When they are present, the hair looks white by reflected light. The root has the same structure as the body of the hair, except at its extremity, which is enlarged into a knob (fig. 133, b); this is composed mainly of soft, growing cells, and fits over a vascular papilla (p), which projects up into the bottom of the follicle. The follicle, like the skin itself, of which it is a recess, is composed of two parts: one epithelial, and the other connective tissue. The epithelial or epidermic part of the follicle closely invests the hair-root, and is often in great part dragged out with it; hence it is known as the root-sheath. It consists of an outer layer of soft columnar and polyhedral cells, like the Malpighian layer of the epidermis-the outer root-sheath (figs. 133, f; 134, e); and of an inner, thinner, horny stratum next the hair-the inner root-sheath (figs. 133, g; 134, f). The inner root-sheath itself consists of three layers, the outermost being composed of oblong cells without nuclei (Henle's layer), the next of flattened polyhedral nucleated cells (Huxley's layer), and the third-the cuticle of the rootsheath-being a thin layer of downwardly imbricated scales, which fit over the upwardly imbricated scales of the hair itself.

The connective tissue or dermic part of the hair-follicle (fig. 134, a, c, d) is composed internally of a vascular layer, separated from the