234

THE RETE MUCOSUM AND THE TRUE SKIN.

soles of the feet and the eyelids. In this respect its use is mechanical. It serves as a protective covering to the parts it envelops, being thick where pressure and hard usage have to be provided for, and thinner where there is a necessity for motion. It consists of an aggregation of nucleated particles adhering together, the deepest being granules, the intermediate more perfect cells, which gradually become flattened scales as they are examined nearer the surface. They undergo constant exuviation, and are as constantly replaced from beneath, the superficial ones becoming dry and horny, thus furnishing a resisting tegument, the operation of which is very well displayed by the action of vesicating agents: a watery discharge from the vessels of the cutis soaks through the lower substance of the cuticle, and raises the dry layers above. The chemical composition of these dry scales is the same as that of nail, hair, horn, and is C1g, H39, N, O18"

48

Rete mucosum

At one time it was supposed that the rete mucosum, or layer of Malpighi, which is the lowest portion of the cuticle, and thereand its color- fore resting on the cutis, is a distinct structure. It is, howing matter. ever, merely the most recently-formed portion of the cuticle. The netted appearance it presents originates in the eminences of the papillary structure below. Many of its constituent particles contain coloring matter, especially in the dark races. The pigment seems to be produced by the agency of the sunlight and continued high temperature, though it disappears gradually as the cells containing it approach the surface. It yields a very large percentage of carbon.

The derma:

tion.

Beneath the epidermis is the derma or true skin. It is composed of fibrous tissue, which also serves to connect it with the parts its construc- beneath, blood-vessels, lymphatics, and nerves. In its areolar tissue both the white and yellow fibrous elements are found, the proportion of each varying according to the mechanical function the part has to discharge, the yellow predominating where elasticity is required, and the white where a resistance to pressure. The derma also contains organic muscular fibres, to which its property of corrugation, as in cutis anserina, is due. On different parts it is of different thickness, being thinnest where motion has to be provided for. A deposit of fatty material, lodged beneath, gives it a yielding support. Its outer surface presents a papillary structure, which is the instrument of touch. This is more perfectly developed on the inner surface of the palm of the hand and fingers. The furrowed aspect of the cutis arises from this. A farther consideration of the mechanism and functions of the papillæ is deferred to the description of the sense of touch.

The photographic engraving, Fig. 99, represents a thin section of the epidermis of the foot of the dog.

The general method of arrangement of the constituent portions of the

skin may be gathered from the perpendicular section of that of the external auditory meatus in Fig. 100. a, the derma; b, rete mucosum; c, horny layer of epiderma; d, coil of ceruminous glands; e, their excretory ducts; f, their apertures; g, hair-sacs; h, sebaceous glands; i, masses of fat. (Kolliker.)

Fig. 101 shows the under surface of the cuticle detached by maceration from the palm, exhibiting double rows of depressions, in which the papillæ have been lodged, with the hard epithelium lining the sudoriparous ducts in their course through the cutis. Some of them are contorted at the end, where they have entered the sweat gland. (Todd and Bowman.)

Fig. 101.

Under surface of the cuticle.

Papillæ of palm, magnified 35
diameters.

Fig. 103, surface of the skin of the palm, showing the ridges, furrows, cross grooves, and orifices of the sweatducts. The scaly texture of the cuticle is indicated by the irregular lines on the surface. (Todd and Bowman.)

The nails.

THE NAILS AND HAIR.

236
The NAILS constitute one of the appendages
of the epiderma. They are horny
coverings protecting the extremities
of the fingers and the toes. They originate
in a fold of the cutis, and become free at their
outer extremity. The nail grows from its
roots, increasing in length, and simultaneously
in thickness. Its rate of growth depends upon
the general rate of nutrition. During periods
of sickness or abstinence, its growth in both
directions is retarded, as is indicated by a mark
or impression on its surface, and so the nail
becomes a register of the condition of nutrition
during the period of its own existence. The

thumb nail is said to occupy about 20 weeks Skin of palm, magnified 20 diameters. in its growth from the root to the extremity; that of the great toe about two years an estimate which is probably too long.

The hair.

THE HAIR. Each hair originates in a flask-shaped follicle, formed by a depression of the cutis, and lined by a continuation of the cuticle, and, like it, presenting scales on its superficies and round cells beneath. The bottom of the follicle is the place of origin. The hair consists of two portions, the outer or cortical, and the inner or medullary, the proportions of which differ very much in different cases. The surface of the hair presents a layer of imbricated scales, within which, at the lower part, are minute cells, but farther from the root the cells become larger and begin to contain pigment, the coloring matter being distributed unequally, sometimes producing a tubular appearance in the axis.

Fig. 104.

Human hair in section.

The hair grows by constant prolongation from the follicle, its color being due to a peculiar colored oil; and in the black varieties, iron predominates. The diameter of the hairs varies from 140 to 1500 of an inch.

In Fig. 104, the structure of the root of a hair and part of its shaft is displayed. Bulb of a small black hair from the scrotum, seen in section: a, basement membrane of the follicle; b, layer of epidermic cells resting upon it, and becoming more scaly as they approach c, a layer of imbricated cells forming the outer lamina or cortex of the hair: they are more flattened and comepressed the higher they are traced on the bulb.

Within the cortex is the proper substance of the hair, consisting, at the base, where it rests on

OF THE SUDORIPAROUS GLANDS.

237

the basement membrane, of small angular cells, scarcely larger than their nuclei. At d these cells are more bulky, and the bulb consequently thicker: there is also pigment developed in them; above d they assume a decidedly fibrous character, and become condensed; e, a mass of cells in the axis of the hair, much loaded with pigment. (Todd and Bowman.)

Fig. 105 is an engraving of a photograph of a transverse section of human hair from the head. The outer line shows the cortex; in some the pigment

Transverse section of human hair, magnified ary axis is seen; in most, however, it is

Fig. 106. d

200 diameters.

sues.

absent.

The SUDORIPAROUS GLANDS originate in depressions of the cutis or tissues beneath, occurring in some parts, as in the axilla, The sudoripamore numerously than in others. They consist of a tube rous glands. wound on itself, and sometimes dividing in convoluted branches. The knot thus arising is contained in a cell, the wall of which is copiously supplied with blood-vessels: the duct passes through the superjacent tisThe tube is formed of a cylinder of basement membrane lined with epithelium. The basement membrane may be considered to be derived from the outer surface of the papillæ, and the epithelium is an external projection of the cuticle. The duct, on its passage outward, loses its basement membrane as it escapes between the papillæ ; and it has a spiral or helical aspect, an arrangement probably intended to keep the calibre open. It is estimated that the number of sudoriparous glands is about seven millions, and the total length of their tubing about 28 miles.

magnified 20 diam

Fig. 106 is a sudoriparous gland from the palm of the hand: a, a, knot of tubes with two excretory ducts, b, b, uniting into a helical canal, which perforates the epidermis at c, and opens on its surface at d: the gland is imbedded in fat vesicles at e, e, e, e. (Wagner.)

The SEBACEOUS GLANDS are distributed in different Sudoriparous gland, abundance in various parts, their office being The sebaceous eters. to lubricate the hair, to keep the skin in a flex- glands. ible condition, and avoid the inconveniences of friction. Their ducts open either into the hair follicles or upon the cuticular surface; the gland consisting of basement membrane lined with epithelium, the cells of which, as they reach maturity, become filled with a sebaceous or oily material. The ear glands of this class secrete a waxy matter.

238

OF THE SEBACEOUS GLANDS.

Such being the construction of the skin, we have next to speak of its action. It discharges a double function: 1st, as an excreting, and, 2d, as an absorbing organ. In this respect it has an analogy with the mucous membrane, which, indeed, is a reflection or continuation of it.

Of the excreting action of the skin. The skin permits water, saline Different kinds and fatty substances, to escape from it in quantities which of perspiration. differ on different portions of its surface, the nature of the secretions varying to meet local requirements. In the examination which we are now entering upon, we shall speak of these substances and their proportions in a general way, overlooking, for the time, the particular variations. Yet that such variations exist is clear on the most superficial observation. The sweat of the feet differs from that of the general surface, as, again, does that of the arm-pits.

the cuticle and

compared.

It has been usual to distinguish the watery transudation into two portions, that which escapes from the perspiratory ducts, and Quantity of water through that passing through the surface of the cuticle. It has even from the ducts been said that the true glandular secretion passing from the ducts is not more than one sixth of the total cutaneous exudation; but this, I believe, is altogether erroneous. When we recall the impermeable nature of the horny and dried scales which constitute the outer portion of the cuticle, and that these are constantly coated over with an oily varnish issuing from the sebaceous glands, we may infer that the cutaneous surface between the mouths of the perspiratory ducts is constructed rather for the hinderance of evaporation than for its promotion; and though the oily matter with which the skin is thus imbued is justly regarded as having for one of its functions the prevention of injury from the admission of external moisture, it must be equally effectual in stopping the escape of water from within. The tardy manner in which water thus escapes is illustrated by the operation of blisters.

Exhalation

Under the form of steam, water continually escapes from the skin. It also, on certain occasions, issues in the liquid state as drops and perspira- of sweat. To its escape under the form of steam the designation of exhalation or insensible perspiration is given; but if under the form of sweat, that of sensible perspiration.

tion.

OF EXHALATION.-On condensing the vapors which arise from the skin, they are found to consist of water containing a little acetate of ammonia. With the water likewise escapes carbonic acid gas. With a view of ascertaining the weight of the matters thus lost, Seguin inclosed himself in an air-tight bag, the mouth of which was gummed Experiments to ascertain the upon his face in such a way as to permit the access of air to amount of wa- the respiratory organs. He then determined the weight of his body and the bag together. After several hours, on reweighing, he ascertained the amount of loss by pulmonary

ter escaping through the skin.