The Organs of the Senses and the Common Integument. Anatomy of the Human Body. It varies in an essay on the inequality of the human races in different parts.
In some situations, as in the palms of the hands and soles of the feet, it is thick, hard, and horny in texture. This may be in a measure due to the fact that these parts are exposed to intermittent pressure, but that this is not the only cause is proved by the fact that the condition exists to a very considerable extent at birth. The free surface of the epidermis is marked by a net-work of linear furrows of variable size, dividing the surface into a number of polygonal or lozenge-shaped areas. Some of these furrows are large, as opposite the flexures of the joints, and correspond to the folds in the corium produced by movements. In other situations, as upon the back of the hand, they are exceedingly fine, and intersect one another at various angles. The function of these ridges is primarily to increase resistance between contact surfaces for the purpose of preventing slipping whether in walking or prehension. The direction of the ridges is at right angles with the force that tends to produce slipping or to the resultant of such forces when these forces vary in direction.
In each individual the lines on the tips of the fingers and thumbs form distinct patterns unlike those of any other person. Fine tubular prolongations are continued from this layer into the ducts of the sudoriferous and sebaceous glands. These are known as prickle cells because of the bridges by which they are connected to one another. They contain fine fibrils which are continuous across the connecting processes with corresponding fibrils in adjacent cells. Between the bridges are fine inter-cellular clefts serving for the passage of lymph, and in these lymph corpuscles or pigment granules may be found.
They are supposed to be cells in a transitional stage between the protoplasmic cells of the stratum mucosum and the horny cells of the superficial layers. According to Ranvier they contain granules of a material which has the characteristics of beeswax. The black color of the skin in the negro, and the tawny color among some of the white races, is due to the presence of pigment in the cells of the epidermis. This pigment is more especially distinct in the cells of the stratum mucosum, and is similar to that found in the cells of the pigmentary layer of the retina. The main purpose served by the epidermis is that of protection, as the surface is worn away new cells are supplied and thus the true skin, the vessels and nerves which it contains are defended from damage. It varies in thickness in different parts of the body.
In the eyelids, scrotum, and penis it is exceedingly thin and delicate. It consists of felted connective tissue, with a varying amount of elastic fibers and numerous bloodvessels, lymphatics, and nerves. Unstriped muscular fibers are found in the superficial layers of the corium, wherever hairs are present, and in the subcutaneous areolar tissue of the scrotum, penis, labia majora, and nipples. In the nipples the fibers are disposed in bands, closely reticulated and arranged in superimposed laminæ.
The distribution of the bloodvessels in the skin of the sole of the foot. Toward the attached surface the fasciculi are large and coarse, and the areolæ left by their interlacement are large, and occupied by adipose tissue and sweat glands. Below the reticular layer is the subcutaneous areolar tissue, which, except in a few situations, contains fat. The papillæ are minute conical eminences, having rounded or blunted extremities, occasionally divided into two or more parts, and are received into corresponding pits on the under surface of the cuticle.
Each ridge contains two rows of papillæ, between which the ducts of the sudoriferous glands pass outward to open on the summit of the ridge. The epidermis and its appendages, consisting of the hairs, nails, sebaceous and sweat glands, are developed from the ectoderm, while the corium or true skin is of mesodermal origin. About the fifth week the epidermis consists of two layers of cells, the deeper one corresponding to the rete mucosum. The subcutaneous fat appears about the fourth month, and the papillæ of the true skin about the sixth. The nails are formed at the third month, and begin to project from the epidermis about the sixth.
The hairs appear between the third and fourth months in the form of solid downgrowths of the deeper layer of the epidermis, the growing extremities of which become inverted by papillary projections from the corium. The central cells of the solid downgrowths undergo alteration to form the hair, while the peripheral cells are retained to form the lining cells of the hair-follicle. The cellular structures of the sudoriferous and sebaceous glands are formed from the ectoderm, while the connective tissue and bloodvessels are derived from the mesoderm. Under the greater part of the body of the nail, the matrix is thick, and raised into a series of longitudinal ridges which are very vascular, and the color is seen through the transparent tissue.
The nails grow in length by the proliferation of the cells of the stratum mucosum at the root of the nail, and in thickness from that part of the stratum mucosum which underlies the lunula. They vary much in length, thickness, and color in different parts of the body and in different races of mankind. When the hair is of considerable length the follicle extends into the subcutaneous cellular tissue. Opening into the follicle, near its free extremity, are the ducts of one or more sebaceous glands. The hair bulb is moulded over the papilla and composed of polyhedral epithelial cells, which as they pass upward into the root of the hair become elongated and spindle-shaped, except some in the center which remain polyhedral. Some of these latter cells contain pigment granules which give rise to the color of the hair. It is composed of rows of polyhedral cells, containing granules of eleidin and frequently air spaces.
The sebaceous gland is situated in the angle which the Arrector muscle forms with the superficial portion of the hair follicle, and contraction of the muscle thus tends to squeeze the sebaceous secretion out from the duct of the gland. Transverse section of hair follicle. Each gland consists of a single duct, more or less capacious, which emerges from a cluster of oval or flask-shaped alveoli which vary from two to five in number, but in some instances there may be as many as twenty. Each alveolus is composed of a transparent basement membrane, enclosing a number of epithelial cells. The outer or marginal cells are small and polyhedral, and are continuous with the cells lining the duct. The ducts open most frequently into the hair follicles, but occasionally upon the general surface, as in the labia minora and the free margin of the lips.
On the nose and face the glands are of large size, distinctly lobulated, and often become much enlarged from the accumulation of pent-up secretion. The tarsal glands of the eyelids are elongated sebaceous glands with numerous lateral diverticula. Body of a sudoriferous-gland cut in various deirections. Longitudinal section of the proximal part of the coiled tube. Transverse section of the same. Longitudinal section of the distal part of the coiled tube. The size of the glands varies.