a greater number of afferent vessels the caliber of which is increased. In regions where the skin is very mobile the arteries are greatly convoluted. All these vascular peculiarities are present in the newborn (Spalteholz).

The lymph-vessels of the true skin are also distributed in two layers a deep and wide-meshed plexus in the subcutis, and a superficial narrow-meshed plexus immediately beneath the papillæ. Into the latter empty the lymph-vessels coming from the papillæ. After treating the skin by certain methods, a fine precipitate may be noticed here and there in the papillary region of the corium, a proof that lymph clefts are present. These are regarded as the beginnings of the cutaneous lymphatic system. They may also be

Fig. 291.-Nerves of epidermis and papillæ from ball of cat's foot; X75.

traced into the epithelium, where they are in direct communication with the interspinal spaces between the epithelial cells (Unna). Cells are also met with in the interspinal spaces of the epidermis; these are migratory cells, or cells of Langerhans.

The skin owes its great sensitiveness to the numerous nerves and special nerve-endings present, not only in the epithelium, but also in the corium and subcutis. In certain regions of the skin the nerves have been traced into the epithelium. In the finger-tip, for instance, numerous nerves are seen in the epidermis, where they branch and end in telodendria with or without small terminal swellings. There is no direct communication between the terminal

nerve filaments and the epithelial cells. (Fig. 291.) In certain peculiarly sensitive regions, as the end of the pig's snout, the nervefibers end in distinct saucer-like discs (tactile menisci) which, as a rule, clasp the lower ends of the basal Malpighian cells.

The special sensory nerve-endings are situated in the corium and subcutis. Of these, we may mention the tactile corpuscles of Meissner, the end-bulbs of Krause, the Pacinian corpuscles, Ruffini's nerve-endings, and the Golgi-Mazzoni corpuscles. All these special sensory nerve-endings with the exception of the two last mentioned have been discussed in a former chapter (p. 154). Meissner's tactile corpuscles are situated in the tactile papillæ of the true skin. They are especially numerous in the hand and foot. In the distal phalanx of the index-finger every fourth papilla is a tactile papilla, containing one or sometimes two corpuscles of

Fig. 292.-Meissner's corpuscle from man; Fig. 293.-Meissner's corpuscle from man;
X 750. Technic No. 295.
X 750. Technic No. 295.

Meissner. They are, however, not nearly so numerous in other parts of the hand or in the foot. These corpuscles are further found on the dorsal surface of the hand and volar surface of the forearm, in the nipple and external genitals, in the eyelids (border), and in the lips. In figures 292 and 293 are shown two Meissner's corpuscles, giving the appearance presented by these end-organs when not stained with special reference to nerve terminations. For the latter see figure 132.

The Krause's end-bulbs, both spheric and cylindric, are, as a rule, situated a short distance below the papillary layer, although they are frequently found in the papillæ. They occur in man in the conjunctiva, lips, and external genitals, and in the mucous membranes previously mentioned (p. 154). See page 154 and figure 131 for their structure.

In the palm of the hand and sole of the foot, the subcutaneous connective tissue contains numerous Pacinian corpuscles. They occur also along the nerve-fibers of the joints and in the periosteum of the extremities. (See Fig. 135.)

Very recently Ruffini demonstrated in the human corium the existence of peculiar nerve end-organs, which consist of a connective-tissue framework supporting a rich arborization of telodendria. They occur side by side with the Pacinian corpuscles and in apparently equal numbers. These nerve terminations resemble in many respects the neurotendinous spindles (see Fig. 140), although they present certain structural differences. Instead of intrafusal tendon fasciculi, the Ruffini end-organ is composed of white fibrous and elastic tissue. In this end-organ the medullated nerves make long and tortuous turns before becoming nonmedullated, and the

Terminal disc of
nerve-fibers.

Epithelial cell.
Connective-tissue
capsule.

Nerve-fiber.

Fig. 294.-Grandry's corpuscles from bill of duck; X 500. Technic No. 296.

The blood vessels of the skin are richly supplied with vasomotor nerves, which terminate in the nonstriated muscle of the vessel walls. These vasomotor nerve - fibers are neuraxes of sympathetic neurones.

In aquatic birds, and more especially in ducks, the waxy skin of the beak and the cornified portion of the tongue contain the socalled corpuscles of Herbst, which resemble the Pacinian corpuscles in general structure, but have cubical cells in the core. In the same tissues are also found the corpuscles of Grandry, 60 μ long and 40 μ broad. They consist of a thin connective-tissue capsule, containing two or three large cells. The nerve-fiber retains its medullary sheath for some distance within the capsule. The axis-cylinder ends in discs situated between the cells inclosed by the capsule.

B. THE HAIR.

The hair and nails are regarded as special differentiations of the skin. Hair is found distributed over almost the entire extent of the skin, varying, however, in quantity and arrangement in different

regions. None whatever is present in the palm of the hand and sole of the foot. In the third fetal month small papillary elevations of the skin are seen to develop in those areas in which the hairy growth later appears. Under each of these elevations there occurs a proliferation of the cells of the Malpighian layer downward into the corium. Although the elevations soon disappear, the epithelial ingrowth continues and finally forms the hair germ. This is soon surrounded by a connective-tissue sheath from the corium, in which two layers may be distinguished. At the lower end of the hair germ the corium is pushed upward, forming a papilla which penetrates into the thickened bulb of the germ. This is called the hair papilla. In the mean time the hair germ itself is undergoing marked differentiation. An axial portion, forming later the hair and inner root-sheath, and a peripheral, constituting later the outer rootsheath, are developed. From the latter are derived also the first traces of the sebaceous glands, which in the adult state are in close relationship to the hair and empty their secretion into the space between the hair and its sheath. As soon as the various layers of the hair are complete it grows outward, breaking through the overlying layers of the epidermis.

The visible portion of the hair is called the hair shaft, and that portion below the skin is the hair root. The lower portion of the hair resting upon the papilla is known as the hair bulb, and the sheaths encircling the root and bulb are called the rootsheaths, the entire structure constituting the hair follicle.

The adult hair is covered by a thin cuticle, consisting of overlying plate-like cells, 1.1 μ thick, most of which possess no nuclei. Beneath the cuticle is the cortical layer, composed of several strata of long, flattened cells from 4.5 μ to 11 μ broad and provided with nuclei. These are also known as the cortical fibers of the hair. Upon treatment with ammonia the fibers separate into delicate fibrils, the hair fibrils (Waldeyer, 82). Scattered between and within the cells of the cortical layer are varying quantities of pigment granules. The axial region of the hair is occupied by the medullary substance, from 16 to 20 μ in diameter. This may be lacking; but if present, consists of from 2 to 4 strata of polygonal, nucleated and pigmented cells. The hair shaft often contains air vesicles.

The inner root-sheath consists of three concentric layers-first, of an outer single layer of clear nonnucleated cells, the so-called layer of Henle; second, of a thicker middle layer, made up of two strata of nucleated cells containing eleidin, the layer of Huxley; and, third, of an inner cuticle, bordering upon the hair.

The outer root-sheath is made up of elements from the stratum germinativum. Here we have to do with prickle cells, surrounded by an outer layer of columnar elements. The connective-tissue portion of the hair follicle is composed of an outer, looser layer of longitudinal fibrous bundles; of an inner, compacter layer of circu

lar fibers; and of an innermost well-developed basement membrane-the glassy membrane.

At a certain distance above the root bulb all the layers of the

Fig. 295.-Longitudinal section of human hair and its follicle; X about 300.

epithelial portion of the hair follicle are well developed and distinct from each other. This condition changes toward the hair papilla