(b.) Note the trabeculae of connective tissue-bounding wide spaces in the cavernous part. The whole is surrounded by a tough capsule, in which, dorsally, are sections of blood-vessels-one vein (DV) and two arteries (DA)—and nerves (fig. 304, N).

SUPRARENAL CAPSULE.

It is well to remember that there are great variations in the structure of this gland in different animals. The suprarenal capsule is a ductless gland, consisting of a cortical zone and a

medulla. It is invested by a fibrous capsule which sends septa into the gland. Especially in the cortex, these septa run so as to give a columnar arrangement to the cells which lie between them. The parenchyma of the organ consists of cells which vary in their characters in different regions. The cells of the cortex (15 μ) are polyhedral, nucleated, granular, yellowish-coloured cells, arranged under the capsule in rounded groups-zona glomerulosa. Next this is the widest zone-the zona fasciculata. Next the medulla

is the zona reticularis. In the medulla the cells are often irregular or polygonal with a clearer protoplasm, which is often tinged of a yellowish or brownish colour. There are numerous vessels and

nerves, the latter with ganglionic cells.

Harden the suprarenal capsules of a guinea-pig in Klein's fluid (5-7 days), and then in alcohol, or fix in Flemming's fluid. Harden a human suprarenal in Kleinenberg's fluid (24 hours), and then in alcohol. Make radial sections, and stain some in hæmatoxylin, and others in picro-carmine. Or stain and cut in paraffin.

12. V.S. Suprarenal Capsule.-(L) Observe the arrangement already described. It is to be noted, however, that there are great variations in the structure of these organs in different species of animals.

(H) Examine the cells in the various zones (fig. 305).

ADDITIONAL EXERCISE.

Termination of Nerves in Suprarenal Capsules.—The literature and most recent results will be found in Fusari's paper1 (with a plate). He used capsules of the mouse, rabbit, pig, cat, and new-born infant. The method employed was the quick method of Golgi (Lesson XXX.), i.e., small fresh pieces are placed in the osmico-bichromate fluid (3-10 days), and afterwards in 1 per cent. silver nitrate solution (1-2 days).

LESSON XXIX.

SKIN AND EPIDERMAL APPENDAGES.

THE SKIN.

THE skin consists of the epidermis and cutis vera, dermis, or corium. The epidermis consists of many layers of stratified squamous epithelium (p. 317). The corium is composed of a basis of fibrous connective tissue-white and yellow fibres-and its surface is thrown into a number of papillæ, which differ in size, number, arrangement, and form in different parts of the body. Undivided conical elevations are called simple papilla, but when these are beset with smaller papillæ, they are called compound papillæ. The epidermis completely covers in the apices of the

1 Archiv ital. de Biol., xvi. p. 262, 1891.

papillæ, and also dips down into the furrows between adjoining rows of papillæ, so that the surface of the skin is smooth, although the arrangement of the papillæ is readily detected by the lines on the palmar aspect of the hand and foot. The fibrous tissue of the cutis, next the epidermis, forms a very thin modified layer with scarcely any fibrils and no corpuscles. This layer acts the part of a basement membrane, and is continuous with the basement membrane of a sweat-gland. In the dermis, the bundles of white fibres interweave with each other, and form a dense tissue; at the lower part of the skin it becomes more open in texture, and gradually passes into the subcutaneous tissue. Elastic fibres in the form of networks exist in large numbers in the cutis; they are finer in the papillæ, and coarser lower down.

The subcutaneous tissue consists of a complex system of trabeculæ of fibrous tissue, and in some of the meshes are lobules of fatty tissue forming a fatty layer, constituting the stratum adiposum.

The arrangement of the blood-vessels is stated at p. 325. There are also numerous lymphatics and nerves-some of the latter with peculiar terminations-glands (sweat and sebaceous), and, in some situations, hairs with their hair-follicles.

It is important to distinguish between the hairy skin and the parts of the skin without hairs. The non-hairy parts are the volar surfaces of the hands, feet, fingers, and toes, nails, lips, mammary papillæ, certain parts of the external genitals, and the inner part of the external auditory meatus. The hairy parts are the remainder of the skin. The non-hairy parts are concerned with direct tactile sensations, the hairy parts with indirect tactile sensations, the hairs themselves being the chief tactile organs (Blaschko1). This observer has shown that the epidermis projects into the cutis vera in the form of septa, varying in form and distribution in different parts of the skin (p. 325).

Methods. The skin must be prepared in various ways according to the particular part which it is desired to study. For a general view proceed as follow:-(a.) Procure a fresh portion of human skin from the palm of the hand or sole of the foot, cut it into pieces about 1 cm. square, and remove most of the subcutaneous fat; pin it, epithelial surface downwards, on a piece of cork, and harden it in absolute alcohol (12 hours). Renew the alcohol for another twenty-four hours. Sections may be cut by freezing, and then stained with hematoxylin or picro-carmine (the latter to be mounted in Farrant's solution). Better still, stain the whole "in bulk” in borax carmine or hæmateïn, and embed and cut it in paraffin.

1 Archiv f. mik. Anat., xxx. p. 495.

Mount in balsam. Or skin so hardened may be double stained in bulk, first in borax-carmine and then in hæmateïn.

(b.) Harden the skin in Müller's fluid.

(c.) For the layers of the epidermis fix say small pieces of the skin in 1 per cent. osmic acid or Flemming's fluid, and harden in alcohol. Stain sections in safranin.

1. V.S. Skin, Palm of Hand.

(a.) (L) The epidermis, consisting of many layers of stratified squamous epithelium, resting on the cutis vera, dermis, corium, or true skin. The latter consists of connective tissue, and is provided with finger-shaped elevations or papillæ, which project into the deeper layers of the epidermis, the latter in the form of septa, dipping in between the papillæ (fig.

306).

(6.) The epidermis, composed entirely of stratified epithelial cells. Proceeding from the outside (fig. 307), observe

(i) The stratum corneum, of variable thickness, consisting of many layers of flattened or slightly fusiform, clear, non-nucleated cells united to each other. As the cells are seen on edge, they are very thin. Those on the surface are about to be shed, and consist of keratin.

(ii.) The stratum lucidum, a thin, narrow, clear, homogeneous layer, composed of two or more layers of flattened cells, containing

Epidermis.

Cutis vera.

sometimes a rod-shaped nucleus. FIG. 306.-V.S. Skin of Palmar Surface

The cells do not stain well with dyes. The eleidin granules seem to

become fused together and form the basis for cornification, as the cells are changed and become corneous.

(iii.) The stratum granulosum, a somewhat thicker layer, composed of ovoid cells two or three rows deep. Each cell is distinctly granular, and usually this layer stands out deeply stained, because its granules of eleidin or keratohyalin are stained with the carmine. The cells, like the foregoing, are devoid of "prickles."

(iv.) The stratum Malpighii, several layers of more plastic cells. At the lowest part, where they rest on the papillæ of the true skin, the cells-prickle-cells are smaller and columnar in shape (with oval, vertically-placed nuclei), but above this they become more spheroidal or polygonal, and each one is distinctly nucleated.

(c.) The cutis vera. The papillæ, conical elevations projecting into the Malpighian layer. They consist of compact fibrous tissue. The rest of the skin consists of bundles of white fibrous tissue interwoven with networks of elastic fibres, and at its lower part masses of fat-cells. The connective tissue and fat-cells below become continuous with the subcutaneous tissue, which is of a more open texture; but there is a gradual transition from the one

to the other. The nuclei of the connective tissue corpuscles appear as red oval dots.

In

In sections of the sudoriferous glands, their coils (in the deeper layers of the corium), their ducts running vertically through the skin, and a corkscrew passage in. the epidermis may be seen. some of the papillæ observe a touch-corpuscle (p. 320), and in granulosum, the subcutaneous tissue occasionally sections of Pacinian bodies (p. 320).

FIG. 307.-V.S. Human Epidermis with

Terminations of Nerve - Fibrils.

n.

(1.) (H) Observe in the epidermis the shape and characters of the successive layers of epithelium. In the Malpighian layers, "prickle-cells," i.e., cells connected with each other by fine "inter-cellular bridges," are better studied in an osmic acid

section (fig. 97). (See also Lesson IV.)

(i.) In the stratum Malpighii the lowest cells are arranged in a single layer of elongated, somewhat columnar cells (6-12 μ), with large oval nuclei surrounded by granular protoplasm.

The

lower ends of the cells frequently exhibit processes which fit into the dermis. The remainder of the cells of this layer are irregularly cubical, and exhibit prickles (Lesson IV., and p. 127). In the dark races the particles of melanin, which give the dark colour to the skin, are present in the cells of this layer, especially in the deepest layer of cells. Nuclei are sometimes seen in process of division.

(ii.) The cells of the stratum granulosum are arranged in two or more layers, and are flattened horizontally, so that they are lozenge