Part 88

The touch corpuscles are the largest in the mucous membrane of the cloaca and smallest in the skin. Hess has found these touch corpuscles in the large filiform papillæ on the side of the tongue and a few on its lower surface. They are also found in the soft skin of the edges and inner borders of the beak.

STRUCTURE OF APPENDAGES

The skin of the fowl is very thin and does not contain oil glands or sweat glands. In the fowl the oil is supplied by a tail or rump gland, the glandula uropygii (Fig. 35, No. 16). This gland is round or oval in shape, and about the size of a pea and consists of two lobes. It is of the tubular variety with a teat, which, in most instances has two openings. A medium septum divides the gland into two halves. The oil secreted by the columnar epithelial cells is collected in a body cavity located in the center of the gland; this has a duct extending to the surface. The bird by squeezing out a quantity of this oil into its beak oils the feathers, passing the beak over them, one by one. The oil renders the feathers practically impervious to water. It is necessary for the bird to give proper attention to its plumage in order that the feathers appear in prime condition. Should there be a disease of the oil gland, or should the bird become ill and neglect its toilet the result is an unkempt appearance of the plumage, the feathers becoming rather rough and more or less injured by the weather.

The subcutis is well developed and furnishes to the cutis great capability for movement, which is necessary for the rising and falling of the feathers. The corneum is very thin. Papillary bodies are present only in a very few places, such as the region about the eye and on the toes. Where the toes touch the ground in walking there are large wart-like thickenings of the epithelium. In most birds the shanks are unfeathered. The epidermis on the feathered parts of the skin is thin, dry on the surface, and abounds in continuous scales. The stratum corneum is very thick on the horny sheath of the beak, on the top of the toes, on the spurs of the cock, and on the scale plates that cover the skin on the shanks. The feathers of birds serve the same protective purpose as hair on mammals. In cold weather the skin muscles controlling the feather movements contract; thus the feathers become ruffled much as we observe the hair standing erect on horses under similar conditions. By increasing the dead air space around the body, the radiation of heat is retarded, and the body kept warmer. The corneum of the skin is not usually rich in blood-vessels; however, in the domestic fowls there is in the comb, in the wattles, and similar appendages of the head, a thick vascular network.

The beak and the claws are modified skin; they are true horn material. At the base of the beak there is often formed scales, which surround the nostrils, in whole or in part, and have a naked, or waxy, appearance, which is known as the cera.

FIG. 79.—Photomicrograph of the section of skin from the sole of the foot of a hen. 1, Horny stratified squamous epithelium. a, Stratum corneum. b, Stratum lucidum. c, Stratum germinativum. 2, Connective tissue supporting membrane, pars reticularis. 3, Blood-vessels. ]

The feathers may be considered of two chief kinds, the quill feathers and the clothing feathers. The most rudimentary of the latter are known as down. A quill feather consists of two principal parts: the quill, or calamus, and the vane, or vexillum. The quill is continuous with the central shaft, called the rachis, the two forming the stem of the feather. Projecting outward from the stem on each side are a large number of pointed and very flexible barbs. These barbs are located nearly at right angles to the quill and have extending from them at right angles smaller processes or barbules. These barbules hook together the barbs and give the web its form. In some feathers, as the hackle, and in the wing bar feathers of the male of some breeds, there is a portion of the upper outer edge of the feather not provided with barbules, which fact gives the feather its characteristic appearance. The down feathers are loose and fluffy. In this kind of feather the shaft is weak, and the barbs are not provided with barbules. The barbs like the shaft may be considered weak. These feathers give great warmth to the body. There are fiber feathers appearing as hair-like filaments, and called filo-plumæ. These are found scattered over the body; they are particularly abundant in the region of the head and the neck.

At each end of the quill is a small opening, or umbilicus. Inside the barrel of the feather there is pulp, which in young feathers is very vascular; the vessels entering by the proximal umbilicus are buried, along with part of the quill, in a papillated follicle of the skin. From this follicle the feather is developed. At the base of the shaft a secondary rudimentary quill is usually formed, which may be represented by a mere tuft of down. On the same general principle the smaller feathers are constructed. These cover the body, the upper parts of the legs, and the head, while the larger feathers and quills are confined to the wings and the tail. The longest quill feathers are those arising from the hand, called the primaries. Those arising from the forearm are called the secondaries. Those that are developed from the proximal part of the arm are called the tertiaries. The rudimentary pollux carries some feathers which form the alula, or bastard wing. The scapularies are feathers covering the scapula and the humerus. Covering the bases of the larger flight feathers are wing coverts consisting of several rows of small feathers. The quill feathers of the tail are called the rectrices. The rectrices have considerable mobility; their bases are covered by a row of tail coverts.

The pedal digits of the natatores are joined by a membrane, covered with scaly skin, which forms the web foot.

The feathers in many parts of the body are developed in rows, there being intervals, or elongated skin areas, between these groups of several rows, which are not provided with feather papillæ, but which are covered over by the feathers developed in front of these spaces. The definite feather lines, or areas, have been called pterylæ. The intervening tracts devoid of feathers are called the apteria.

The first outer covering of the bird, or baby chick, is a temporary one consisting of fasciculi of long filaments of down. These fasciculi on their first appearance, are enveloped in a sheath, which soon becomes ruptured and are entirely cast off by the time the baby chicks are ready to be taken from the nest or the incubator.

FIG. 80.—Photomicrograph of the skin of the neck of a S. C. White Leghorn hen. 1, The skin possessing an outer stratum, the stratum corneum, next the stratum lucidum, next the stratum granulosum and underneath the stratum germinativum. Beneath the outer dark band representing the upper skin strata is the pars reticularis of the derma. 2, Papilla showing from inside to outside the hyaline feather wall, the stratum corneum, the Malpighian layer of the follicle, the corium. Inside the feather is noted the pulp-like material. ]

The down fasciculi, each emerging from its small quill, are succeeded by the feathers, which they apparently guide through the skin.

Feathers do not spring from all parts of the body alike; especially devoid of feathers are those parts where chafing and friction is greatest, as under the wings and in the groin.

At the end of the quill there is a small opening, the inferior umbilicus, into which projects a papilla of the dermis. Where the quill emerges from the skin there is another small opening, the superior umbilicus, from which springs frequently a small feather called the hyporachis. The shaft, or rachis, has a groove extending along that surface which lies next the body.

In the newly formed chick the first indication of feathers is the formation of papillæ, which is constituted by the upward growth of the dermis, or the sensitive and vascular parts of the skin. Then the skin immediately around the papilla sinks downward, so that later the papilla is inclosed in a follicle of the skin. The epidermis over the papilla is the same as over the rest of the surface. The horny outer layer of the epidermis forms for the growing feather a protective sheath which is cast off as the feather is formed. The feather proper develops from the underlying germinative layer, which as the feather develops, forms a cylinder of cells. The lower part of the cylinder is in touch with the papilla; this later becomes the quill (Fig. 80, No. 2). The upper part of the cylinder develops the web portion of the feather. As soon as the feather is fully developed the papilla, which has projected into the quill and nourished it, is withdrawn, and the quill becomes filled with a pithlike material forming septa, which extend in different directions.

Once a year, usually in the late summer or in the fall, the entire feather coat is changed. This process is called molting. During this time the bird appears in a somewhat depressed condition, the hen almost always ceases laying. Birds also molt in the spring, to a limited extent. The male at this time takes on the so-called breeding plumage, which is the most beautiful of the year. It has recently been established by Rice, that the young fowl, in reaching a stage of egg production, molts five times before the laying period begins.

The =structure of the skin= of fowls is similar to that of the skin of mammals. The skin consists of two layers the outer portion, or epidermis, and the inner true skin, the cutis, corium, or dermis. If we study a section of skin from the shank region of a fowl we find the outer portion is differentiated into two distinct regions, the rete Malpighii and the stratum corneum. The stratum corneum is the outer horny layer. The cells making up this portion are fusiform, flattened, and in regular rows. The nuclei in these layers of cells are not pronouncedly visible, and the outline of the cells not clear in a section such as used in our ordinary methods of study. The corneum is a compact mass of remnant cells which have lost the appearance and their texture of living cells. It thus becomes modified into scales upon the skin surface.

Between the stratum corneum and the stratum Malpighii there is another zone which consists of the cells undergoing a transitional stage from a cubical shape to a more flattened appearance, and gradually becoming more granular and hyaline.

The various parts of the epidermis are in close genetic relationship to one another. The upper layer of epithelium is constantly being desquamated. This casting off is compensated for by a continuous upward pushing of its lower elements. Cell proliferation occurs in the basal cells and in adjacent cellular strata of the stratum germinativum, or stratum Malpighii, where the elements are often seen in process of mitosis, or cell division. The young cells are gradually pushed upward. During their course they assume the general characteristics of the elements composing the layers through which they pass. This process is as follows: each cell changes first into a cell of the stratum Malpighii; then, when it commences the formation of keratohyalin, it changes into a cell of the stratum granulosum; later still, into a cell of the stratum lucidum; and finally into an element of the stratum corneum, where it gradually loses its nucleus, cornifies, and at last drops off.

The mesodermic portion of the skin consists of loose, subcutaneous connective tissue containing some fat. The amount of adipose tissue in the subcutaneous layer is subject to great variation; there are a few places in which there is little or no fat. The upper portion of this layer contains a few elastic fibers, which interlace and run in all directions. Numerous round or oval cells are found in the upper region. The lower and middle portions of the corium are richly supplied with blood-vessels terminating into capillaries, which penetrate the portions bordering the epidermis. Nerves giving off terminal branches also occur.

The various colors of the skins of fowls are due to the distribution of various quantities of two colors, orange-yellow and brownish-black.

The yellow pigment is probably carotin and xanthophyll, two pigments contained in association with the chlorophyll of plants, which the bird obtains in its feed. These coloring matters were formerly called lipochrome; but as lipochrome may be any coloring matter of fat, it is not sufficiently definite. This yellow pigment, when present, is diffused through all parts of the cell. When dilute, it gives a yellow hue; when concentrated, orange. It is found in the epidermis and in the fatty masses of and beneath the corium, and is probably identical with the yellow color of fat in other portions of the body and in the yolk of the egg. A yellow shank, in a heavily laying hen, soon loses a part of its pigment; this is also noted of the coloring matter in other parts of the body. This fact indicates that the coloring matter of the fatty part of the egg yolk is from the same source as that of the fat. The draught of this substance is more than normally and the reserve is being drawn upon. Feed rich in zanthophyll and carotin cause intense yellow colored yolks, and feeds poor in these substances cause the yolks to be a pale yellow. Cotton seed meal contains two pigments, one a yellow crystalline substance and the other a brownish resinous substance. Both of these pigments are probably deposited in the egg yolk. The eggs of some hens contain a large quantity giving a light brownish-yellow color to the yolk.

The brown or black-brown pigment is carried in microscopic pigment granules, which may be scattered through the ordinary cells or may be confined to special pigment cells. The former are confined to the epidermis; the latter may be found in both layers, but infrequently in the epidermis. When granules are present in the flattened cells of the corium, they occupy the nuclear region. They lie in short thin lines while those of the under portion of the Malpighian layer occur in oval groups. Where these granules occur in the rete layer, they cluster around the nuclei. In the colored skin there are dark pigment granules found in the corium and to a less extent in the rete layer. Hanau has described a definite cellular body which he found densely packed among the black-brown granules. There is a central body which sends out branches in all directions. In very dark skinned shanks these ramifying strands interlace, and form a compact network, which in many cases is so thick as to give the impression of a homogeneous mass. Here and there occur round or oval pigmented bodies, which Hanau concluded were the star-shaped cells with their pseudopod-like appendages contracted. Pigment cells commonly lie around blood-vessels, clearly indicating their course. They frequently form a compact tube, but more often are limited to fragments which only partly enclose the vessels. Pigment cells occur in several well-defined localities: in the upper portion of the cutis among the closely interwoven strands of connective tissue; in the region bordering the blood-vessels; in proximity to nerves, in nerve endings and in surrounding fat masses. Isolated granules are frequently scattered throughout the lower section of the corium. Barrows concludes that the lower bodies of pigment play little or no part in the color of the external shank, as they lie far below the opaque connective tissue. Melanin pigment granules are always contained in the pigment cells. When found in the Malpighian layer, the pigment cells are of an oval form.

Immediately below the epidermis in the shank skin, extends a space less in width than the row of columnar cells, which is devoid of pigment. The brown pigment is melanin, which in large quantities takes on a black hue.

White skin does not contain superficial pigment. Melanin has been observed in the study of white shank skin, but it lay at considerable depth or in quantities insufficient to be noticeable. In some breeds of fowls, as the Mottled Houdan, there are areas in which much melanin is irregularly deposited, which circumstance gives the leg its mottled appearance.

The yellow color is the result of a deposit of the yellow pigment in the fat of the shank. This may be deposited in both layers of the skin, or in the corium alone. When present it is diffused throughout the entire cell as well as throughout the intercellular substance. In young birds the Malpighian layer contains much yellow pigment. Old hens have only small quantities in the corneum. In breeds with normal yellow shanks old hens that have never laid eggs show a deep orange color in both the dermis and epidermis.

In blue shanked birds melanin is present only in the corium. The black pigment is seen through the semi-transparent Malpighian stratum, making it appear bluish instead of brown or black.