Book - Vertebrate Zoology (1928) 17

CHAPTER XVII

THE SKIN, AND ITS DERIVATIVES

The skin forms the outermost layer of the body, and its functions are protective, excretory, and sensory ; for all information which the animal receives concerning the outer world must come through the skin. Correlated with these functions, it is found that the constituents of the skin may undergo various modifications.

The skin is formed of an outer ectodermal layer, the epidermis, and an inner mesodermal layer, the dermis. In Amphioxus the epidermis is only one-cell thick (as in most invertebrates) ; while in all Craniates it is several layers of cells in thickness. Of these, the innermost form the stratum germinativum (or stratum Malpighi) which constantly pro- duces new cells, while the outermost layers tend to become horny forming the stratum corneum. As the cells become horny the protoplasm within them dies, and they become worn away by friction with the environment and replaced from the stratum germinativum. In many reptiles and amphibia, it is common for the superficial layer of the epidermis (overlying the horny scales) to be sloughed off all at once and replaced.

The epidermis may be ciliated in early stages of develop- ment in the lower forms, such as Amphioxus and the frog tadpole.

The epidermis covering the eye becomes very thin and transparent forming the conjunctiva. Sensory cells are present in the stratum germinativum, and it will be remembered that the sensory epithelium of the nose, of the eye, the ear, the lens and the placodes which contribute nerve-cells to the cranial ganglia, are all formed from the epidermis.

The skin excretes by means of glands which may be com- posed of single cells or many cells. Examples of the latter are to be found in the mammary, sebaceous, and sweat-glands of the mammals. These glands arise in the epidermis and project inwards into the underlying dermis. In some animals the glands may be modified into poison-glands ; and in deep- sea fish they may produce a luminous secretion.

The epidermis may be modified into a variety of structures such as horny scales (corneoscutes) which are present in reptiles, birds (chiefly on the feet) and mammals (all over the body of the Pangolin, at the base of the tail of the rat). The epidermis also gives rise to feathers which are characteristic of birds (see p. 224) ; hairs which are characteristic of mammals (see p. 234) ; the termination of the digits which may take the form of claws, nails, or hoofs ; and the horny covering of the beak in tortoises and birds. The " horns " of cattle are formed of a layer of epidermal horn overlying a central dermal bony core. The horn of the rhinoceros is made of fused hair. Special epidermal structures on the edge of the mouth of Petromyzon, frog tadpoles, and Ornithorhynchus, give rise to the so-called " horny teeth," which have nothing to do with true teeth. Lastly, the epidermis produces the cap of enamel which forms a covering to the dentine of denticles and true teeth.

Hairs and feathers are commonly moulted at intervals and replaced.

The dermis forms the leathery layer of the skin. It con- tains blood-vessels which serve to supply the cells of the epidermis as well as those of the dermis, and especially the papillae at the bases of hairs and feathers, and the glands. In amphibia this dermal circulation also serves respiratory pur- poses, and in the mammals it forms part of the mechanism for regulating the heat of the body. In amphibia the dermis is separated from the underlying muscles by lymph-spaces, but in higher forms the skin is firmly attached to the muscles by connective tissue. In higher forms, special muscles arise in connexion with the dermis. Some of them are attached to scales, feathers, or hair-follicles, which they move. It is by the contraction of these (smooth) muscles in mammals that hair is made to " stand on end," and the puckering of the skin round the hair-follicles gives rise to the condition known as " chicken-skin. " In addition to these dermal muscles, there are in the higher forms, and especially in the mammals, sets of muscles beneath the skin and which move the skin as a whole. The panniculus carnosus muscles are in the region of the trunk and they serve to shake the skin. (They are of somatic origin.) In the head and neck regions the platysma muscles (of visceral origin) move certain parts of the skin such as the lips, eyebrows, and ears. In man, these are the muscles of expression. The smooth dermal muscles are innervated by sympathetic fibres, the panniculus carnosus by ventral nerve-roots, and the platysma by the facial nerve.

Just as the cells of the epidermis seem to be prone to the production of horn and horn-like structures, so the cells of the dermis seem to run to the formation of bone and dentine. Dentine is the substance of which denticles and teeth are formed, under the epidermal cap of enamel. The bone pro- duced in the dermis takes the form of dermal or membrane- bone, bony scales, or fin-rays (lepidotrichia). In Selachii the dermis forms dentine but no bone.

Dermal bones are widely distributed over the body in forms above the Selachii. They play an important part in the formation of the skull, and of the pectoral girdle. In some animals, the body may be entirely covered by an armour of bony plates, as in the Labyrinthodonts, or the armadillos. These bony plates are osteoscutes, and remnants of them are to be found in the carapace and the ventral shield (or plastron) of the tortoise, and in the so-called abdominal ribs or gastralia of Sphenodon, crocodile, Plesiosaurs, Ichthyosaurs, Pterosaurs, and Archaeopteryx (see Fig. 160). Osteoscutes are also present in Gymnophiona, lizards, and crocodiles.

In the fish, the dermal bones come into relation with the overlying denticles, forming complex scales. In the Osteolepidoti and primitive (extinct) Dipnoi, the denticles have fused together forming a layer of " cosmin," and this is attached to the underlying bony plate, which forms the so-called " isopedin " layer. This is the " cosmoid " scale. In the primitive (extinct) sturgeons (the Palaeoniscoidea) and in Polypterus, the layer of cosmin is not only covered by bone underneath (the isopedin), but also on top, the superficial layer of bone being called the ganoin. This type of scale is called palaeoniscoid. In Lepidosteus the structure of the scale is similar, but the layer of cosmin has disappeared, and the scale consists simply of a layer of ganoin overlying a layer of isopedin. This is the lepidosteoid type of scale. The palaeoniscoid and lepidosteoid scales are of course beneath the epidermis since the layer of ganoin (bone) is a mesodermal structure. The epidermis overlying these scales may possess true denticles. It is also worth noticing that the structure of the dermal bones and of the dermal fin-rays (lepidotrichia) in a given animal tends to be identical with that of the scales.

In the higher bony fish or Teleosts, the scales lose the layer of ganoin. The scales form in the dermis, but the bone cells become lost and the scales are very thin. It is obvious that these dermal scales together with the dermal scales of Gymnophiona and lizards (osteoscutes) must not be regarded as having anything in common with the epidermal scales (corneoscutes) of higher forms. Dermal scales are retained throughout life ; epidermal scales and denticles are shed.

Other examples of dermal ossifications are to be found in the bone (os corneum) which forms the core of the " horn " of cattle, and which becomes attached to the frontal bone of the skull. Similar little bones form the knobs on the head of the giraffe, while large bony structures in this position give rise to the antlers of deer. Antlers are restricted to the males, they may be forked, and they are shed every year. The size of the antler often bears an interesting relation to the size of the body (see p. 482). Horns, on the other hand, may be present in both sexes, and, except in Antilocapra (the American prong-buck), they are neither forked nor shed.

Lastly, when dealing with the skin, mention must be made of colour. Pigment-cells may occur in the epidermal and the dermal layers of the skin. In some cases, the pigment-cells are capable of altering the distribution of their pigment, with the result that the animal may change colour (as, for example, the frog, or the chamaeleon). Pigment may also be present in feathers and in hair, but in these structures the texture of the surface may also produce effects of colour without any pigment being there.

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Fig. 120. — Sections through the skin of Scyllium embryos, showing the mode of development of the placoid scales or denticles.

d, dentine ; e, ectoderm ; ec, modified ectoderm cells which produce the enamel ; en, enamel ; m, mesoderm ; 0, odontoblasts, mesoderm cells which produce the dentine ; pc, pulp-cavity.

Literature

Goodrich, E. S. Vertebrate Craniata, Cyclostomes and Fishes. Black, London, 1909.

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