-cuticle 



secretion of outer layer (striated) 



m . 



tri — pigment granules 



epidermal (sensory and secretory cells) 



outer fibrous layers (at right angles) 



middle gelatinous layer with 

 ^cutaneous canal (lined) 



dermal cell 



inner fibrous layer 

 inner cellular layer 



Figure 8-2. Section through the skin of omphioxus. 



derived. New generations of cells are delaminated from this 

 layer and move outward with each subsequent generation 

 until they reach the surface and are sloughed off. 



The dermis is a fibroelastic connective tissue in which the 

 multicellular glands of the epidermis are embedded. This 

 layer of the skin is formed around interlaced collagenous 

 and elastic fibers in a gelatinous matrix. The outer layer is 

 usually the more densely fibrous, while deeper parts are more 

 open. This tissue may become bony or filled with fat cells. 

 Its surface capillary net is of importance in the maintenance 

 of the epidermis and, in some animals, for respiration. 



The skin functions in a number of ways. It forms an enve- 

 lope for the body, and a surface of contact with the environ- 

 ment. It protects the body from abrasion, presents a barrier 

 to disease, acts as a barrier to free exchange of material with 

 the environment (mainly water), and becomes involved in 

 body temperature control with the production of feathers 

 and fur. 



Mammals 



The epidermis of the mammal is covered externally by 

 the stratum corneum, a layer of flattened, keratinized, dead 

 cells. The thickness of this cornified layer varies according 

 to the amount of wear to which the skin is exposed. Thick 

 or thin, this layer functions as a protection against drying 

 and is lubricated by sweat and by sebaceous (oil secreting) 

 glands. In the epidermis of man there is a sequence of 

 cell layers. This sequence is as follows: stratum germinati- 

 vum at the base, stratum spinosum, stratum granulosum, 

 stratum lucidum, and stratum corneum. The cells of the 

 stratum germinativum are generally pigmented. This pig- 

 ment is produced by melanoblasts (melanin is the name of 

 the black or brown pigment) and passed from these cells to 

 chromatophores (literally "I carry color"). The term chro- 

 matophore is sometimes used as a synonym for melanoblast. 



The dermis, or cerium, has an outer papillary layer in 

 which there is a dense mat of fibers parallel to the surface. 



Below this is a reticular layer with much intercellular 

 material. The two layers grade one into the other. The col- 

 lagen fibers of the papillary layer are finer than those of the 

 reticular part. The cells of the dermis are mainly fibro- 

 blasts with a few macrophages and pigment cells (melano- 

 blasts). Fat cells vary in number and are particularly abun- 

 dant in the deeper part. 



Sweat glands are found over most of the body in many 

 mammals; they may be lacking in some rodents and rab- 

 bits, or limited to the pads of the feet in other rodents. In 

 man, these glands are particularly numerous in the thick 

 skin of the palms and soles. They are tubular glands extend- 

 ing from the surface far down into the dermis through the 

 interpapillary pegs of the papillae. Sweat glands are of two 

 basic types: the apocrine, which secretes a milky fluid, and 

 the eccrine, which produces a watery fluid. The eccrine, the 

 smaller of the two, is the typical sweat gland of the body 

 surface. 



The apocrine type occurs in the armpit, on the external 

 genitalia, around the nipples of the breasts, and around the 

 anus. Modified sweat glands produce the wax of the exter- 

 nal auditory meatus (outer ear opening), and give rise to the 

 Moll's glands of the eyelid. The sweat is expelled from the 

 secretory basal part by the contraction of the enclosing myo- 

 epithelial cells. 



The thin skin of the general body surface has irregular 

 dermal papillae; each of these has an interpapillary peg of 

 epidermis penetrated by a sweat gland. The body is covered 

 by hair. Each hair is embedded in a follicle and each fol- 

 licle has sebaceous glands opening into it. Sebaceous glands 

 are simple, branched, alveolar (little sacs) glands (Figure 

 8-3). Such glands are also found in the lips at the corner of 

 the mouth, on the glans penis and the internal fold of the 

 prepuce, on the labia minora, and on the nipple. In these 

 areas they ojjen directly on the skin surface and are not re- 

 lated to hairs. The Meibomian glands of the eyelid edge are 

 similar. 



The hair may be short or long and of different textures. 

 It is deciduous and continually replaced. Hairs may be modi- 

 fied into spines in the Echidna, the hedgehog, or porcupine. 

 Scales occur on the tails of rodents and over the entire dor- 

 sal and lateral aspect of the body of the pangolin (Figure 

 8-4). Hairs grow from between these scales, and, when 

 scales are lacking, the groups of hairs have the same pattern 

 as when scales are present. 



The stratum corneum is modified in the production of 

 horns (Figure 8-5), hoofs, claws, and nails. In these, and in 

 the scales of pangolin, a harder, more homogeneous keratin 

 layer is produced, which grows outward from a basal area 

 of origin. 



The horn of the rhinoceros is described as being formed 

 of hair, but it can also be described as formed of cornified 

 fibers or tubes embedded in a keratin matrix. The nature of 

 the follicles of these hairs is generally not described. The 

 horn grows upward from a basal generative area of the skin 



218 • THE SKIN AND ITS DERIVATIVES 



