INTEdUMENT OF MAMMALS 139 



INTEGUMENT OF BIRDS. 



In birds the skin is thin hut is covered by scales and feathers 

 formed by the epidermis. The epidermis has a superficial layer 

 of dead cornified squamous cells resting on a few layers of poly- 

 hedral cells. The scales covering the legs and the claws of the toes 

 are formed by accumulation of cells in the corneum. Glands are 

 limited to the base of the tail, where the uropygial glands secrete 

 an oily substance used mainly in dressing the feathers. The dermis 

 is composed of dense fibrous connective tissue adjoining the epi- 

 dermis, but in the deeper region there is a looser fibroelastic con- 

 nective tissue carrying blood vessels and containing numerous fat 

 cells. Pigment cells are generally absent from the corium, the 

 coloring of birds being due to pigment secreted in the epidermal 

 cells of the feathers. The bill, or beak, of lairds is formed by the 

 accumulation of horny epidermal cells. 



Feathers.— The beginning of feather development is very similar 

 to that of scale development: a dermal pajiilla thrusts out with a 

 covering of epidermis and then the base sinks into the dermis. The 

 dermal pajjilla carries blood vessels and nerves to this epidermal 

 covering which alone is responsible for the feather formation. 



INTEGUMENT OF MAMMALS. 



Among the mammals, both layers of the skin are usuall\' much 

 thicker than in the forms previously described. IVIulticellular 

 glands and epidermal formations, the hairs, are abundant. The 

 structin-e of the epiflermis varies considerably in different parts of 

 the mammalian body. In exposed regions where there is consider- 

 able friction, the epidermis is extremely thick because of an accu- 

 mulation of cornified cells in the corneum. Over the body generally 

 the skin proper is broken by numerous projecting hairs, as well as by 

 the ducts of sweat and sebaceous glands deri^'ed from the activity 

 of the stratum germinativum. Pigment may be found in the ejji- 

 dermal cells of most mammals. 



The dermis composed of interlacing bundles of fil)roelastic con- 

 nective tissue divisible into two blending layers, an upper narrow 

 one, called the subepithelial or ])a])illary layer, and a dee])er, denser, 

 wide reticular layer. Immediately below the basal epithelial cells 

 of the epidermis lies a fine network of reticular fibers and reticu- 

 lar cells. These fibers intermingle with the elastic and collagenous 

 fibers to form a network generally paralleling the skin surface. 



