INTEGUMENT 37 



The corium of birds is thin and consists of irregularly interlaced fibres; it is 

 rich in sense (tactile) organs and smooth muscle fibres, which are largely used in 

 elevating the feathers. The colors of feathers depend in part upon pigment — 

 red, yellow, orange, brown, and black — deposited in them, but the iridescent 

 colors are due to interference spectra. 



MAMMALS have a skin relatively thicker than have other verte- 

 brates, both layers contributing to the thickness and the whole is 

 rather loosely attached to the lower tissues. There are numerous 

 glands, and the hair, abundant in all orders except the whales and 

 sirenians, is found in no other class. Other cuticular structures as 

 horn and claws (p. 31) are widely distributed, and scales occur in 

 several forms. 



I iG. 28. — btereogram of part of developing contour feather; compare with fig. 24. b, 

 developing barbs; pc, pith cavity; per, periderm; s, rhachis. 



The corium is thick and composed of irregularly interlaced fibres with mus- 

 cles, blood vessels, etc. Its outer surface is frequently thrown into papillae or 

 ridges, especially on the palms and soles, these carrying the epidermis with them. 

 In the thick epidermis several strata may usually be recognized: at the base a 

 thick Malpighian layer; then a thin stratum lucidum in which distinct cells 

 cannot be recognized; and on the outside the stratum corneum. One or more 

 others are sometimes present. As will readily be understood a cell passes 

 through all of these layers before it is worn from the surface of the skin. 



Hair. — The epidermis takes the initiative in the formation of 

 hair. It thickens in spots, the thickenings pushing into the corium 

 and each being cupped at the tip, blood-vessels extending into the 

 cup. The basal cells of the ingrowth, thus richly nourished, pro- 

 liferate rapidly and the new cells thus formed are forced outward, 

 rming the hair. While this is going on the ingrowth splits around 

 the hair, forming the follicle, while another ingrowth of the Mal- 

 pighian layer forms the sebaceous gland which oils the hair. 



\ section through a hair and its follicle gives the following layers (fig 29). 

 round all is the connective-tissue envelope, formed from the corium; next 

 -ide is the outer root sheath formed of the Malpighian layer and extending 



