INTEGUMENT. 



33 



there are formed a series of rods set at an acute angle to the undivided dorsal strip, 

 the future shaft. When set free, as before, by the rupture of the periderm, these 

 rods straighten out, forming the vane. In the region of the shaft there are two 

 longitudinal ridges on the ventral side. These gradually roll together, thickening 

 and strengthening the shaft, the groove between them forming the umbilicus. As 

 will be understood, the dorsal and ventral sides of the feather were the outside and 

 inside of the stratum corneum of the papilla. 



The corium 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 interfer- 

 ence spectra. 



FIG. 25. Stereogram of part of developing contour feather; compare with fig. 24. b 

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



MAMMALS have a skin relatively thicker than have other verte- 

 brates, both layers contributing to the thickness and the whole 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. 27) are widely distributed and scales occur in several forms. 



The corium is thick and composed of irregularly interlaced fibres with muscles, 

 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, proliferate rapidly and the 



