130 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Certain members of each class of vertebrates bear scales except the 

 amphibians, and in them scales are unknown. Most fishes and all reptiles 

 are more or less covered with scales; birds have them on their legs; many 

 mammals bear them on their tails — mice, rats, ground moles, opossums, 

 beavers; and armadillos have them on their bodies and tails (Fig. 8.3). 

 Fishes and reptiles are the typically scaly animals. In the yellow perch, sal- 

 mon, and other bony fishes, the scales grow out from pockets of connective 

 tissue in the dermis and overlap one another like shingles. Fishes do not 

 molt and scales keep growing and wearing off as long as the animals live. 

 The scales of reptiles are formed by the thickening and hardening of the 

 cornified epidermis. Those of turdes lie flat over the bony plates beneath; 

 those of snakes partly overlap one another. Turtles never shed their scales 

 but each one increases in size as the animal grows. The cornified scaly epi- 

 dermis of snakes and lizards forms a complete armor that is shed in early 

 summer. It is then that reptiles appear most sleek and burnished in their 

 new skins. 



Feathers. These are slender upgrowths from the dermis. A feather carries 

 the epidermis with it and at its base sinks into a depression or pit in the skin. 

 Feathers are cellular structures but only near the level of the skin do they 

 remain alive as the feather grows. Nearly all of the feather consists of cornified 

 walls of microscopic air spaces that once were living cells. Thus each feather 

 is an extraordinarily complex horny air trap, an insulation, whose light weight 

 is only a part of its great efficiency. The habits and successes of birds are 

 peculiarly bound up with their feathers. (See also Chap. 36.) 



Hair. The most striking development of mammalian skin is hair, an in- 

 sulation as characteristic of mammals as the feathers of birds. Among the 

 very few almost hairless mammals are the armadillo, the hippopotamus with 

 a few bristles around the snout, elephants, and whales that are covered with 

 hair before birth but afterward have only a few bristles about the lips. 



A hair is a shaft of purely epidermal cells which projects outward obliquely 

 from its bulb-shaped root that extends down into the dermis (Fig. 8.2), 

 Below the surface of the skin a hair is a column of rapidly multiplying cells; 



Fig. 8.3. Hairs and overlapping scales on the tail of a rat, section of it magnified. 



