PROTECTION, SUPPORT AND MOVEMENT 505 



Velvet 

 slou6hinb off 



Fronted bone 

 Antler of deer 



Core of bone 



Horn, of co^^ 



Male and. female 



Figure 25.3. A diagram to show the differences between antlers (deer) and horns 

 (cow). Antlers are annual growths that are shed in the winter; horns are permanent 

 outgrowths. 



cattle, in contrast, do not branch, are not shed and occur in both sexes. 

 These horns have a core of bone covered by a highly cornified skin. 



Horny skin derivatives develop by the accumulation of keratin in 

 the cells of the epidermis. Reptiles have a covering of horny scales that 

 reduce water loss through the skin. As the animal increases in size, the 

 horny scales are periodically shed and newly formed ones are exposed 

 beneath them. Bony scales, in contrast, are not shed but increase in size 

 by the addition of new bone. Except for their retention in such regions 

 as the feet of birds and the tails of certain mammals, horny scales are 

 not present in most birds and mammals, though a prominent stratum 

 corneiun persists. 



Feathers are believed to be modified horny scales, but the hairs of 

 mammals are regarded as a different kind of horny skin derivative. A 

 hair lies within a hair follicle (Fig. 25.1), which is composed of a tubular 

 invagination of the epidermis supported by surrounding fibers of the 

 dermis. A hair papilla, containing blood vessels and nerves, protrudes 

 into the base of the follicle and nourishes the adjacent epithelial cells. 

 These proliferate rapidly and add to the base of the hair, which extends 

 up through the follicle as a column of keratinized cells. A small smooth 

 muscle, the arrector pili, is associated with each follicle. These muscles 

 contract when temperatures fall and pull the hair follicles and hairs 

 into a more erect position, thereby increasing the depth of the hair layer 



