90 The Vertebrate Organ Systems 



Periodically the bird sheds its feathers in a process known as 

 molting. The dead feather falls out from its socket, and the living 

 cells of the stratum germinativum form a new feather. During molt- 

 ing, the feathers in the center of the feather tracts fall out first. 



The remarkable colors often seen in bird feathers may be due 

 either to pigments deposited during the growth period or to physical 

 structures which refract the light. The iridescent sheen seen on some 

 birds' plumage is due entirely to light refraction. 



Hair. — As feathers are characteristic of birds, hair is character- 

 istic of mammals. Even such seemingly hairless mammals as the 

 whales have a coat of hair during their early development. For most 

 mammals, the hair is an effective insulating mechanism, one which makes 

 it possible for this group to live in even the coldest regions. 



The individual hair (Fig. 21) projects at an angle from the hair 

 follicle in the skin. Opening into this pit are the sebaceous glands 

 which secrete sebum for maintaining the typical softness of the skin 

 and hair. The hair itself has two main portions : the root within the 

 follicle, and the shaft, the portion outside the follicle. Except at its 

 very base, the hair consists of the heavily keratinized epidermal cells 

 which are dead. Nutrition is supplied to the hair follicle by a hair 

 papilla which is in close contact with the follicle. 



In cross section the hair is seen to have three areas : the inner 

 core or medulla which is lacking in human hair, a ring or cortex 

 which makes up the bulk of the hair, and a thin outer layer of cuticle 

 cells. When the shaft of the hair is rounded in cross section, the indi- 

 vidual hair is straight ; if flattened, the hair is curly. 



Each follicle is supplied with a muscle w^hich can raise the hair 

 when contracted. The action of this muscle is very clear if one should 

 happen to see an angry dog. The hair on the scruff of the neck stands 

 upward, while the cat (which may be the source of the trouble) ap- 

 pears to have a much larger tail. In man this muscle is of little value, 

 causing "gooseflesh" when he becomes very cold or frightened. These 

 muscles are not under voluntary control. 



Mammalian hair is arranged in vortices or streams over the sur- 

 face of the body. The vortices are often around structures such as the 

 tail or horns. The many different streams and vortices are best seen 

 in an animal such as a short-haired dog or on a horse. 



Hairs are often specialized for particular functions. Thus the eye- 

 brows and eyelashes protect the eyes, hairs in the ears and nose catch 



