The Integumentary System 91 



dust particles. The cat possesses special stiff hairs, the vibrissae about 

 its snout. These serve as feelers. The heavy pelt of the fur-bearing 

 animal is an excellent insulator as well as a shedder of rain. The fur 

 of these animals consists of dense soft hairs with some longer stiff 

 guard hairs scattered throughout. The spines of the porcupine are 

 hard rigid hairs, and the bristles of swine are stiff elastic ones. 



The growth of hair may be either definitive or indefinite. Definitive 

 hairs such as are found over most of man's body grow to a certain 

 length. Then the individual hair is pinched off from its source of 

 nutrition in the follicle. This hair is then shed, and a new one starts 

 growth. The hair of the head grows indefinitely in length. With many 

 animals molting, the shedding of the hair, occurs at regular intervals, 

 while in man it occurs at all times. 



Hair color is due to pigments deposited in the cortex during growth. 

 Graying occurs when the amount of pigment is decreased, and the 

 light-reflecting areas between the cells become more numerous. 



Other Integumentary Derivatives. — Many other unusual struc- 

 tures are derived from the ingenious integument. Some examples are 

 the strange horny beaks of birds which serve a tremendous variety of 

 uses to these highly modified animals, and the similar beaks which are 

 found among the turtles. The rattle on the rattlesnake's tail is also 

 derived from the epidermis. Each time the snake molts, a button of 

 cells from the stratum corneum is left behind. These rings are quite 

 loose and rattle when the tail is vibrated. The successful outcome 

 of a cock fight is often dependent upon which cock has the best spur, 

 another epidermal derivative. 



