GLANDS OF THE SKIN 55 



more open fleeces. The reason for this is that the wool is a non- 

 conductor of heat and hence protects against variation in tem- 

 perature, and when it is dense and oily it prevents the rain from 

 penetrating to the skin. 



Variations in Hairy Covering. With the exception of the 

 Barbadoes breed, all of the domesticated breeds in the United 

 States are covered with wool over all parts save the head, legs, arm- 

 pits and groins. In nearly all of these breeds wool extends partly 

 over the head and legs, and in the Merinos it is not uncommon for 

 it to grow over the surface of the armpit and most of the groin. 



Breeds exhibit marked variation in length, fineness and density 

 or thickness of wool. Density may vary from 600 to 1500 fibers 

 to each square inch of skin; fineness from one three-hundredth or 

 more to one three-thousandth of an inch in diameter; and length 

 attained in twelve months from less than one inch to fifteen inches 

 or more. As a rule the shortest wool is the finest and densest. 

 White wool prevails among the domesticated breeds of the United 

 States and of other countries giving special attention to the pro- 

 duction of wool, but the color of the hair on face and legs varies, the 

 most common shades being white, reddish brown, light soft brown, 

 and deep brown or black. In various parts of the world there are 

 domesticated breeds, in most cases not highly improved, however, 

 that grow gray, brown and black wool. 



Wool is rarely if ever uniform in length, fineness and density 

 over all parts of the sheep. The finest and densest wool is in the 

 regions of the shoulders, about midway between the top and bottom 

 lines of the body; the coarsest wool grows on the outer thighs and 

 at the dock ; and the shortest wool is to be found on the belly. 



The horns and hoofs are modifications of the epidermis. Most 

 of the modern breeds are hornless ; in a few breeds only the males 

 have horns, and in a few others they appear in both sexes, but the 

 males always have them much more strongly developed than the 

 females. Males unsexed while young resemble the females of the 

 breed in the degree of development of the horns. Hoof tissue and 

 also horn tissue, if present, are white only in those breeds in which 

 the naked skin at the muzzle is pink. 



Glands of the Skin. The sweat glands, secreting water and 

 potassium salts, and the sebaceous glands, secreting a fatty or oily 

 substance, are the most important glands in the skin. Their com- 

 bined product, less most of the water secreted by the sweat gland, is 



