354 THE ANIMALS AND MAN 



Hair. — Hair is similar to the nails in formation, that is 

 it is epidermal in origin. A section of a hair shows it to be 

 a long hollow shaft embedded at its base in a socket of 

 dermis called a hair follicle (fig. 178, k). The socket is lined 

 with an epithelial layer. The growing part of the hair is 

 the papilla at its base. This is supplied with blood-vessels 

 and sensitive nerves. These nerves are stimulated when a 

 hair is pulled. Connected with each hair follicle is a tiny 

 oil-secreting gland called a sebaceous gland (fig. 178, 1). 

 Its duct opens into the follicle and the secretion from the 

 gland is poured out on the surface about the base of the 

 hair, thus keeping the skin from becoming hard and dry, 

 and, on the head, rendering the hair glossy. 



The care of the hair means the care of the skin where 

 the hair 'grows. The scalp must be kept clean and rubbed 

 that a good circulation may be maintained, and the hair 

 must be well brushed. 



The scales of fishes and snakes, the hoofs and horns of 

 cattle, and the feathers of birds are, like nails and hair, 

 modifications of the epidermis and outgrowths from it. 



Sweat glands. — The sweat glands of the skin are among 

 the most important glands of the body, and with the kidneys, 

 function as excretory organs. 



These glands (fig. 178, f, g, h,) open in the tiny pits that 

 may be seen covering the surface of the skin. The base of 

 the gland lies coiled up in a little compact knot, deep in the 

 dermis or true skin (g, h,). Here it is surroimded by lymph 

 and numerous blood capillaries and is supplied with nerve 

 fibers. These glands are microscopic but very numerous, 

 probably 2500 or more to the square inch, so that their 

 combined secretion amounts to a great deal. 



The skin as an excretory organ. — The principal waste 

 given off by the skin is water in the form of perspiration. 

 The amount varies under different conditions. Usually 

 it evaporates immediately so as to become invisible. During 



