THE SKIN 237 



The cells which constitute this part of the tube are the true 

 secreting cells, and this glandular portion is surrounded by 

 a net-work of blood capillaries. The ducts from different 

 glands do not join one another, but each passes nearly straight 

 up through the dermis, then through the epidermis in a wavy 

 course, to open by a minute pore on the surface of the skin. 

 The pores are too small to be seen with the naked eye, but 

 are easily visible if the skin is examined with a good lens. 



Like the other glands in the body sweat glands are under 

 the control of definite nerve fibres, called secretory nerves. 

 These are stimulated by various means; by reflex influences 

 when strenuous muscular work is being performed; by the 

 condition of the surrounding air; by fright or by special con- 

 ditions of the internal organs. Like other glands, too, their 

 action is not within control of the will. 



Functions of the Sweat Glands. The sweat glands have 

 two very important functions: (1) Excretion of water and 

 other materials. Large amounts of perspiration, which is 

 water in great measure, are secreted daily, thus making the 

 skin second only to the kidneys in regulating the amount oi 

 water in the blood. There are also small amounts of various 

 salts, traces of fat and other substances in perspiration. If 

 on account of a disease of the kidneys, urea cannot be properly 

 excreted from them, it may be partially disposed of through 

 the skin, which thus helps to purify the blood. (2) Regula- 

 tion of body temperature. This is a matter of such importance 

 that it must be discussed in considerable detail. 



BODY TEMPERATURE 



The life processes in all animals and plants are closely 

 related to temperature. At the freezing point all their 

 activities stop; at temperatures slightly above this they begin 

 and with increase of temperature become more vigorous. 

 For the human body 98 F. seems to be a very favorable tem- 

 perature. Even the simple functions of digestion are checked 



