The Integumentary System 77 



The skin also is able to conserve heat by making certain physiolog- 

 ical responses to decreased temperatures. The skin tends to contract 

 into "goose pimples" when chilled. This results in diminishing the 

 amount of blood circulating to the skin surface due to the contraction 

 of the capillaries. 



When environmental temperatures are high, heat loss becomes 

 important. Here again the skin plays a major role. The capillaries 

 tend to expand, thus bringing more blood to the skin surface for heat 

 loss. 



Of great importance in regulating heat loss is the cooling of the 

 skin that results from the evaporation of water from the surface. This, 

 of course, can occur only in those animals such as man that possess 

 sweat glands over the body surface. Sweat is composed chiefly of 

 water, some salts, and traces of urea. During strenuous work, up 

 to 5,000 c.c. of sweat may be lost in a single day. More usual is a 

 loss of from 500 to 2,000 c.c. daily. 



In animals such as the dog, there are but few sweat glands. In- 

 stead, these animals rely on panting for their heat loss. This allows 

 for evaporation of saliva from the tongue's surface. Due to its com- 

 parative lack of sweat glands, the dog cannot withstand the high 

 temperatures that cause but little discomfort to man. 



3. Fat Storage. The fat stored in the deeper layers of the skin 

 of many mammals and birds serves a double purpose : not only is it 

 an efficient insulating device, but it also provides reserve food. Be- 

 fore starting on their migrations in the fall, many birds deposit large 

 quantities of fat which they draw upon during their long flights. Hiber- 

 nating mammals, such as the woodchuck, rely on these stored food 

 sources for energy during their long sleep. 



Among human beings, fat lends the familiar contours to the body. 

 There are sexual differences in the points of deposition. 



4. Respiration. Among a few vertebrates, the skin is important 

 as a medium of gaseous exchange. This is especially true among the 

 amphibians such as the salamanders and frogs. 



Among the fish, the gills are often considered to be merely ex- 

 tensions of the skin. These gills are richly supplied with blood vessels, 

 and provide the surface for respiratory exchange between the water 

 and the blood. 



5 Sensation. To receive sensations from the environment is 

 one of the primary functions of the integumentary system. For the 



