ACTION OP THE SKIN 361 



root. The visible part consists of a body or bed of the 

 nail which is attached to the dermis, and a free edge. 

 Near the root is a half moon, or limula, which is whiter 

 than the rest. This difference in color is due not to the 

 nail itself which is transparent, but to the fact that the 

 blood papilliE are less numerous under this part of the 

 nail. 



General physiology of the skin. — The principal uses of 

 the skin may be classified under the following heads: 



(a) It affords protection to the underlying muscles and 

 organs, and at the same time, by its loose connection, per- 

 mits free movement of these underlying parts. 



(6) It permits the evaporation of water from the sur- 

 face of the body, and thus regulates the temperature of 

 the body. In hot weather, or when one is exercising, the 

 excess of internal or external heat is attended with profuse 

 perspiration, and this heat is used to evaporate the water 

 of the perspiration, and hence does not raise the body 

 temperature. By this means the body temperatm'e is 

 maintained at an even rate of about 98° Fahrenheit. 



(c) Its underlying layers of fat also prevent too great 

 a loss of heat from the underlying parts. 



{d) It contains sensory papilla; of various kinds which 

 are connected with the central nervous system (spinal 

 cord and brain), and these papillae permit the response of 

 various parts of the surface to external stimuli. 



(e) It is an excretory organ, and by means of the sweat 

 and sebaceous glands it is able to extract wastes from the 

 blood and remove them to the outside of the body. 



Action of the skin as an excretory organ. — The precise 

 composition of sweat is difficult to determine, as it is 

 usually mixed with the secretion of the sebaceous glands 



