THE SKIN. 161 



be firm for picking and touching; and, therefore, these 

 little stiff backs are made to grow in them. The root of 

 the nail is the upper end beneath the skin. The matrix 

 is the bed on which it lies, and from which it grows. The 

 nail may be shed or torn out, and, if the matrix is sound, 

 a new nail will grow. If the whole matrix is destroyed, 

 there will be no new one. But a very small portion of 

 the matrix remaining, will produce a new nail, though it 

 may be an imperfect one. 



USES OP THE SKIN. 



11, Understanding the structure of the skin, we are 

 ready to notice its uses. 



1. It protects the parts beneath. Being tough and 

 elastic, it can bear hard knocks. The cuticle, which is 

 without nerves, covers and guards the sensitive parts. 

 Neither fluids nor gases pass through it easily. If the 

 skin is whole, we can safely put our hands in poisonous 

 fluids, which would enter the blood, and do us harm, if it 

 were scratched. 



2. It gives off waste matters from the body. Perspira- 

 tion is chiefly water, but it contains some other substances 

 dissolved in it. The amount of water given off in a day 

 is different at different times and in different persons, but 

 is ordinarily about a quart. In this amount of water, two 

 or three spoonfuls of solid matter are dissolved. 



3. It regulates the heat of the body. 



BODILY HEAT. 



12, Our bodies are always making heat ; and yet, if we 

 put the bulb of a thermometer in the mouth of a healthy 

 man, it will never rise more than a degree or two above 



