THE EXCRETORY SYSTEM 217 



from lung and kidney troubles are frequent. Death from 

 skin troubles rarely occurs, since the skin is not apt to be 

 attacked over the whole body at once, but it has been dis- 

 covered that if a person's body is painted over with a varnish 

 that interferes with skin functions, death will inevitably 

 result. (See "Body Temperature," Chapter XV.) 



When we speak of the portion of the waste material ex- 

 creted through the intestine, we do not refer to the undigested 

 parts of the food that simply pass through to be discharged, 

 but to materials actually excreted from the body into this 

 part of the digestive tract. Most of these come from the 

 liver, which, as we have seen, pours quantities of bile 

 into the intestine. This bile aids somewhat in digestion, 

 but is, after all, chiefly a waste product which passes from 

 the body with the faBces. 



Agency of the Blood in Removing Wastes. In the building 

 and economy of any great city two systems of piping are con- 

 nected with every house, and are at the service of every in- 

 dividual: the water pipes which supply the fresh water, and 

 the sewers which take away the waste and the polluted 

 water. To be sure, water could be taken from a well in the 

 cellar of each house, and the waste could be poured on the 

 ground outside; but these practices, sooner or later, would 

 almost certainly cause diseases, if not death, in the house or 

 community. All up-to-date houses are provided with special 

 water supplies and sewage outlets. 



In the human body the same system of tubes serves to 

 bring in fresh food and water supplies and to take away the 

 wastes. The blood, which serves both these ends, is thus a 

 very complex liquid. It contains all of the food absorbed 

 from the intestine, and in its circulation it receives all the 

 wastes from the various parts of the body, carrying them 

 away to the organs that are to excrete them. Living involves 

 such constant activity that waste matters are continually 

 thrown into the blood. A certain amount of blood can ab- 



