EXCRETION AND SEWAGE 2 53 



451. Sewers. In cities the houses are too near together 

 to permit the use of cesspools, and so underground tubes 

 or tunnels, called sewers, are built at public expense in 

 order to conduct the sewage outside the town. There it 

 should be treated in a sewage disposal plant, although it 

 is often emptied into the nearest body of water (p. 407). 



452. Plumbing. In houses pipes are arranged to carry 

 off the sewage as fast as it is formed in the sinks, wash 

 bowls, and closets. Since they open into a common sewer 

 of the town, sewer gas can readily enter the houses. To 

 keep it out, each pipe is bent into a loop which remains 

 full of liquid and prevents the entrance of gas. 



453. Cleanliness. No matter how good the natural or artificial 

 drainage may be, if decaying matter is left in cellars, it may poison the 

 air. Sinks may become clogged and poison the air, while slops and 

 dirty dishes may be carriers of disease. So cleanliness is of great im- 

 portance, aside from its mere looks. 



454. Choice of a house Site. In choosing a site for a dwell- 

 ing house we should consider the natural drainage of the ground. If 

 the soil is low and marshy, or if the subsoil consists of clay or rock, the 

 sewage may not soak away readily. Mosquitoes breeding in marshy 

 ground may cause malaria (p. 225). The site for a house should be 

 such that the barnyard and outhouses can be put so they will drain 

 away from the house and well. Attention to these details of drainage 

 is of far more importance than the natural beauty of a site. 



SUMMARY 



1. Excretions are waste and poisonous substances ex 



pelled from the body. The principal ones are car- 

 bonic acid gas, water, urea, and mineral matters. 



2. Sweat, or perspiration, is formed in tubes in the skin. 



It contains some urea and mineral matters. 



3. The kidneys are collections of minute tubes which sepa- 



rate urea, mineral matter, and water from the blood. 



