RESPIRATION AND EXCRETION 349 



sides of the chest so as to compress the chest. A third 

 method is to lay the subject face downward, placing a thick 

 folded garment beneath the chest. "The operator then puts 

 himself athwart or at the side of the subject, facing his head 

 and places his hands on each side of the lower part of the 

 back. He then slowly throws the weight of his body for- 

 ward to bear upon his own arms, and thus presses upon the 

 thorax of the subject and forces air out of the lungs. This 

 being effected, he gradually relaxes the pressure by bring- 

 ing his own body up again to a more erect position, but 

 without moving the hands. These movements are repeated 

 regularly, twelve to fifteen times per minute until normal 

 respiration begins or its possibility is abandoned. It may 

 require a half hour or more before independent breathing 

 movements start." 



Ventilation. Ventilation is the circulation of the air 

 of rooms or buildings by means of which fresh air is brought 

 into the room and impure air removed. It may be accom- 

 plished by opening windows or by a mechanical "system" 

 such as is often used in public buildings. An open fire place 

 is a good ventilator. Ventilation must be rapid enough to 

 remove vitiated air but not so rapid as to cause a draft. 



Where many people occupy the same room some system of 

 ventilation must be maintained or the supply of oxygen 

 would soon be exhausted. If we always lived in the open 

 air, as our ancestors did, there would be no need for arti- 

 ficial ventilation. 



Foreign substances in the air. In the air there is a 

 large amount of floating dust, as we can see by observing the 

 path of a ray of sunlight in the room. The cilia and hairs 

 that line the air passages normally prevent this dust from 

 entering the bronchial tubes. When rooms are carefully 

 and frequently cleaned and dusted with a damp cloth the 

 amount of dust in the air in an ordinary room is at a mininum 

 and is not injurious. If the room is carelessly swept and 



