80 HUMAN PHYSIOLOGY 



brought from all parts of the body, and in part 

 derived from the lungs themselves. 



VENTILATION. One of the great problems of human 

 life is to secure an adequate and a constant supply of 

 pure air. This task is much more difficult than 

 might at first sight be supposed. Our rooms and 

 abodes as constructed in modern times rarely pre- 

 sent facilities for securing free and perfect ventila- 

 tion, that is without exposing us to injurious draughts 

 and so exposing us to risks of cold and chill. The 

 problem of ventilation can only be adequately solved 

 in a scientific fashion by mechanical means whereby 

 through the use of ventilating fans fresh air is 

 brought in and the foul air expelled. Such means 

 are, of course, not applicable to ordinary dwellings, 

 and therefore we have to fall back in the latter case 

 on simple and crude expedients, represented by the 

 opening of windows or the fixing of ventilators which, 

 however well they may act in theory, rarely come up 

 to the expectations of their inventors in the matter of 

 practice. One of the most simple fashions in which 

 a room may be ventilated at practically no cost at 

 all, is that of raising the bottom sash of a window 

 four or five inches and of placing in the open space a 

 bar of wood on which the lower sash rests. The 

 result of this arrangement is to raise the top of the 

 bottom sash above the bottom of the upper sash. 

 Air comes in between the two sashes with much less 

 draught than is the case when the window is simply 

 drawn down from the top. Outlet ventilators may 

 be placed near the ceiling, the ordinary form of these 

 ventilators allowing air to pass out into the chimney, 

 whilst the smoke is prevented by a valve arrange- 

 ment from entering the room. Such ventilators, how- 

 ever, only act when the pressure of air in the room 



