THE VENTILATING SYSTEM OF THE BODY 271 



breathes a minute depends somewhat upon circumstances. 

 Young children may breathe as often as forty times a minute. 

 Adults breathe from fifteen to eighteen times a minute, 

 though when violently exercising the number of breaths a 

 minute may rise to sixty or seventy. The muscles which 

 control breathing are involuntary, though by taking thought 

 we may for a time modify or even stop their action. It is 

 impossible for one to hold his breath for more than a cer- 

 tain time. After that the involuntary system asserts itself. 

 Vigorous exercise, calling for an increased amount of oxygen, 

 causes one to breathe faster, but if the exercise is begun sud- 

 denly, we may find ourselves "out of breath," because not all 

 the lung cells are in use. If the exercise is continued all the 

 lung cells are soon brought into action and we find we have 

 our "second wind" and can then exercise without becoming 

 breathless. 



230. Ventilation. The need for an abundant supply of oxy- 

 gen in the air we breathe accounts for the attention that is 

 everywhere paid to ventilation. Nobody can work or study 

 well in an atmosphere depleted of its oxygen, though when the 

 supply falls short it is not noticed so quickly if the air is kept 

 moving. Every effort should be made to have one's surround- 

 ings well ventilated. One should sleep with the window open 

 at night, even in the coldest weather. Night air is no more 

 harmful than day air. Many people, by the use of sleeping 

 porches, now sleep in the open air throughout the year. 

 Open air treatment is one of the recognized methods of treat- 

 ing tuberculosis or consumption. Those who are compelled 

 to pass much of their time indoors should manage to take 

 a brisk walk of a mile or two daily. Next in importance to 

 ventilation is dust and dusting. A number of diseases are 

 spread by dust, and dusty locations and occupations should 

 be avoided whenever possible. In dusting a room a feather 

 duster should never be used. This simply stirs up the dust 



