228 THE HOUSE I LIVE IN. 



nearly the whole of it, leaving only a small 

 chamber at one side for the heart. It blows 

 its blasts at the rate of twenty or twenty-five a 

 minute in an adult and at a greater rate still 

 in children and continues them, whether we 

 sleep or wake, as long as we live. I refer, as 

 you will readily know, to the lungs. 



I have already spoken briefly of the lungs. 

 I have told you about the windpipe, which 

 leads by its various branches to the ten thou- 

 sand little cells within ; and I have told you 

 that all these cells were lined by mucous mem- 

 brane, a membrane constructed like the skin, 

 though thicker. But I believe I have not yet 

 told you how much air these chambers of the 

 human body will hold, nor how great are the 

 superficial contents of the membrane on which 

 the air is spread to be purified. 



So numerous are the pipes and cells in the 

 lungs, that it is commpnly thought the extent 

 of the mucous membrane which lines them 

 must be equal, at least, to the extent of the 

 skin. This is, in a middling sized adult, about 

 fifteen square feet. Over all this surface the 

 fresh air which we breathe may circulate and 





