i 4 4 THE ENGINES OF THE HUMAN BODY 



regulating mechanism would be destroyed if the chambers 

 were quite emptied. Nay, if emptied their walls would 

 collapse, and the flow of blood through the capillary fields 

 of the lungs would thus be blocked. Death would soon 

 follow from a complete stoppage of the pulmonary 

 circulation. 



Before bringing this chapter to an end, there is one 

 very important matter to which I must allude — one which 

 has to do with maintaining the lungs in a healthy con- 

 dition. Like all wise engineers, Nature designs every one 

 of her fabrics so as to fulfil the law known as the " factor 

 of safety." When an engineer builds a boiler or designs 

 a crane, he builds and designs so that the boiler will with- 

 stand a pressure of steam and the crane will support a 

 load ten times the amount required in the course of 

 ordinary use. Nature builds her engines, levers, pumps, 

 and also her bellows, on this principle, In ordinary 

 sedentary occupations men and women do not use the 

 respiratory chambers of their lungs to one-tenth of their 

 full capacity. If lungs are always used, day after day, at 

 this low rate, they are more likely to suffer damage than 

 any other part of the human machine. They are so con- 

 structed that we may use only one part of them fully, 

 the rest being employed to, merely a minimum degree. 

 Numerous sets of respiratory chambers, under such con- 

 ditions, may be regarded as temporarily shut down. 

 Unfortunately, we have always been taught, have always 

 supposed, that a lung expands equally in all its parts when 

 we take a breath, much as if it were a simple bellows. 

 Every movement of an ordinary bellows, however slight, 

 diffuses the indrawn air throughout its entire chamber. 

 The lungs, as we have seen, are set with millions of 

 miniature air sacs ; the pipes or bronchi which conduct 

 air to them are stout ; they radiate out into the lungs 

 like the branches from the trunk of a tree, or, if 

 I may alter the simile, like the rays of a lady's fan. 

 Between these fan-like bronchial rays are set myriads 

 of air chambers. When we take a full breath the 

 thoracic bellows expand in such a way that all the rays 



