124 THE ENGINES OF THE HUMAN BODY 



will also follow if the inlet passage is choked, which 

 corresponds to an obstruction in the windpipe of the 

 human machine. Similarly vitiated air which will not 

 support the needs of the human body will also bring 

 the motor cycle to a sudden stop. The more work we 

 compel the human machine to do, the quicker it must 

 breathe ; the more air and oxygen must it consume. 

 This is also true of the engine of the motor cycle. 

 When tested in all of these ways we thus see that a 

 series of movements, which may be named respiratory, do 

 go on in the internal-combustion engine of a motor cycle. 

 We have already laid open the cylinder of the engine 

 of a motor cycle and seen its respiratory chamber and the 

 manner in which air is drawn in and forced out (fig. i). 

 We are now to lay open the respiratory chambers of the 

 human machine — in order to examine the air passages 

 and the parts which serve to set the sides of the bellows 

 in motion. The most instructive view is got when the 

 thorax is laid open from the side, and as the man depicted 

 in Plates I. and II. has turned his left side towards us, we 

 shall make our approach from this side. Our first aim is 

 to see the machinery employed in expanding and con- 

 tracting the bellows' wall. Before that can be done a 

 most severe surgical operation has to be performed — the 

 left arm and shoulder have to be removed in order that 

 the upper part of the wall of the thorax can be displayed. 

 The shoulders not only hide but actually rest on the 

 upper part of the respiratory bellows, which at first sight 

 appears a bad arrangement, for their, weight must clog 

 respiratory movements. We have already seen how 

 Nature has overcome this difficulty by suspending the 

 shoulders by living muscular straps or engines which 

 come into action when we sit up or stand (p. 25) ; but all 

 of us know that the shoulders have to be lifted or elevated 

 when a deep breath is taken. Nay, if these suspending 

 engines become weak or lazy from inattention and the 

 shoulders are allowed to droop or slope, then the upper 

 part of the respiratory bellows is pressed upon and 

 rendered almost useless. 



