A TRANSPORT SYSTEM 171 



and regulation of the transport or traffic along the 

 alimentary canal. 



There is no need to describe how petrol is carried or 

 transported from the petrol tank to the engine of a motor 

 cycle. The petrol tank or stomach is set under the 

 horizontal bar of the frame, well above the level of the 

 engine ; gravity is the force which carries petrol along the 

 feed-pipe, which we have already compared to the bowel 

 of the alimentary system. If the fuel supplied to our 

 bodies were as liquid as petrol and we kept ourselves always 

 in the upright posture, then a transport system similar to 

 that of a motor cycle would have met our needs. Gravity 

 would have been sufficient to transport food along the 

 28 feet of tubing which forms our alimentary canal. If 

 the passage of the food had to be delayed in certain 

 sections or compartments of the canal, as we know it has, 

 then this could have been accomplished by the use of a 

 system of automatic stopcocks or float chambers. The 

 food which has to be transported along the " feed-pipe " 

 of the human machine cannot flow like petrol, but, as we 

 have seen, is reduced in the mouth to the consistency of 

 a semi-fluid pulp ; it cannot be made to flow along a 

 system of collapsible pipes which are kept compressed 

 tightly against each other, as bowels are in the abdomen ; 

 rigid tubing is out of the question in the human machine ; 

 every part of it, save the bony levers, must be supple. 

 Nor can its posture be guaranteed ; a motor cycle can run 

 only as long as the petrol tank is above the level of the 

 engine, but the human machine must be able to work in 

 every posture. 



Engineers have frequently to design a system for 

 transporting a semi-fluid pulp from one workshop to 

 another, but they never have been able to copy Nature's 

 means of propulsion. For such purposes Nature always 

 employs a pump with muscular walls. We have seen 

 the plan on which the heart is built, an elaborately fitted 

 muscular pump for propelling blood through the vast 

 capillary fields of the human machine. We can picture 

 the principle on which the heart works by compressing 



