WORKSHOPS AND LABORATORIES 161 



the alimentary tract — stopcock or sphincter mechanisms 

 which regulate the rate of progress of the alimentary 

 contents. The bowel, then, of a motor cycle ends 

 abruptly at the jet aperture of the carburettor ; there 

 is no vent as in the human machine. That is because 

 petrol is a perfect fuel ; there is no need for a vent or 

 rectum because there is no refuse to discharge. 



The alimentary tract of a motor cycle is a simple 

 affair, very different from the bewildering arrangement of 

 parts we meet with in the same system of the human 

 machine. If mere simplicity be a merit, the motor cycle, 

 as far as an alimentary system is concerned, seems to 

 hold an advantage over the human machine. The 

 stomach of a motor cycle is, however, a delicate thing ; 

 it must be fed with fuel already digested — already in a 

 condition suitable to form the basis of a combustion 

 mixture. Petrol, the food of engines, if modern geologists 

 are rightly informed, is the fuel which was stored up 

 in the livers of extinct animals, vertebrate beasts which 

 lived in marshes and seas, now buried beneath strata 

 of solid rocks hundreds of feet in thickness. In the 

 livers of living sharks and certain other fish we know 

 that there is stored up a carbohydrate material, very 

 similar to petrol, which is used by them as fuel to drive 

 their muscular engines in swimming. The petrol which 

 carries a cyclist so swiftly along we are justified in re- 

 garding as fuel which extinct monsters had stored up 

 in their livers many millions of years ago to carry them 

 through old-world seas in search of prey. The fuel of a 

 motor cycle is already digested. It is otherwise with the 

 human machine ; it has to be provided with a plant or 

 outfit which can convert the raw material provided in 

 the natural products of the earth into a form of fuel 

 suitable for combustion in the tissues of the body. 

 Hence the elaborate arrangement of its alimentary system. 



Before actually setting out to examine the machinery 

 of the alimentary system, there are one or two matters 

 connected with its general management which require 

 to be mentioned. If the petrol tank should run empty 



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