1 64 THE ENGINES OF THE HUMAN BODY 



shall see, like a petrol tank ; its walls are built of living 

 muscle which can relax as well as contract. Its capacity 

 and power to waste can be increased by force — just as 

 pressure could be brought to bear on a petrol tank, so 

 that oil is forced within the engine cylinder to a wasteful 

 extent. In the designing of the human machine Nature 

 has exercised a degree of care for our welfare which may 

 well be called grandmotherly. She has set up the con- 

 trivance called appetite to regulate the consumption of 

 fuel to the needs of the body. We may circumvent her 

 designs and enjoy life for a time by flooding our combus- 

 tion chambers with a wasteful supply of fuel, but sooner 

 or later she levies a fine on the exchequer of our health. 



We are now to commence a rapid survey of the various 

 chambers or workshops — the mouth, stomach, duodenum, 

 small bowel, and great bowel — in which tissue-fuel is pre- 

 pared from the food which is passed through them. 

 These workshops, we shall find, are linked together by 

 a well-managed transport system, designed on a plan quite 

 different from any employed by man and yet regulated on 

 the same lines as a modern railway system, except that 

 the traffic is all in one direction, from lips to vent. The 

 length of the system varies from one moment to another ; 

 when one section is shortening another may be lengthen- 

 ing ; but taking an average condition in a man of medium 

 build, the total length may be estimated at about 28 feet. 

 As the food makes this long journey, which it usually 

 does in the course of from twenty-four to thirty-six hours, 

 it passes through a series of elaborately fitted and busy 

 factories, and undergoes many changes before the essential 

 fuel becomes separated from the dross or refuse. 



None of the factories set along the alimentary tract of 

 the human machine is more effectively contrived than the 

 first work-shop — the mouth. There are two doorways : 

 one is guarded by movable muscular shutters or lips ; 

 the other is seen when the mouth is widely opened 

 leading into the throat or pharynx. The doorway to the 

 throat is opened and closed by delicately adjusted side 

 curtains — the movable pillars of the fauces. When we 



