THE BIOLOGY OF HEALTH L33 



only one kind of lock, and the lock seems to have a 

 positive attraction for the appropriate key. Again 

 we must say : We are fearfully and wonderfully made. 



It is very useful to compare our body to an engine, 

 but the comparison breaks down. In particular, the 

 regulation of the living body is something that we can- 

 not explain in terms of anything else. If the body is 

 an engine, it is a self-stoking, self-repairing, self-regulat- 

 ing engine ; it can profit by experience ; it can behave 

 as a self-willed agent ; it can work along with others ; 

 it may give rise to another engine like itself, as parents 

 to their children. But, above all, the Hving body is a 

 Mind-body. 



The general idea to be grasped is that the body is a 

 material system for transforming the energy of food 

 into the energy of motion, whether that be the move- 

 ment of limbs in locomotion or the beating of the heart. 

 The furnace, the boiler, the piston are all, as it were, 

 condensed in the muscular system. To keep the fire 

 burning there must be a supply of fuel, which is prepared 

 in our food-canal and liver. To keep the fire burning 

 there must be air, and the lungs are the ventilating 

 structures. To keep the fire burning there must also 

 be a removal of the ashes, and this is effected by the 

 l3anph and hj the blood which sweep the waste-products 

 to filters like the kidneys. There must also be arrange- 

 ments for regulating the supply of fuel, the draught, the 

 speed, and so on. Anticipating a little, we may say that 

 the obvious conditions of the continued efficiency of 

 the bodily engine are plenty of varied exercise, plenty 



