274 THE ENGINES OF THE HUMAN BODY 



only for healing wounds, but for repelling the germs 

 which invade them. She mobilises her army of defence 

 at the point of danger. She uses means which dope 

 or lower the vitality of the enemy so that they fall more 

 easily a prey to her fighting forces ; she can also raise 

 the power with which her soldiers carry out their attack. 

 Medical men have studied closely the battles which are 

 fought between the defensive forces of the body and the 

 advance guards of bacterial invasions and have discovered 

 means of assisting the defence. 



But the means they have discovered and applied are 

 those used by Nature. In this case again medical men 

 assist recovery by playing the part of Nature's assistants. 

 The r61e of the surgeon in the base hospital and of the 

 mechanic in the base repairing shed are totally diverse. 



We shall cite a parting instance to show how different 

 the nature of living flesh is to that of dead metal. In 

 previous chapters I have striven to convey some impression 

 of the manner in which countless myriads of living units 

 are linked together to form the human machine. I have 

 also sought to make clear that the kind of society in which 

 these units spend their lives may be compared to the 

 social organisation of a highly governed modern state. 

 Whether we are coal-miners, transport workers, farm- 

 labourers, cotton spinners, or even men of "independent" 

 means, we are, one and all, units in the organisation of a 

 great machine. I must freely admit, however, that the 

 units of the human machine are arranged upon a caste 

 basis ; the laboratory attendants in the stomach and their 

 descendants have to follow their allotted vocation during 

 the lifetime of the machine. So it is with every system 

 of the body ; there is a muscle caste, a nerve or governing 

 caste ; there are bone-building and cartilage-building 

 castes. The caste system is rigidly enforced by a mechan- 

 ism which, unfortunately, we have not yet discovered. 

 The units of one caste rigidly follow their special vocation 

 and strictly observe the rights of neighbouring units 

 belonging to another caste ; all do their best for the 

 machine as a whole. This is the rule, but occasionally it 



