THE BIOLOGY OF HEALTH 143 



cells of the skin, of the lining of the food-canal, of the 

 glands, and of the blood get worn-out and others take 

 their place. We are said to have about 25 trilHons of 

 red blood corpuscles, occupying 3,300 square yards if 

 spread out on a surface. These cells are continually, 

 as it were, wearing out, and being replaced by fresh 

 units, but we never get a new nerve-cell after birth. 

 The moral is plain, that over-fatigue is more serious for 

 nerve-cells than for other cells, and an injury to a cluster 

 of them may be very serious indeed. 



§4. The Regulative System 

 We have seen that the muscles correspond to the 

 motor parts of the bodily engine ; that the nervous 

 system is the storage battery, the sparking apparatus, 

 the steering gear, and the observant driver besides ; that 

 the fuel is the food which the stomach prepares and the 

 liver stores ; that the air for combustion is supplied by 

 the lungs ; that the blood distributes oxygen, digested 

 food, and lubricant substances, and collects carbon 

 dioxide and nitrogenous waste-products ; and that the 

 latter are filtered out by the kidneys. We must now 

 consider the regulative system. 



Paul the Apostle had a very vivid conception of the 

 correlation or harmonious working together of parts. 

 *' There are many members and one body. The eye 

 cannot say to the hand * I have no need of you,' nor 

 again the head to the feet ' I have no need of you.' 

 Quite the contrary. Yes, God has tempered the body 

 together, with a special dignity for the inferior parts, 



