The Body-Machine and Its Work 835 



waste matter from the tissues can get back into the blood. Even 

 this is a far more complicated matter than is generally supposed. 

 The cells in each tissue of the body must somehow select their 

 own food and oxygen, and even the union of oxygen with carbon 

 in the working muscle does not take place as we find it in 

 ordinary combustion. 



A Wonderful Apparatus 



At the point where the artery subdivides into the finest 

 tubelets (the capillaries), there is a wonderful apparatus, a sort 

 of "stopcock," for regulating the supply of blood. Muscular 

 fibres are coiled round the artery, and, as the artery enlarges or 

 contracts, the supply of blood to that particular tissue is increased 

 or lessened. When you sit down to a meal, for instance, the 

 stopcocks are opened full to your digestive organs and partially 

 closed against your muscles and brain. When you stand up and 

 move about the room various muscles have to work, and the cocks 

 are duly turned on to them. When your muscles need all the 

 blood they can get, your brain and digestive organs get less. 

 When you stand erect for some time, the regulative system has 

 to see that blood does not accumulate in your legs at the expense 

 of your head; but if you overdo it if you stand long in a close 

 crowd or a stuffy room even this admirable system breaks down, 

 and your brain, which is particularly sensitive about oxygen, 

 runs short of blood. You "faint." 



Here again science has only made a great discovery to be 

 confronted with a mystery. We know that there are nerves from 

 the muscles of the arteries to the spinal cord, and that the stop- 

 cock we have spoken of is regulated by a reflex nervous message 

 from the cord ; but how these unconscious elements of the human 

 mechanism work so perfectly together we do not know. When 

 we remember how densely ignorant of all these things men were 

 only a few generations ago, we may be sure that much will be 

 discovered by our children and grandchildren. 



