68 LETTERS TO BROTHER JOHN. 



rather the vital blood contained in them) to restore 

 what the lymphatics have eaten away ; and that of 

 the veins to carry back to the heart the refuse of 

 the blood; that is, what remains of it, after the 

 arteries have done with it When the blood has 

 parted with its living elements while in the arteries, 

 the veins carry it away, in order that it may receive" 

 a fresh supply of these living elements. But the 

 arteries could not carry the blood from the heart, 

 nor the veins return it to the heart, if they were 

 not supplied with the power of moving. This mo- 

 tive power is afforded them by the nerves or 

 rather, a fluid conveyed by the nerves. This fluid, 

 however, does not, I conceive, travel along the 

 nerves like a tangible fluid in a tube, but like the 

 electric fluid along a wire. The nervous fluid, 

 therefore, is to the organs of the body what steam 

 is to a steam-engine. And as this fluid is conveyed 

 by single filaments of nerves, it is clear, that where- 

 ever there is an artery, vein, or absorbent, there 

 must also be a nerve to enable those vessels to 

 convey their fluids, which they do by a motion of 

 their own, or of neighbouring parts. 



You may conceive the universality of the nerves 

 and blood-vessels, by the fact, that you can scarcely 

 insert the point of the finest needle into any part of 



