CIRCULATION.] PHYSIOLOGY 83 



the streets, but the work within the houses is hidden 

 from your view. Yet you know that, busy as seems 

 the street, the turmoil and press which you see there 

 are but tokens of the real business which is being 

 carried on in the house. 



So is it with any piece of the body upon which you 

 look through the microscope. You can watch the 

 red blood jostling through the network of capillary 

 streets. But each mesh bounded b; red lines is filled 

 with living fliesh, is a block of tiny houses, built of 

 muscle, or of skin, or of brain, as the case may be. 

 You cannot see much going on there, however strong 

 your microscope ; yet that is where the chief work 

 goes on. In the city the raw material is carried 

 through the street to the factciy, and the manufac- 

 tured article may be brought out again into the street, 

 but the din of the labour is within the factory gates. 

 In the body the blood within the capillary is a stream 

 of raw material about to be made muscle, or bone, or 

 brain, and of stuff which, having been muscle, or bone, 

 or brain, is no longer of any use, and is on its way to 

 be cast out. The actual making of muscle, or of bone, 

 or of brain, is carried on, and the work of each is done, 

 outside the blood, in the little plots of tissue into which 

 no red corpuscle comes. ^^.^ -^^..v^-.:. 



The capillaries are closed tubes ; they keep the red 

 corpuscles in their place. But their walls are so thin 

 and delicate that they let the watery plasma of the 

 blood, the colourless fluid in which the corpuscles 

 float, soak through them into the parts inside the 

 mesh. You probably know that many things will 

 pass, through thin skins and membranes in which no 



holes can be found even after the most careful search. 



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G 2 



