THE FLUIDS OP THE BODY 39 



various other structures, the walls of capillary vessels, while 

 offering practically no resistance to water and diffusible salts 

 which can pass through membranes, prevent proteid substances 

 from passing from blood to lymph, except in extremely small 

 quantities. In this way an exquisite balance is automatically 

 maintained. Water and salts pass out as they are needed. 

 But they never pass out in excess, because the protein-contain- 

 ing blood-stream tends to keep them in, in virtue of the same 

 attractive force which enables it to suck in the oxidized pro- 

 ducts thrown into the lymph by the tissues. Whatever a tissue 

 needs it takes from the lymph. Suppose that bone is being 

 formed. Large quantities of lime and phosphates are needed for 

 the calcification of the cartilage in which it is modelled. The 

 cartilage absorbs lime and phosphates from the lymph which 

 bathes it. Lime salts and phosphates immediately begin to 

 diffuse from blood into lymph. The hurrying blood-stream 

 brings up further supplies from the walls of the intestine, 

 products of digested milk and other foods. Lymph contains 

 (although not in the same proportions) everything which blood 

 contains. Many an analogy may be found in the world of 

 economics, although no illustration would be sufficiently 

 complete. From the lymph tissues take the fuel that they need, 

 the oxygen with which to burn it, the foods for their own 

 repair, the raw materials for their arts. Into it they throw 

 their smoke, their drainage, the slag and refuse of their fac- 

 tories. The blood replaces the supplies as they disappear. It 

 absorbs all waste. Lymph occupies streets, market-place, 

 passages, corridors. The blood-stream is a closed system, 

 rolling down the streets and through the market-place, on its 

 never-ceasing circuit from port and mine to open air and open 

 sea. From the alimentary canal it picks up food and fuel ; 

 the lungs give it oxygen, and disperse its carbonic acid ; the 

 kidneys purge it of non-gaseous waste. 



The facility with which the constituents of blood pass out 

 to the lymph, and the constituents of lymph pass into the 

 blood, depends upon the condition of the walls of the capillary 

 vessels. Water and substances dissolved in water might pass 

 through the wall of a capillary vessel in either of three ways 

 by filtration, by osmosis, or by secretion. A filter is a porous 

 barrier, which allows water and all substances dissolved in 



