154 ANATOMY FOR NURSES [Chap. IX 



tissue. There is thus a continual interchange going on between 

 the blood and the lymph. This interchange is effected in two 

 ways : — 



{\) By dialysis. — The lymph becomes altered by the metabolic 

 changes of the tissues which it bathes, and we have two different 

 fluids, separated by the moist membrane which forms the walls 

 of the blood-vessels, — the lymph in the tissues outside the walls of 

 the capillaries and the blood inside the capillary walls. Some of 

 the constituents of the lymph pass into the blood, while some 

 of the constituents of the blood pass into the lymph, by the process 

 of dialysis.^ 



These constituents, which, as we cannot too often emphasize, are 

 products resulting from the activity of the tissues, are carried away 

 by the blood to other tissues, which will either make use of them, 

 or, as in the kidneys, take them up to make excretory fluids, 

 and so remove them. 



(2) By osmosis. — The blood, on account of the higher pressure, 

 loses more liquid to the lymph than it receives back by the process 

 of osmosis. This excess lymph gathers up the waste materials 

 of the cells surrounding the lymph spaces and through the medium 

 of the lymphatics pours this waste into the blood, to be eliminated 

 by the skin, lungs, and kidneys. 



In consequence of the different wants and wastes of different 

 tissues at different times, both the lymph and blood must vary in 

 composition in different parts of the body. But the loss and gain 

 is so fairly balanced that the average composition is pretty con- 

 stantly maintained. 



The chyle, or lymph of digestion, absorbs nutrient materials 

 (mostly fat) from the intestines and pours this food into the blood 

 current, to be distributed to all parts of the body. 



1 See page 12. 



