THE LYMPH AND ITS MOVEiMENT 73 



blood and the lymph and between the lymph and the cells, the movements 

 being through the capillary walls and the membranes inclosing the cells 

 (Fig. 27). Oxygen and food materials, which are found in great abun- 

 dance in the blood, are less abundant in the lymph and still less abun- 

 dant in the cells. According to the principle of osmosis, the main flow 

 of oxygen and food is from the capillaries toward the cells. On the 

 other hand, the wastes are most abundant in the cells where they are 

 formed, less abundant in the lymph, and least abundant in the 

 blood. Hence the wastes flow from the cells toward the capillaries. 



Solutions. Neither the blood plasma nor the lymph, as already 

 shown, are simple liquids ; but they consist of water and different sub- 

 stances dissolved in the water. They belong to a class of substances 

 called solutions. The chief point of interest about substances in solu- 

 tion is that they are very finely divided and that their little particles 

 are free to move about in the liquid that contains them. Both the 

 motion and the finely divided condition of the dissolved substances 

 are necessary to the process of osmosis. All substances, however, that 

 appear to be in solution are not able to penetrate membranes, or take 

 part in osmosis. 



Kinds of Solutions in the Body. The substances in solution in the 

 body liquids are of two general kinds known as colloids and crystal- 

 loids. The crystalloids are able to pass through membranous parti- 

 tions, while the colloids are not. An example of a colloid is found in 

 the albumin of an egg, which is unable to penetrate the membrane 

 which surrounds it. Examples of crystalloids are found in solutions 

 of salt and sugar in water. The inability of a colloid to penetrate a 

 membrane is due to the fact that it does not form a true solution. Its 

 particles (molecules), instead of being completely separated, still cling 

 together, forming little masses that are too large to penetrate the mem- 

 brane. Since, however, it has the appearance, on being mixed with 

 water, of being dissolved, it is called a colloidal solution. The crystal- 

 loid substance, on the other hand, completely separates in the water 

 and forms a true solution one which is able to penetrate the partition 

 or membrane. 



Osmosis not a Sufficient Cause. The passage of materials through 

 animal membranes, according to the principle of osmosis, is limited to 

 crystalloid substances. But colloid substances are also known to pass 

 through the various partitions of the body. An example of such is 

 found in the proteids of the blood which, as a colloidal solution, pass 

 through the capillary walls to become a part of the lymph. Perhaps 



