72 THE VITAL PROCESSES 



and from the cells toward the capillaries is not entirely 

 understood. Blood pressure in the capillaries undoubtedly 

 has much to do in forcing the plasma through the capil- 

 lary walls, but this tends to prevent the movement of the 

 lymph in the opposite direction. Movements between the 

 blood and the lymph are known to take place in part 

 according to a general principle, known as osmosis, or 

 dialysis. 



Osmosis. The term " osmosis " is used to designate the passage of 

 liquids through some partition which separates them. Thus, if a vessel 



with an upright membranous partition 

 be filled on the one side with pure 

 water and on the other with water con- 

 taining salt, an exchange of materials 

 will take place through the membrane 

 until the same proportion of salt exists 



FIG. 31.- Vessel with an on the two sides (Fig. 31). The cause 

 upright membranous partition for rf ^^ . g ^ ^.^ rf ^ ^^ 



illustrating osmosis. . . . x , A . 



cules, or minute particles, that make up 



the liquid substance. If the partition were not present, this motion 

 would simply cause a mixing of the liquids. 



Conditions under which Osmosis occurs. Osmosis may be shown by 

 suitable experiments (see Practical Work) to take place under the fol- 

 lowing conditions : 



1. The liquids on the two sides of the partition must be unlike either 

 in density or in composition. Since the effect of the movement is to 

 reduce the liquids to the same condition, a difference in density causes the 

 flow to be greater from the less dense toward the denser liquid, than in 

 the opposite direction ; while a difference in composition causes the 

 substances in solution to move from the place of greater abundance 

 toward places of less abundance. 



2. The liquids must be capable of wetting, or penetrating, the par- 

 tition. If but one of the liquids penetrates the partition, the flow will 

 be in but one direction. 



3. The liquids on the two sides of the partition must readily mix 

 with each other. 



Osmosis at the Cells. In the body osmosis takes place between the 



