124 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



that pressure depends upon the density of the solution. In 

 course of time, however, the crystalloid passing itself through the 

 membrane, conditions of equal volume and density will be estab- 

 lished on the two sides of the membrane, and osmosis in either 

 direction will cease to be apparent. But if the membrane be non- 

 permeable to the dissolved substance, an excess of water will pass 

 to the colloid side and will continue so to pass until finally it will 

 be inhibited by hydrostatic pressure on that side. This is taken 

 as the measure of osmotic pressure for the colloid. 



All substances in solution, whether crystalloids or colloids, 

 exert a certain osmotic pressure; that is, they may be said to 

 interfere with the passage of a current from their side of the 

 membrane, and that interference depends on the number of 

 molecules in solution, or, in other words, upon the density Oi 

 the fluid. A fanciful but striking illustration refers the explana- 

 tion to the continual molecular motion: the molecules of the 

 dissolved substance act as a screen to protect the membrane 

 from the water molecules, which are incessantly moving against 

 it, and consequently, in a given time, more molecules of water 

 will strike and pass through the membrane on the unscreened 

 than upon the partially screened side. Evidently the number of 

 molecules in solution (the density) has a material influence upon 

 the escape of water from that side. Of course, since a crystal- 

 loid finally passes to the less dense side in sufficient quantity to 

 establish an equilibrium, the effect of its osmotic pressure is only 

 temporary; but while the osmotic pressure of a colloid may be 

 less than that of a crystalloid, its effect is inclined to be perma- 

 nent. For instance, if a hypertonic solution (one whose density 

 is greater than that of blood serum) of sodium chloride be 

 injected into the blood, the first effect is to cause an increased 

 flow of water to the vessels, but soon enough sodium chloride 

 passes out by osmosis to raise the density of the extravascular 

 fluids, and thus to cause an escape of water/row the vessels. On 

 the other hand, the osmotic pressure exerted by the proteids of 



