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 established on the two sides of the membrane, and osmosis 

 in either direction will cease to be apparent. But if the 

 membrane be nonpermeable 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 os- 

 motic 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 of 

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

 planation 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 mem- 

 brane 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 crystalloid finally passes to 

 . the less dense side in sufficient quantity to establish an equi- 

 librium, 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 permanent. 

 For instance, if a hypertonic solution (one whose density is 

 greater than that of blood serum) of sodium chloride be in- 

 jected 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 extravascu- 

 lar fluids, and thus to cause an escape of water from the ves- 

 sels. On the other hand, the osmotic pressure exerted by the 



