258 THE HUMAN BODY 



phenomenon which is of great importance in the working of the 

 Body. At every step in the complex process of supplying the 

 living cells with nourishment and removing from them their 

 harmful waste products membranes stand in the way of the 

 liquids involved and must be traversed by them. Thus the di- 

 gested food must pass through the membranous lining of the 

 digestive tract before it can enter the blood; the oxygen of the 

 air must pass through a membrane in the lungs on its way to the 

 same medium. The juices which are secreted or excreted have 

 to be forced through membranes in passing out from the organs 

 from which they come. The movements of liquids through the 

 membranes of the Body take place for the most part in accordance 

 with certain physical principles which may conveniently be stated 

 at this point. 



FILTRATION. If a membranous bag such as an ox bladder be 

 filled with a liquid and pressure be applied to the liquid in the 

 bag a point may be reached where the liquid is squeezed through 

 the membrane and appears in drops on its outer surface. This is 

 an example of filtration. When a liquid is filtered in this way any 

 solid particles which may have been suspended in it are left be- 

 hind, but any substances which may be dissolved in it pass through 

 as part of the liquid. Thus a salt solution which contained some 

 particles of sand might be filtered and the sand removed, but the 

 solution would have just as much salt dissolved in it after filtra- 

 tion as before. 



OSMOSIS. If we should take such a membranous bag as de- 

 scribed above filled with salt solution and dip it into a vessel of 

 pure water, so that the surfaces within and without the bag are 

 at the same level and taking care that no water could run over 

 the edge into the bag, it would be seen after a while that the level 

 of liquid within the bag had risen while that in the vessel outside 

 had correspondingly fallen. That is, there would have been an 

 actual movement of water into the bag with sufficient force to 

 overcome the pressure resulting from the change of water level 

 on the two sides of the membrane. Whenever two solutions of 

 different concentrations are separated by a membrane which is per- 

 meable to water there will be a flow of water through the membrane 

 in the direction of the greater concentration. This phenomenon is 

 known as osmosis. The force which drives the water is called 



