18 THE HUMAN BODY 



through the membrane. We shall learn later how much the mem- 

 branes affect the cells which they enclose through the influence 

 they have on the passage of materials into and out of the cells. 



Intercellular Spaces and Intercellular Fluids. In the grouping 

 of cells into tissues (p. 8) we find, even in those that are most 

 compact, minute spaces among the cells. There are points of 

 union between cell and cell, holding the tissue together, but these 

 involve only relatively small portions of the total cell surfaces. 

 Every cell has a large part of its surface fronting on intercellular 

 spaces. These spaces are filled with watery fluid called lymph, 

 which bathes the individual cells, and, in fact, forms their sole en- 

 vironment. The nourishment of the cells reaches them by way of 

 the lymph; the discharges of waste materials from the cells are 

 into the lymph. The interchanges between the cells and lymph 

 are therefore the fundamental interchanges of the Body. They are 

 subject, in part, at least, to certain definite laws given below. 



Filtration, Osmosis, and Dialysis. At every step in the com- 

 plex process of supplying the living cells with nourishment and 

 removing from them their harmful waste products the membranes, 

 described above, stand in the way of the substances involved and 

 must be traversed by them. There are membranes between the 

 protoplasm of the cells and the lymph which surrounds them. 

 The digested 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 



