EMULSION-STRUCTURE OF PROTOPLASM 291 



tissue-fluids. Evidently the intestinal epithelium permits the passage 

 of certain dissolved substances into the tissue-fluids behind it, but not 

 the migration of dissolved substances in the reverse direction. 



The maintenance of one-sided permeability in tissues is dependent 

 upon the maintenance of the unimpaired structure of the cells. Thus 

 the phenomenon of one-sided permeability is nowhere more clearly 

 illustrated than it is in the kidneys, where the dissolved constituents 

 of Urine are constantly excreted against a high pressure, the tissues of 

 the kidney being much more permeable for dissolved substances in the 

 direction blood - urine than in the direction urine - blood. If, how- 

 ever, the epithelium of the renal tubules is injured by perfusion with 

 solutions of certain substances, for example Sodium Fluoride, it loses 

 this power and comes to resemble much more closely a membrane of 

 parchment. This is very clearly illustrated by the following experi- 

 ment by Bottazzi. One kidney in a dog was injured by perfusion with 

 sodium fluoride solution. The ureters of the two kidneys were then 

 separately catheterized and the freezing-points of the samples of urine 

 collected from the two kidneys were determined from time to time. 

 The following were illustrative results : 



Depression of Depression of 



Urine c.c.: freezing-point. freezing-point 



Time. .. . of blood. 



Normal. Injured. Normal. Injured. 



2.30 to 3.00 



3.30 to 4.00 



4.50 to 5.20 



5. 30 to 6.00 



6.00 to 6.30 



9.30 to 10.00 



8.00 to 8.30 



9. 12 1.616 0.979 0.572 



14. 20 1.118 0.294 



14. 22 0.584 0.240 



10. 22 0.570 0.224 



12. 20 0.572 0.212 0.560 



4. 9 1.002 0.206 



2.5 6 1.304 0.302 0.569 



It is evident that the phenomenon of one-sided permeability must be 

 dependent upon a heterogeneous structure of the membrane which 

 displays it. The phenomenon is not, and could not be displayed by 

 structureless membranes, or by membranes having a uniform structure 

 in the direction of penetration, i. e., perpendicularly to their surface. 

 For instance, consider a membrane formed of successive columns, of 

 two different materials, one of which permits the passage of substances 

 soluble in water while the other does not. Then if the arrangement 

 of these two components were th,at displayed in Fig. 15 substances 

 soluble in water could penetrate the unshaded channels just as easily 

 from below as from above the membrane. But if the membrane were 

 curved, so as to bring the columns of impenetrable material closer 

 together on the under than on the upper surface, or if they were 

 pyramidal in shape so as to achieve the same end, so that the arrange- 

 ment would be that displayed in Fig. 16 then it is evident that the 

 penetrable area on the under surface of the membrane would be a 

 much smaller proportion of the whole area than on the upper surface 

 of the membrane, so that substances penetrating from above would 

 do so with comparative ease, while substances issuing from below the 

 membrane would do so with difficulty. 



