644 THE MECHANISM OF ABSORPTION 



the cells and large quantities in the subepithelial layer, it seems 

 possible that such may be the case. Hober extended MacCallum's 

 research and failed to demonstrate the salts of the other heavy 

 metals, Hg, Pb, Cu, &c., within the cells by micro-chemical tests, 

 although they are known to be absorbed. He treated sections of 

 epithelium which had absorbed NaCl or MgS0 4 with alcoholic 

 AgN0 3 and BaCl 2 solutions respectively ; but the results were in- 

 conclusive as to whether or not any of the salts had passed into 

 the cells. He demonstrated within the cells certain basic dyes, 

 such as methylene-blue, which are known to produce intra- 

 vital staining of cells, but others which have not this property 

 were found only intercellularly. In order to explain the path 

 taken by different substances Hober adopted Overton's theory of 

 the chemical permeability of the superficial layer of cell proto- 

 plasm. According to this theory only such substances can pass 

 through this layer as are soluble in mixtures of cholesterin, lecithin, 

 cerebrin, protagon, &c. Overton's lipoids. Such substances are 

 water, alcohol, some basic dyes, urea, ammonium salts, and the 

 products of fat digestion. On the other hand, most salts and 

 carbohydrates are insoluble in lipoids. Hober therefore explained 

 that urea was absorbed at a rate greater than corresponded to 

 its diffusion velocity, because, unlike most salts, it could be 

 absorbed by the cells as well as by the cement substance. The 

 difficulties of this view are obvious. For instance, inorganic iron 

 salts are not soluble in lipoids and yet are absorbed intracellularly. 

 But the greatest difficulty of all is the one-sided permeability of 

 the intestine to NaCl. If this salt passes only by an intercellular 

 path, then we have to believe that the apparently structureless 

 and dead cement substance is of such a nature that it allows nor- 

 mally the passage of NaCl in only one direction. Such a mechanism 

 may of course exist, but we have no knowledge of a physical 

 membrane with such a one-sided permeability. Although some 

 substances may be absorbed through the intercellular cement, 

 Hober' s view cannot be held to explain satisfactorily the different 

 absorption rates of various substances. And it is better in the 

 present state of our knowledge and for the sake of simplicity to 

 consider that the main path of absorption of all substances is 

 through the cells. 



We must now deal with the experiments which have been 

 considered to be inexplicable by physical processes, and which 



