60 INTRODUCTION 



fluids bathing them; e. g., animal cells generally contain more 

 potassium and less sodium than the fluids surrounding them. 

 The inorganic constituents of red cells are totally different from 

 those of the plasma r the corpuscles not containing any calcium at 

 all, while the magnesium seems to enter them freely ; in other 

 words, the red corpuscle seems to be impermeable to calcium 

 and permeable to magnesium. If the salts in a corpuscle are in 

 smaller proportion than in the surrounding fluid, it indicates that 

 the cell membrane is not freely permeable for them ; if in 

 greater proportion, that some constituent of the cell is holding 

 them in combination, possibly as ion-proteid compounds. Prob- 

 ably inorganic salts are present in the cell by virtue of both 

 physical and chemical influences, some simply diffusing in and 

 out, others combining with the proteids and being held chemi- 

 cally. 



The intercellular substance varies greatly in different 

 tissues. In the case of the supportive tissues it is the impor- 

 tant element, and the cells seem to exist chiefly for the purpose 

 of forming and keeping it in repair as it is worn out. In the 

 epithelial and secreting tissues, however, the intercellular sub- 

 stance is reduced to a minimum, except in so far as a cement 

 substance is required, and the cells generally lie in almost 

 immediate apposition. It is probable that there is a greater or 

 less amount of cement substance, even between the most closely 

 applied cells, and this substance seems to be related to mucin. 

 It can generally be brought out by staining with silver nitrate, 

 and Macallum l points out that this reaction is merely a micro- 

 chemical test for chlorides, and indicates that the cement sub- 

 stance contains them in larger proportion than does the cyto- 

 plasm. 



1 Proceedings of the Koyal Society, 1905 (76), 217. 



