FOOD DIGESTION AND RESPIRATION 59 



of their molecules, compounds containing them, so that they will 

 be taken as food by the animal along with the other parts. The 

 higher plants are able to make these compounds for the animal 

 from inorganic salts, sulphates and phosphates in the soil. The 

 animal also, to some extent, uses inorganic compounds of sulphur 

 and phosphorus. 



We may note again here that there are certain constituents in 

 protein food which cannot be made by the animal organism itself, 

 and must be supplied. They are probably required for the 

 replacement of particular parts of the cell machinery, although 

 it is also possible that important chemical products need certain 

 chemical groupings to be provided, the animal cell being unable 

 to make them. 



Salts 



Inorganic salts are found to be present in living cells, and in the 

 food taken by living organisms. But the question may be asked, 

 Are they necessary, or only present because food materials always 

 contain them ? 



The relationship of colloids and salts, briefly discussed in the 

 preceding chapter, indicates that salts must play an important part 

 in the colloidal changes of the cell. It has been found by 

 experiment that certain inorganic elements are necessary, not only 

 for growth, but for the proper working of the activities of the 

 living cell. 



The heart of a frog can be made to continue beating if supplied 

 with a solution of inorganic salts only. We have learned in our 

 study of the osmotic pressure of cell contents that such a solution 

 must possess a particular osmotic pressure, otherwise the tissue 

 cells either swell or shrink. We can give this value to our solution 

 by sugar or by sodium chloride. It is usual to do so with the 

 latter, because of its convenience ; but there is good evidence that 

 sodium chloride is somewhat toxic, and that a part of it may, with 

 advantage, be replaced by its osmotic equivalent in cane sugar. 

 Even if we do this, however, we find that we cannot keep the heart 

 beating normally for more than a short time (E., p. 187). We find 

 that both calcium and potassium are necessary in small amounts, 

 and that there is a certain proportion between the three cations 

 that gives the best results. It may have been noticed that no 

 mention was made of any particular salt of these metals ; in fact, 

 the anions may be of various kinds indifferently. This must not 

 be taken to imply that the anion plays no part, but rather that its 

 function is one common, more or less, to all anions, apparently due 

 to the sign of the electrical charge. Certain special properties, 

 on the other hand, are required in the cations. Calcium ions, 



