218 HUMAN PHYSIOLOGY 



Non-protein foods are represented in the plasma but 

 more scantily than would be expected. The sugar is 

 usually not much more than one part in a thousand, while 

 the fat content is higher but still a mere fraction of 1 

 per cent. The salts of the plasma are kept nearly con- 

 stant by the reciprocal adjustment of excretion to ab- 

 sorption. The conspicuous one is sodium chlorid or 

 " common salt/' the only one which we are at pains to 

 add to the diet. Salts of calcium and potassium are 

 present in very small quantities. It might be thought 

 that their being in the blood was purely accidental and 

 of no moment. This is far from being the case for the 

 removal of either calcium or potassium from the plasma 

 is most disturbing to many of the activities of the tissues. 



An interesting suggestion has been made that the 

 salts of the plasma are the same in variety and pro- 

 portion as those that existed in the prehistoric sea. 

 The simpler marine animals might be expected to have 

 their body fluids based on sea water. Their descendants 

 would inherit this standard of composition. Now the 

 sea water of our age is perhaps three times as concen- 

 trated as the blood of vertebrate animals, but geology 

 teaches that it was formerly dilute and must always be 

 gaining in salts as they are washed from the rocks and 

 soil. It is a fascinating thought that races of animals 

 may have more power to maintain fixity in their makeup 

 than the inorganic world around them. 



We should naturally examine the plasma for the 

 presence of compounds clearly recognizable as waste- 

 products. These are in fact to be found but in singularly 

 small amounts. This condition points to an extra- 

 ordinary efficiency on the part of the excretory organs, 

 especially the kidneys. One waste-product is indeed 

 found in relative abundance; this is carbon dioxid 

 and it is carried both in the plasma and in the corpuscles. 

 We infer the existence in the plasma of many bodies, 

 such as hormones, not because we can detect them by 

 chemical tests, but because the blood has certain actions 



