CRYSTALLOIDS OF PLASMA 311 



plasma and in sea water are not similar, yet there is a remarkable 

 resemblance in the proportions of the main salts present in both. 



The similarity in proportion is not very striking because the 

 figm'cs given are from analysis of the ocean as it is to-day. What 

 we should have is the analysis of the ocean in pre vertebrate days. 



Not only has the concentration of the salts of the sea undergone change, 

 but alterations have taken place in the proportion of its constituents. Sodium 

 and magnesium have increased in concentration and are still increasing. 

 Material lixiviated from river beds, etc., is rich in those salts. 



On the other hand, potassium and calcium have decreased. The formation 

 of soil leads to the abstraction of potassium from the river water. Water 

 evaporated from the ocean contains potassium in not inconsiderable amounts. 

 The rain falling in the region of Caen is responsible for an annual increment 

 to the land of 1"23 tons of potassium per square mile. Rivers discharge more 

 calcium into the ocean than they do of sodium, magnesium or potassium, and 

 yet the concentration of this element appears to be fairly steady. The cause 

 for this lies in the formation of rock-beds of gypsum (CaS04) and limestone 

 (CaCOg) and in the formation of calcareous skeleta (CaHPO^). 



From the study of fossil seas and of lakes surrounded by pre- 

 Cambrian formations as well as from other geological considerations 

 it has been decided that the ocean of Cambrian days had a ratio of 

 salts somewhat as follows. 



Present day 



Cambrian 



Serum 



The salt content of the plasma is regulated by the kidneys. It 

 is interesting to note that while the blood is Cambrian the tissues 

 are decidedly pre-Cambrian in their salt content, e.g. Muscle. 

 Na = 100, K = 4.00, Ca = 9-3, Mg = 26-4. This may be in part 

 accounted for by the adsorption of salts by the protoplasmic 

 colloids. Of these salts, one of the most important is sodium 

 bicarbonate on account of its power of neutralising acid. This 

 has been termed its "buffer" value — a term which, although 

 faulty, has crept into the writings of physiologists and clinicians 

 and seems firmly ensconced there because it is handy. As Prof. 

 Bayliss points out, a railway buffer absorbs shock but not the 

 engine, while NaHCOg absorbs the acid. 



The amount of NaHCOg present in plasma has been called the 

 alkali reserve of the body. How does it act ? The addition of 

 acid to bicarbonate may be represented by the equation 



NaHCOg + HA - NaA + H^O + CO,. 



