712 SPECIAL PHYSIOLOGY. 



stimulating, or excretive; they all represent stages of chemical retro- 

 gression, from less to more oxidized compounds, and may, by their 

 further oxidation, assist in the evolution of heat. As completely effete 

 and non-nutritious, or even poisonous, must be classed the urea, de- 

 tectable, in minute traces, in the blood, as well as the ammonia which 

 escapes from it when it is drawn, and the uric acid, all of which are 

 excretory substances. 



The salts of the blood are said to prevent its decomposition, and also 

 to regulate its chemical characters and its specific gravity or density, 

 so as to adapt it to the healthy condition of the liquor sanguinis and 

 blood corpuscles floating in it. It is well known that certain salts are 

 preservative, and, likewise, that if the blood corpuscles be suspended 

 in a fluid of too low a specific gravity, they immediately become dis- 

 tended, a process of endosmosis going on into them ; whereas, if the 

 fluid be of too high a specific gravity, they shrink by exosmosis. But 

 neither of these supposed uses, explains the great variety of the saline 

 constituents of the blood; for one saline substance alone, say common 

 salt, would have sufficed for both these purposes. Several uses are 

 probably served by this variety. Thus, some salts, perhaps, are ne- 

 cessary for the maintenance of the properties of blood ; others are 

 destined for the nutrition of certain tissues, or the formation of certain 

 secretions; whilst others again appear to be the result of the disinte-, 

 grating and oxidating processes going on in the tissues and organs of 

 the body, during the exercise of their respective functions. 



Thus, common salt or chloride of sodium, appears to be present in 

 the blood of all animals, and in every tissue. Its great importance is 

 evidenced by the tenacity with which it is held in the bodies of animals, 

 and accumulates in their blood and tissues, even when, as in the cases 

 of the herbivorous species, the food which they consume, contains com- 

 paratively minute traces of it. The strong necessity and appetite for 

 salt, felt by the herbivorous Mammalia, is shown by their licking lumps 

 of that substance, or boiled bones, scattered about their pastures; and 

 also by the periodical migrations of herds of cattle to the salt dis- 

 tricts in South America, facts which indicate that salt is indispensable 

 for the healthy condition of the blood, and of the tissues which are 

 nourished from it. It would seem, indeed, that chloride of sodium is 

 associated with every important act of tissue formation and change. 

 Of the secretions, all exhibit minute traces of salt; but the gastric 

 juice in particular, contains an acid the hydrochloric derivable only 

 from the salt of the blood; for the quantity of chloride of potassium, 

 as compared with the chloride of sodium, in the blood, is so small, that 

 it may be inferred that the latter is the source of this acid of the gastric 

 juice. The large quantity of soda present in the bile, in combination 

 with its fatty acids, is probably -also derived from the common salt in 

 the blood ; the separation of the chlorine from this, for the formation 

 of the hydrochloric acid of the gastric juice, may be accompanied by 

 the transference of the sodium, in the shape of soda, to the hepatic 

 cells, for combination with the biliary acids. Again, the lime salts in 

 the blood, chiefly the phosphate of lime with carbonate, held in solution 

 in lactic and carbonic acids, are highly important nutritive substances, 



