THE CHEMISTRY OF THE ANIMAL BODY. 965 



SODIUM, Na = 23. 



Sodium salts belong particularly to the fluids of the body (see p. 948), 

 blood-plasma containing 0.4 per cent, calculated to Na 2 O. 



Sodium chloride, NaCl, is found in all the fluids of the body. It is found 

 in blood and lymph to an extent of about 0.65 per cent., in the saliva, gastric 

 juice, milk, sweat, urine, etc. Sodium chloride, like potassium chloride, melts 

 at a low red heat, hence the fluids of the body yield a fluid ash, with the single 

 exception of milk, which contains a high percentage of infusible calcium phos- 

 phate. Sodium chloride is very readily soluble. In the blood it acts as a 

 solvent on serum-globulin and other proteids, and its inert presence in proper 

 concentration affords a medium in which the functional activity of cells and 

 tissues is maintained. (For " physiological salt-solution " see p. 948.) From 

 sodium chloride the hydrochloric acid of the gastric juice is prepared (see p. 

 952) ; it is also a necessary addition to every food where potassium salts are 

 in great preponderance (see p. 964), but it is taken by most races in amounts 

 far above these physiological necessities. 



If a mixture of necessary food-stuffs proteid, fats, starch, salts, and water in proper 

 proportion, but without flavor, be set before a dog, he will starve rather than touch it. A 

 man will attempt its digestion, but the permanent support of life is impossible. A food 

 to support life must be a well-tasting mixture of food-stuffs, for, through the action of the 

 flavor on the mucous membrane of the mouth and stomach there is established reflexly a 

 nervous influence causing a proper flow of the various digestive juices. Hence salt, 

 pepper, mustard, beer, wine, and other condiments are taken with the food. What the 

 change is, when a substance acts on the taste-buds of the tongue, for example, start- 

 ing a motion such as is afterwards interpreted in the brain as flavor, is unknown. 

 Chemical constitution gives no hint how a body will taste or smell. 



In carnivora every trace of sodium chloride is absorbed by the villi from 

 the intestinal tract. This is a proof that absorption does not depend on simple 

 physical osmosis, in which case the intestinal contents would tend to have the 

 same percentage composition as the blood, but upon the selective capacity of the 

 exposed protoplasm of the villi. Sodium chloride is the principal solid con- 

 stituent of sweat and of tears. Usually, however, it is lost to the body 

 through the urine, of whose ash it forms the chief constituent. The quantity 

 of salt in the urine is decreased during gastric digestion (see p. 952). Sodium 

 chloride does not pass to the urine as soon as it rises above a certain quantity 

 in the blood, but the tissues retain or give it up according to circumstances. 

 Experiments l have been made on a man who ate normally 27 grams of salt 

 daily; on reducing this to 1.4 grams the following daily excretions occurred 

 in the urine : 9.9, 6.5, 3.8, 4.1, 3.2, 2.9, 2.9, 2.5. Then, on returning to 27 

 grams daily : 3.4, 7.9, 11.2, 15.8, 17.4. Experiments of abstention have never 

 been carried so far as to produce vital disturbances, but the physiological min- 

 imum is probably very low. A dog weighing 35 kilograms may live on 0.6 

 gram of salt daily. 2 Sodium chloride, fed, produces of itself alone an increase 



1 Klein und Verron : Sitzungsberichte der Wiener Academic, Mathematisch-physikalische Classe, 

 1867, iv. (2), p. 622. 



2 Voit: Hermann's Handbuch, 1881, vi. 1, p. 367. 



