In a chemical point of view there is no more important mineral constituent in the human body than the chloride of sodium, for it occurs nearly in every part of the system, both solid and fluid, in close and intimate relation with the organic compounds, and it materially influences their chemical and physical properties; for instance, the albumen partly owes its solubility to the presence of salt, the quantity of which causes the differences which it presents as regards its coagulation; pure caseine, which is quite insoluble, is dissolved at once on the addition of common salt; and if it is added in increased proportions it impedes the coagulation of the fibrine. Another remarkable physiological fact is that the chloride of sodium is not only uniformly present, but its various proportions are nearly definite and constant, both in the fluids and tissues; and the existence of a provision for the limitation of the quantity kept in the system causes the proportions to be little affected, in the way of excess at least, by the amount of salt the food may contain, that is, if the diet is wholesome and the individual healthy.
According to Lehmann, who experimentalised on himself, the blood may contain in a normal state 4·14 parts of the chloride of sodium in 1000, and after a meal of very salty food it may be only increased to 4·15; he says it only rose to 4·18 when two ounces of salt had been taken an hour before, and two quarts of water had been drunk in the interval. The blood will not receive more than a certain amount; and as an over-amount of salt will produce extreme thirst, a quantity passes through the kidneys with the water that has been drunk; frequent drinking of course causes frequent micturition. If we take the mean of numerous observations by several experimenters, the average quantity of urine voided by healthy adult males is about 52-1/2 fluid ounces; this quantity may contain urea to the amount of 512·4 grains, chlorine 105·0 grains, and soda 125·? grains, besides other salts and extractives. If such an amount passes out with the urine we can easily account for the slight increase in the blood of Lehmann.
The quantity of the chloride of sodium in the blood is liable to great variation in different diseases; and there can be little doubt that this variation is closely connected (though whether in the relation of cause or in that of effect, we are not exactly entitled to surmise) with the histological and other transformations of the component parts of the blood. The proportions of salt greatly differ in several tissues, and also at different periods of the development of the same tissue. “Thus in muscle,” according to Enderlin, “100 parts of the ash left after incineration of ox-flesh yielded nearly 46 per cent. of the chloride of sodium and potassium; which, as this ash constitutes 4·23 per cent. of the dried flesh, would give 1·94 as the proportion of the chloride of sodium in 100 parts of the latter; and reckoning this dried residue to constitute 28 per cent. of the whole substance of the muscle (the remaining 77 parts being water) the proportion of chloride of sodium in the latter will be 0·44. These figures, as will be presently seen, bear a remarkably close correspondence to those which represent the proportion of chloride of sodium in the ash, solid residue and entire mass of the blood.”[57]
Next to muscle, cartilage contains the largest amount of the chloride of sodium, and this especially in the temporary cartilages of the fœtus, its place being taken by the phosphate of lime as it approaches the time of birth. The percentage of the chloride of sodium contained in the ash of the costal cartilage of an adult is about 8·2; in the laryngeal cartilage 11·2; but as the ash does not constitute above 3·4 per cent. of the entire substance the percentage of the chloride of sodium in the latter is, at the most, 0·38 of the whole, or less than that of blood and muscle. Only from 0·7 to 1·5 per cent. could be extracted from the ash of bone.
Besides the important uses of the chloride of sodium in the blood to which we have already adverted, it serves the purpose of furnishing the hydrochloric acid required (by many animals, at least) for the gastric secretion; and it likewise supplies the soda-base for the alkaline phosphate, whose presence in the blood appears to serve a most important purpose in the respiratory process. Moreover, there is reason to think, from the experiments of Boussingault upon animals, as well as from other considerations, that the presence of salt in the blood and excreted fluids facilitates the deportation of excrementitious substances from the blood. The proportion in which it occurs in the principal animal fluids is represented by the following table, constructed by Lehmann chiefly from his own analyses; it is highly interesting, and shows us at a glance that it is more important in the economy than any other substance, and is significant of the fact that health cannot exist long if the chloride of sodium is below the normal amount.
Percentage of Chloride of Sodium in various Animal Fluids, their Solid Residue and their Ash.
| Liquid. | Solid Residue. | Ash. | |
| Human Blood | 0·421 | 1·931 | 57·641 |
| Blood of Horse | 0·510 | 2·750 | 67·105 |
| Chyle | 0·531 | 8·313 | 67·884 |
| Lymph (Nasse) | 0·412 | 8·246 | 72·902 |
| Serum of Blood (Nasse) | 0·405 | 5·200 | 59·090 |
| Blood of the Cat (Nasse) | 0·537 | 2·826 | 67·128 |
| Chyle (Nasse) | 0·710 | 7·529 | 62·286 |
| Human Milk | 0·087 | 0·726 | 33·089 |
| Saliva | 0·153 | 12·988 | 62·195 |
| Gastric Juice of Dog | 0·126 | 12·753 | 42·089 |
| Human Bile | 0·364 | 3·353 | 30·464 |
| Mucus (Nasse) | 0·583 | 13·100 | 70·000 |
| Serum of Pus (Nasse) | 1·260 | 11·454 | 72·330 |
We have thus proved physiologically that the chloride of sodium holds a most prominent position among the other constituents of the body; that it is present in considerable quantities in muscle as well as in the blood; and that it furnishes the acid, which is necessary for the stomach to perform its functions of digestion. It holds the albumen partly in solution, and its coagulation is dependent more or less on the amount of salt which is present in the blood, and it also possesses the power of preventing the coagulation of the fibrine. In fevers the blood is generally thicker, and has a tendency to coagulate by reason of the partial absence of salt, because a good deal passes off with the perspiration; and fever patients always prefer salt to sugar, for while one refreshes them and helps to restore the usual healthy tone of the palate, by constringing the papillæ of the tongue, the other raises feelings of disgust.
It is also present in cartilage, though in a lesser degree than in blood or muscle, because in cartilage there is no disintegration or waste of tissue, and therefore it does not require such a perpetual supply; there is, on the contrary, a continual loss going on in muscle, especially during exertion. Perspiration is to a certain extent the principal medium which carries off the chloride of sodium, owing to its being held in solution by the liquor-sanguinis; during fatigue, particularly if prolonged, a greater quantity passes off, producing various degrees of thirst. That the normal proportion of the chloride of sodium should be regularly maintained must be obvious.
In febrile disease the fibrine of the blood is materially increased, and there is also a marked decrease of salt, which is dependent on a greater or lesser intensity of the attack, rendering the blood denser, owing to the fact of the tendency of the fibrine to coagulate by reason of the diminution of the chloride of sodium, causing the blood to circulate slowly and with difficulty.