494 NUTRITION. 



with food in the condition in which it is found in the body. It is contained in the sub- 

 stance of all kinds of food, animal and vegetable ; but, in the herbivora and in man, this 

 source is not sufficient to supply the wants of the system, and it is introduced, therefore, 

 as salt. The quantity which is discharged from the body has been estimated by Barral 

 to be somewhat less than the quantity introduced, about one-fifth disappearing ; but 

 these estimates are not exactly accurate, for the amount thrown off* in the perspiration has 

 never been directly ascertained. It exists in the blood in connection with the phosphate 

 of potassa, and a certain amount is lost in a double decomposition which takes place 

 between these two salts, resulting in the formation of chloride of potassium and phos- 

 phate of soda. It also is supposed to furnish the soda to all the salts which have a soda 

 base, and a certain quantity, therefore, disappears in this way. 



Existing, as it does, in all the solids and fluids of the body, chloride of sodium is 

 discharged in all the excretions, being thrown off in the urine, faeces, perspiration, 

 and mucus. 



Chloride of Potassium. Chloride of potassium, although neither so important a proxi- 

 mate principle as the chloride of sodium nor so generally distributed in the economy, 

 seems to have an analogous function. It is found in the muscles, liver, milk, chyle, blood, 

 mucus, saliva, bile, gastric juice, cephalo-rachidian fluid, and urine. It is exceedingly 

 soluble, and in these situations it exists in solution in the fluids. Its quantity in these 

 situations has not been accurately ascertained, as it has generally been estimated in 

 connection with the chloride of sodium. In the muscles, it exists, however, in a larger 

 proportion than common salt. In cow's milk, Berzelius has found 1'7 part per 1,000; 

 Pfaff and Schwartz, 1-35 per 1,000 in cow's milk, and 0'3 per 1,000 in human milk. 

 Of the function of this principle, little remains to be said after what has been stated 

 with regard to the chloride of sodium. The functions of these two principles are prob- 

 ably identical, although the latter, from its greater quantity in the fluids and its univer- 

 sal distribution, is by far the more important. 



Origin and Discharge of Chloride of Potassium. This substance has two sources ; 

 one in the food, existing, as it does, in muscular tissue, milk, etc., and the other in a 

 chemical reaction between the phosphate of potassa and the chloride of sodium, forming 

 the chloride of potassium and the phosphate of soda. That this decomposition takes 

 place in the body, is evident from the fact that the ingestion of a considerable quantity 

 of common salt has been found, in the sheep, to increase the quantity of chloride 

 of potassium in the urine, without having any influence upon the amount of chloride 

 of sodium. The chloride of potassium is discharged from the body in the urine and 

 mucus. 



Phosphate of Lime. This salt is found in all the solids and fluids of the body. As it 

 is always united, in the solids, with organic substances as an important element of consti- 

 tution, it is hardly second in importance to water. It differs in its functions so essen- 

 tially from the chlorides of sodium and potassium, that they are hardly to be compared. 

 It is insoluble in water, but is held in solution in the fluids of the body by virtue of free 

 carbonic acid, the bicarbonate^, and the chloride of sodium. In the solids and semi' 

 solids, the condition of its existence is the same as that of water ; i. e. it is incorporated, 

 particle to particle, with the organic substance characteristic of the tissue and is one 

 of its essential elements of composition, and cannot be completely separated without 

 incineration. Nothing need be added here with regard to this mode of union in the 

 body of organic and inorganic substances, after what has been said under the head of 

 water. 



The following table gives the relative quantities of phosphate of lime in various situa- 

 tions : 



