356 LECTURE XVI. 



maintain the same proportion of salts to organic material as in milk. Fed 

 with such a mixture, six mice lived 20, 23, 29, 30, and 31 days, or about 

 the same length of time as the mice fed with the former mixture containing 

 alkali carbonate. Two mice, fed entirely upon cow's milk for a period of 

 2J months, remained in good health at the end of the experiment. 



These experiments apparently prove that it is not possible to keep mice 

 alive without feeding them salts, and, moreover, that an artificial mixture 

 of salts fails to sustain the lives of mice for more than a short time. This 

 result may be accounted for in a number of different ways. It has not 

 shown that the early death of the mice in the first experiment was actually 

 due to a lack of salts in the diet. It is equally conceivable that certain 

 necessary organic constituents were wanting.' Milk always contains, 

 besides casein, a certain amount of albumin. It is possible that the 

 albumin is of great importance for certain functions. Again, perhaps 

 lecithin and cholesterol are essential. Possibly milk contains organic 

 compounds of a nature unknown to us. Above all we must remember 

 that the exceedingly important Law of the Minimum * holds for the nour- 

 ishment of animals as well as for that of plants. In any diet the amount 

 of each constituent required by the organism must be regulated in accord- 

 ance with this principle. Perhaps some inorganic element, such as fluor- 

 ine was lacking, and for this reason the other inorganic constituents were 

 not sufficiently utilized. Naturally the same law holds with regard to the 

 organic constituents. 



Until recently but little was known concerning the physiological impor- 

 tance of salts in the plant and animal organism. It was known that they 

 took part in the anabolism of the cells. In fact, potassium, sodium, calcium, 

 magnesium, iron, phosphoric acid, fluorine, and chlorine are invariably 

 found in every cell. In some cases, manganese, silicic acid, iodine, and 

 arsenic are found in the animal cells. The plants receive their nourish- 

 ment from the ground, and under certain conditions may contain other 

 elements; for example, copper, zinc, and aluminium. Silicic acid in many 

 cases of plant life is an important source of rigidity. The plant cell 

 requires inorganic material. There is no doubt that the physiological 

 importance of the salts is the same in both the animal and vegetable king- 

 doms, and the assumption that they form merely a passive building 

 material for the cells is not justifiable. This is evidenced by the fact that 

 the distribution of the inorganic material is not uniform throughout the 

 organism. Thus we find in certain cells more potassium and less sodium, 

 while in the liquids of the body, for example the serum, the relative 

 amounts of the two are reversed. In the cells there is more phosphoric 

 acid, but less chlorine. It is evident from this that the taking up of salts 



1 J. Liebig: Die Chemie in ihrer Anwendung auf Agrikultur und Physiologie, 

 p. 332 (1876). 



