NUTRITION 



36i 



nerve, milk-curdling, and growth is of supreme importance. 

 The distribution of the salts throughout the structure is remark- 

 ably regular, sodium being found in the blood plasma, potassium 

 and iron in the red cells, sulphur in hair and horn, potassium in 

 sweat, sulphur in protein, and lime in bones, etc. Animals fed 

 on a diet which is as far as possible rendered free from salts soon 

 die. When a deficiency in salts occurs, the body apparently for 

 some time draws on its own store, and then nutritive changes 

 follow. These are more likely to show themselves earlier with 

 young than with adult animals, but the influence of inorganic 

 food on the nutrition of the skeleton is little understood. 



The chief salt used by herbivora is potassium, whilst sodium 

 is used by carnivora. Both carnivora and herbivora obtain in 

 their natural diet a sufficiency of these salts, though there is a 

 general impression that the wild herbivora long for sodium. It 

 is quite certain that under the conditions of domestication horses 

 can be kept in perfect health without receiving any sodium 

 chloride, other than that naturally contained in the food, and 

 the amount of this in vegetable substances is small. The iron 

 required by the blood is probably furnished in some organic 

 combination. It is evident that the daily quantity of salts 

 required must depend upon the age of the animal, young growing 

 animals requiring more than adults. 



Storage of Tissue. — Every diet must contain the food principles 

 we have been considering, viz. : 



Protein. 



Fat or carbohydrate, or both. 



Salts. 



It is interesting to learn in what proportion these are stored 

 up in animals being fattened, also the amount of food required 

 for a definite increase in weight, and the rate at which that 

 increase occurs . This is shown in the following table from the 

 classical experiments of Lawes and Gilbert : 



Proportion of Food Principles stored up for Every 

 100 Pounds Increase of Body Weight. 



