374 ESSENTIALS OF PHYSIOLOGY. 



The vitamines in milk are destroyed by prolonged boiling, and the 

 condition of scurvy rickets, which occurs in infants, may be due to their 

 being fed on a diet devoid of, or deficient in, vitamines. 



Diseases which are brought about by the absence from the diet of 

 some essential constituent are called " deficiency diseases " ; they do 

 not occur when the diet is sufficiently varied and contains fresh, 

 uncooked food. 



The presence in the food of small traces of certain substances seems 

 also to be necessary for growth in young animals. It has been found 

 that when young rats are fed upon an artificial milk containing 

 perfectly pure caseinogen, fat, and milk sugar in the same proportions 

 as in milk, together with salts and water, the animals fail to grow, 

 although their diet is adequate both as a source of energy and as 

 regards the amount of protein present. On the addition to the diet of 

 very small quantities of fresh milk, growth takes place in a normal 

 manner. Evidently the natural food contains something essential to 

 growth, which is removed in the purification of the constituents of the 

 artificial milk ; the nature and mode of action of these substances is 

 quite unknown. 



We see, therefore, that in order to maintain health the diet must 

 fulfil the following conditions. In the first place, it must provide 

 sufficient potential energy to replace that lost as work and heat. 

 Secondly, it must contain sufficient protein to replace the breaking 

 down of the tissues, and to provide the complex chemical groupings 

 which the body cannot make for itself. Thirdly, it must contain the 

 substances known as vitamines. 



Salts and water also must be present in the diet, although they 

 do not supply energy. Farther, in young growing animals the diet, 

 especially protein food, must be relatively more abundant than in 

 adults. Not only is metabolism more active in young animals, but an 

 additional amount of protein is required to provide material for the 

 laying down of new tissue during growth. 



THE COMPOSITION OF FOOD-STUFFS. 



MILK. 



Milk contains all the alimentary principles of a dietary, combined in 

 the proportions necessary for the early stages of life. The proportions 

 of the constituents of milk vary somewhat with the species, as will be 

 seen in the following table : 



