VI GELATIN AS A FOOD-STUFF 277 



of water (86 p. c.) in which proteins, casein and some 

 albumin, are dissolved, as also a carbohydrate, milk-sugar 

 or lactose, and inoi-ganic salts such as chlorides and 

 phosphates of sodium, potassium and calcium. The fat 

 present in milk is emulsified or suspended in the water in 

 the form of extremely minute globules, and the white 

 appearance presented by milk is due to the great amount 

 of light reflected from these minute particles of fat. Milk, 

 however, is deficient in one essential constituent of diet, 

 that of iron. We have seen that iron is an important 

 constituent of haemoglobin (p. 97). A child is born with 

 sufficient iron in it3 body to provide for the formation of 

 red blood corpuscles for nine months to a year. After 

 this period it must get iron from its diet. Milk does not 

 furnish this. 



4. The Effects of the several Food stuffs.— When 

 some protein food is given to an animal, such as a dog, 

 which has been previously starved, the larger part of the 

 nitrogen given in the protein is not retained in the body 

 but is excreted almost immediately. This is the nitrogen 

 in what we have termed the "exogenous " urea (p. 189). 

 The nitrogen excreted during the starvation was of cour.se 

 solely of "endogenous " origin. If another larger meal of 

 protein be given the amount of nitrogen excreted is still 

 further increased, less and less being retained in the body. 

 By proceeding in this way it is possible to increase the 

 excretion of nitrogen to such an extent that it ultimately 

 becomes equal to the amount administered in the food : 

 the animal is then said to be in " nitrogenous equilibrium." 



The eflfects of carbohydrates and fats as foods cannot 

 be studied by feeding an animal with these alone, as 

 is possible with proteins. But this difficulty may be 

 got over by administering a small, fixed quantity of protein 

 with a variable amount of either carbohydrate or fat. In 

 this case it is found that an increase of the carbohydrate 

 in food very soon leads to the laying on of fat ; and this 

 corresponds to the everyday experience which is frequently 



