DEVELOPMENT 639 



The complete emptying of the udder at milking is 

 necessary, not only to obtain the valuable fat in the last 

 portion, but to maintain the full activity of the gland. In 

 incomplete milking the amount secreted diminishes. 



(3) Food. — (1) The percentage composition of tlie milk of 

 any individual animal is within wide limits independent of 

 the relative proportions of the constituents of the food. Only 

 small deviations from the normal standard can be obtained 

 by feeding excessive amounts of one constituent. So great 

 is the tendency to secrete milk of normal composition that 

 when one of the constituents is deficient in the food, the animal 

 draws upon its own body for material, salts being supplied by 

 the skeleton and protein and fat by the tissues. The amount 

 secreted is however diminished. This prevents undue 

 depletion of the body. The body possesses only a small 

 reserve store of carbohydrate (p. 353). In an experiment in 

 which the carbohydrate of the food was reduced, and the 

 reserve store depleted by drawing off sugar through the 

 kidneys by means of phloridzin, it was found that instead of 

 milk deficient in sugar being secreted the quantity decreased. 



(2) Nature of Fat. — The composition of fat can be 

 modified by the food. Abnormal fats fed may appear in the 

 milk. Feeding stuffs rich in oils, such as linseed cake, 

 produce butter which is deficient in the higher fatty acids 

 and consequently soft at a low temperature. 



(3) Yield.— So long as food is given to supply (1) the 

 maintenance requirement of the animal and (2) suflficient 

 material for milk formation, the jueld depends upon the 

 capacity of the animal as a milk producer much more than 

 on the food. When, however, cows are put to graze on 

 young pasture-grass increased flow almost invariably occurs, 

 and in certain experiments, increasing the proportion of 



