PHYSIOLOGY OF MILK SECRETION. 207 



accepted method of interpretation, not over 17 pounds of fat could have 

 been produced from this amount of neutralized protein. For the greater 

 part of the milk fat, they could draw no other conclusions than that it 

 is formed partially at least, from the carbohydrates. Several experimenters 

 have proven that body fat may be formed from carbohydrates, and the 

 foregoing experiment only strengthens the analogy between milk and body 

 fat in this mode of formation. This experiment completely disproves the 

 transudation theory, as the conditions under which it was conducted 

 were wholly under control. It further offers an explanation for the results 

 of many other experiments conducted along the same line, but not so com- 

 pletely carried out. 



The transudation theory also meets a serious set back in the fact that 

 the fats in the milk are unlike the fats in the food or body, and that casein 

 and milk sugar are not found in the blood or the gland itself. 



Another theory that has had many supporters, is that milk is the result 

 of the separating of part of its constituents, as the water serum and salt 

 from the blood, and part due to a fatty degeneration of the cells lining 

 the alveolar cavities, the fat globules being due to the degenerated cells 

 and the casein due to the undegenerated portion of the cells. This theory 

 is actively supported by many of the best physiologists. Smith, after 

 examining all the phases of milk secretion, sums up the whole as follows: 

 " The process of milk secretion may therefore be regarded as a process of 

 metabolism of the epithelial cells, which undergo decomposition, and dis- 

 charge the resulting products into the excretory ducts." He regards " fat 

 as a product of fatty degeneration of the protoplasmic cell contents, for 

 it is not increased but actually diminished by an increase of fat in the foods. 

 On the other hand an increase of proteids in the diet will cause an increase 

 in milk fat. In microscopic examination of the epithelial cells of the mam- 

 mary gland, oil globules may be actually seen to increase in size and number, 

 until often the protoplasmic content becomes almost entirely replaced 

 by oil globules which entirely agree in their characteristics with the oil 

 globules found in milk." In feeding animals on a highly albuminous diet, 

 they increase in weight and produce more fat in the milk, at the same time 

 showing that they cannot be filling the pail from adipose tissue. However 

 in herbivora not enough albuminoids are being taken up to account for 

 this fact, so that some must be derived from the blood. 



P. Collier made an investigation 1 to determine the number of fat 

 globules found in milk in a given time. He made his observations on a large 

 number of cows and found on an average each secreted seven-tenths of 

 a pound or nearly 19.6 cubic inches of milk per hour, and that there were 

 152 fat globules in each .0001 cubic inch of milk. He concluded that this 

 was equivalent to secreting 136,000,000 fat globules per second. He 



1 New York State Experiment Station, Rept. 1891, 



