MILK PRODUCTION 465 



Subsequent investigation, however, has generally failed to 

 show satisfactory evidence of cell division. A modification 

 of Virchow's theory still held is that while there is no cell division, 

 the outer portion of the protoplasm is sloughed off and dissolved, 

 forming the milk, and is again renewed by the growth of new 

 protoplasm. The weight of opinion, however, regards milk 

 production as a true secretion, entirely analogous to that ob- 

 served in other glands. It is not believed that there is normally 

 a breaking down of cells, but that the latter extrude their 

 secreted materials into the alveolus precisely as do the secreting 

 cells of other glands. This is held to apply to the fat globules 

 as well as to the other ingredients of milk. The process is in 

 many ways analogous to that of the resorption of digested 

 material by the epithelial cells of the small intestine, the obvious 

 difference being the direction in which the materials move. 



The secretion of milk in the active udder is a more or less 

 continuous process, the product accumulating in the cavities 

 and passages of the gland. Fleischmann long ago showed, 

 however, that the cavities of the udder cannot possibly contain 

 the amount of milk produced in a single milking by a reason- 

 ably productive cow, and it is well recognized that a rapid secre- 

 tion of milk occurs during suckling or milking. In other words, 

 the milk gland, like other glands, reacts to a specific stimulus. 



551. Sources of ingredients of milk. While the ultimate 

 source of the material contained in the milk is of course the 

 feed, the milk gland draws its supply of material for milk pro- 

 duction immediately from the blood, while at the same time it 

 brings about extensive chemical transformations in the sub- 

 stances thus supplied. Probably all the ingredients of the 

 milk should be regarded as products of the chemical activity 

 of the epithelial cells of the glands, although the extent to which 

 the original material is modified varies. 



552. Origin of milk proteins. The albumin and globulin 

 of milk are quite similar to the corresponding substances in the 

 blood. The casein, on the other hand, is radically different. 

 In the first place, it is, as already stated, a conjugated protein 

 containing some phosphorus-bearing radicle. Whether the 

 latter is derived exclusively from the organic phosphorus com- 

 pounds of the feed has not been demonstrated, although it 

 appears probable that inorganic phosphorus compounds (phos- 



2 H 



