PHYSIOLOGY OF MILK SECRETION. 209 



however, that it ignores any special constructive power in the gland itself, 

 and treats milk as an excretory product. 



The latest theory is to regard milk as a product of metabolism of the 

 cells of the mammary gland. It is in all essential characters a secretory 

 product, and not an excretory product. In viewing the physiology of the 

 formation of milk in such a light, it is only regarding it in the same way 

 as saliva, gastric and pancreatic juices. It may be argued that these glands 

 secrete a special product to be used in the animal economy, while milk is 

 not so used. All excretory glands, as the kidneys, liver and sweat glands, 

 find their material already prepared in the blood, the result of activity in 

 other parts of the body, and they serve as a means of eliminating it. Secre- 

 tory glands, as the pancreas, salivary glands, etc., do not find their active 

 principles in the blood, but construct them within their own especial cells. 

 The mammary gland does not find fat, casein and lactose in the blood, 

 but constructs them within its own tissues. The recognition of the mammary 

 gland as an organ having a special function will explain fully all the dif- 

 ficulties met in trying to reconcile all other theories with the facts as they 

 are observed. 



The theory of special cell metabolism is supported by the behavior 

 of the gland, viewed from an anatomical standpoint. The cells differ 

 when at rest and when active. When at rest the cells lining the alveoli 

 lie flat and close to the wall. Their nuclei are small and spindleform. Dur- 

 ing a period of activity they are much enlarged, filling nearly the .entire 

 cavity, and the nuclei are prominent. The cells may be seen in all stages 

 of reproduction, and in these particulars the gland shows the same char- 

 acters as seen in the secreting glands already mentioned. 



This theory is further sustained by the antecedents of the milk. When 

 fat is taken into the intestine and assimilated, it no longer has an existence 

 as fat, but is broken up into various combinations. Fat as deposited in 

 the body is not the same as the fat in the food. The proportions of olein 

 and stearin have been changed to meet the peculiarity of the animal. Where 

 the analytic and synthetic process take place is not known. It is now 

 recognized that it is not necessary that the fat in the body be derived from 

 the fat of the food, but that the carbohydrates supply the necessary ma- 

 terials. With these proofs of synthetic process going on, to produce body 

 fat, it is not unreasonable to suppose that a similar process may take place 

 in the formation of milk. 



The milk sugar or lactose is a product of metabolic activity of the proto- 

 plasm of the secreting cells of the mammary gland. This particular form 

 of sugar occurs nowhere else in the body. It is a typical carbohydrate, 

 and is found in the milk of animals fed exclusively upon meat, thus show- 

 ing that the carbohydrates of the food are wholly unnecessary. Of all 

 the constituents the milk sugar is least affected by external conditions. 



The casein of milk is thought to be formed the same as the fat, although 

 authorities differ on this point. The evidence seems to be in favor of this 



