MILK SECRETION 7 



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 mammary 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 character- 

 istics with the oil globules found in milk.' In feeding animals 

 on a highly albuminous diet they increase in weight and pro- 

 duce 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." — 

 Bitting. 

 Present theory of milk secretion (Bitting). 



The latest theory is to regard milk as a product of metab- 

 olism of the cells of the mammary gland. It is in all essential 

 characters a secretory 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 and 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. Secretory glands, as the pancreas, salivary 

 glands, and the like, do not find their active principles in the 

 blood, but construct them within their own special cells. The 

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

 blood, but constructs them within its own tissues. The rec- 

 ognition of the mammary gland as an organ having a special 

 function will explain fully all the difficulties 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. 



