10 MANUAL OF MILK PRODUCTS 



minoids, fat, and sugar. The weight of the total solids of a 

 gland producing that amount of milk solids is only about 1.16 

 kilograms (2.25 lb.), which would necessitate a complete re- 

 newal of tissue 2.09 times a day. He might have added that 

 the epithelial cells constitute only a small part of the gland 

 structure, and it would therefore require even more rapid re- 

 newal. This would require an almost incredible cell growth, 

 so that we are forced to assume that, although the growth and 

 disappearance of certain cells is of the greatest importance, the 

 organic substances in milk are modified from substances in the 

 blood and lymph into the forms we find them in milk by the 

 functional activity of the cells. The estimates upon the rate 

 of cell multiplication as made by Collier are only approximate, 

 but are certainly near enough to the truth to warrant drawing 

 the conclusion that fat is not the result of fatty degeneration 

 of the cells. In fact, such a process is incompatible with our 

 knowledge of the physiology of the cell reproduction or dis- 

 integration. 



FACTORS WHICH AFFECT MILK SECRETION 



Since the function of milk secretion is the normal accom- 

 paniment of maternity, the mammary gland does not become 

 fully developed or active until the time of parturition. At this 

 time the blood which has been used to nourish the foetus is 

 turned into the arteries supplying the udder, and this increased 

 flow of blood stimulates the action of the secretory cells. Bitting 

 and Woods discuss the factors affecting milk secretion as 

 follows : 



" As milk is dependent upon the metabolism of the mammary 

 gland, this is in turn dependent upon the quantity of blood 

 passing through it. For large milking capacity it is necessary 

 that there should be large glandular development; but, more 

 important still, a large circulation of blood in the part. The 

 cow must receive an ample supply of food and have the capacity 



