Incentives to Secretion. 9 



through a chemical change in the contents of these 

 lining cells, since but minute quantities of sugar 

 are found in the blood. 



Incentives to secretion. — Maternity is the prime 

 incentive to the secretion of milk. While there is 

 a distinct increase in the development of the mammae 

 upon attaining pubertj^ it is not until pregnancy is 

 well advanced that the organ attains anywhere near 

 its full development, or that there is any activity 

 in the true secreting cells. In the virgin .animal, 

 and up to within a short time of parturition, the 

 cavities and ducts of the udder contain a watery 

 saline fluid, but true milk does not appear until a 

 short time before, and in some cases not until after, 

 parturition. The immediate stimulus to the produc- 

 tion of milk is the turning of the blood that went 

 to nourish the foetus from the arteries of the uterus 

 to the arteries of the udder. The pressure of blood 

 in the vessels of the udder stimulates the secreting 

 cells to great activity, and the cells, hitherto dor- 

 mant, begin to multiply rapidly. When this activity 

 is first set up, the various processes of secretion are 

 more or less incomplete, so that the milk first se- 

 creted is verj^ different in character from that se- 

 creted afterwards, and is known as colostrum. The 

 colostrum contains in the first place considerably 

 less water than normal milk; in the second place, 

 the transformation of albumin into casein is only 

 partial, so that colostrum contains large amounts of 

 albumin; and finally, when secretion of milk begins, 

 the cells of the follicle multiply more rapidly than 



