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 mamme 
upon attaining puberty, 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- 
ereted is very different in character from that se- 
ereted 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 
