82 SECRETION. 



glands is obscure. The particles are not produced in cells 

 and set free by their rupture, by a process analogous to that 

 which takes place in the formation of the fatty particles 

 found in the sebaceous matter, for during the time when 

 the secretion of milk is most active, the epithelium of the 

 secreting culs-de-sac has entirely disappeared. The butter 

 is produced by the action of the amorphous walls of the 

 vesicles, in the same way, probably, that fat is produced 

 by the vesicles of the ordinary adipose tissue. At least, this 

 is all that is known regarding the mechanism of its pro- 

 duction. 



As regards the mechanism of the formation of the 

 peculiar and characteristic constituents of the milk, the 

 mammary glands are to be classed among the organs of 

 secretion, and not those of elimination or excretion; for 

 none of these elements preexist in the blood, but all appear 

 first in the substance of the glands. 



During the period of secretion, the glands receive a much 

 larger supply of blood than at other times. Pregnancy 

 favors the development of the secreting portions of the 

 glands, but does not induce secretion. On the other hand, 

 when pregnancy occurs during lactation, it diminishes, mod- 

 ifies, and may arrest the secretion of milk. The secre- 

 tion is destined, however, for the nourishment of the child, 

 and not for use in the economy of the mother an important 

 point of distinction from all other secretions and its produc- 

 tion presents one or two interesting peculiarities. 



In the first place, the secreting action of the mammary 

 glands is nearly continuous. When the secretion of milk 

 has become fully established, while there may be certain 

 periods when it is formed in greater quantity than at others, 

 there is no absolute interim ttency in its production. 



Again, in all the other glandular organs, the epithelial 

 cells found in their secreting portion seem to be the active 

 agents in the production of the secretions ; but in the mam- 

 mary glands, as we have already noted, the epithelium 



