LACTATION 569 



supply of milk in strong, healthy women may last almost inde- 

 finitely. As already mentioned, menstruation not infrequently 

 commences to recur during the lactation period, and the latter 

 may overlap gestation until within a short time of delivery. 



THE DISCHARGE OP MILK 



The ^'discharge of the milk from the lactiferous ducts which 

 occurs during sucking is due partly to the direct mechanical 

 pressure, and partly to the action of the muscular tissue which 

 is present in the walls of the ducts and in the nipple. The 

 muscular mechanism appears to be stimulated reflexly by the 

 action of sucking. The contraction of the muscles in the nipple 

 causes this structure to stiffen, and it is suggested that this 

 action has the effect of keeping open the orifices of the ducts, 

 and thus permitting the free outflow of milk. 1 



It is probable also that the discharge of the secretion is 

 assisted in some degree by the swelling of the entire mammary 

 gland resulting from a reflex dilatation of the vessels ; but if 

 the secretory process is very active, and the ducts are heavily 

 charged, the flow of milk may take place almost automatically, 

 and with hardly any external stimulus. 



THE FORMATION OF THE ORGANIC CONSTITUENTS OF MILK 



The principal organic constituents of milk are peculiar to the 

 secretion, a fact which shows that they are elaborated in the 

 mammary glands themselves, and not elsewhere in the body. 

 It is stated, however, that a relatively small amount of caseinogen 

 is present in the secretion of the sebaceous glands, from which, 

 as already remarked, it is commonly supposed that the mammary 

 glands 2 have been derived in the course of evolution. Nothing 

 appears to be definitely known regarding the method of forma- 

 tion of the caseinogen of milk, 3 but it has been suggested that it 

 is derived from the degenerate nuclei of the gland cells. 



1 Schafer, loc. cit. 



2 Neumeister, Lehrbuch der physiologischen Chemie, vol. ii., Jena, 1895. 



3 Thierfelder, " Zur Physiologie der Milchbildung," Pfluger's Archiv, 

 vol. xxxii., 1883. 



