LACTATION 365 



ment which permits the flow of blood only from right to 

 left. The increased pressure in the left auricle closes the 

 valve, which is soon sealed. The placental circulation 

 ceases owing to the Ugature of the umbiUcus. Finally, the 

 lumina of the ductus arteriosus and ductus venosus become 

 obUterated. 



LACTATION 



The Mammary Glands 



These consist of a number of lobes subdivided into 

 lobules. The lobules are composed of alveoh, separated 

 by connective tissue. From the alveoH run ducts which 

 join together to form the lactiferous ducts, of which about 

 fifteen or twenty open on the nipple. At their proximal 

 ends the lactiferous ducts are dilated so as to allow of 

 the accumulation of milk in the intervals between suckhng. 

 Some unstriped muscle fibres are found in the walls of 

 the ducts. The gland owes its rounded appearance to a 

 layer of fat which hes between it and the skin. It is 

 plentifully supphed with blood-vessels and nerves. 



The nipple is an erectile organ containing unstriated 

 muscle fibres. On its surface are papillae connected with 

 sensory nerves. 



The secretory cells hning the alveoh form a single 

 layer. Their appearance varies according to the physio- 

 logical condition of the gland. When the gland is at rest 

 they are flattened, when it is active they are columnar. 

 They contain protein granules and fatty globules. The 

 latter are found also in the lumen of the alveoh, together 

 with free granular cells. 



The act of secretion is provoked directly by the negative 

 pressure produced in suckling, aided probably by reflex 

 contraction of the unstriped muscle. The lactiferous 

 ducts are kept patent in face of the pressure of the suck- 

 hng's hps owing to the nipple becoming erectile. During 

 suckhng the vessels of the gland are reflexly dilated. 



