[TA] a circumscribed spindle-shaped dilation of the lactiferous duct just before it enters the nipple. In nursing mothers, this dilation stores a droplet of milk that is expressed by compression as the infant begins to suckle; this is thought to encourage continual suckling while the let-down reflex ensues. SYN: sinus lactiferi [TA], ampulla lactifera, ampulla of lactiferous duct, ampulla of milk duct, lactiferous ampulla.