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Medical Definition of Sinus lactiferi
1. A circumscribed spindle-shaped dilation of the lactiferous duct just before it enters the nipple. In nursing mothers this dilatation stores a droplet of milk which is expressed by compression as the infant begins to suckle; this is thought to encourage continual suckling while the let-down reflex ensues. Synonym: sinus lactiferi, ampulla lactifera, ampulla of milk duct, lactiferous ampulla. (05 Mar 2000)