The nipple becomes erect because of such stimuli as a cold environment, breastfeeding, and sexual activity. The nipple of the post-partum female is used by the infant to breastfeed.
The small darkened (pigmented) area around the nipple is called the areola. (The word "areola" is the diminutive of the Latin "area" meaning a small space.) In pregnancy the areola darkens further and spreads in size. The areola contains small modified sweat glands (Montgomery's glands) that secrete moisture that acts as a lubricant for breastfeeding.
