The term breast refers to the upper ventral region of an animals torso, particularly that of mammals, including human beings. The breasts of a female primates body contain the mammary glands, which secrete milk used to feed infants. This article deals with the human breast; for other animals, see udder and mammary gland.

Breasts are more visible on adult women, but male humans also have breasts which, although usually less prominent, are structurally identical (homologous) to the female, as they develop embryologically from the same tissues.

The breasts are modified sudoriferous (sweat) glands, producing milk in women, and in some rare cases, men. Each breast has one nipple surrounded by the areola. The areola is colored from pink to dark brown and has several sebaceous glands.

In women, the larger mammary glands within the breast produce the milk. They are distributed throughout the breast, with two-thirds of the tissue found within 30 mm of the base of the nipple.[3] These are drained to the nipple by between 4 and 18 lactiferous ducts, where each duct has its own opening.

The network formed by these ducts is complex, like the tangled roots of a tree. It is not always arranged radially, and branches close to the nipple. The ducts near the nipple do not act as milk reservoirs; Ramsay et al. have shown that conventionally described lactiferous sinuses do not, in fact, exist.

The remainder of the breast is composed of connective tissue (collagen and elastin), adipose tissue (fat), and Cooper's ligaments. The ratio of glands to adipose tissues rises from 1:1 in nonlactating women to 2:1 in lactating women.

The breasts sit over the pectoralis major muscle and usually extend from the level of the 2nd rib to the level of the 6th rib anteriorly. The superior lateral quadrant of the breast extends diagonally upwards towards the axillae and is known as the tail of Spence. A thin layer of mammary tissue extends from the clavicle above to the seventh or eighth ribs below and from the midline to the edge of the latissimus dorsi posteriorly.

The arterial blood supply to the breasts is derived from the internal thoracic artery (formerly called the internal mammary artery), lateral thoracic artery, thoracoacromial artery, and posterior intercostal arteries.

The venous drainage of the breast is mainly to the axillary vein, but there is some drainage to the internal thoracic vein and the intercostal veins. Both sexes have a large concentration of blood vessels and nerves in their nipples. The nipples of both women and men can become erect in response to sexual stimuli, and also to cold.

The breast is innervated by the anterior and lateral cutaneous branches of the fourth through sixth intercostal nerves. The nipple is supplied by the T4 dermatome.

Breasts vary in both size and shape, and their external appearance is not predictive of their internal anatomy or lactation potential. The shape of a womans breasts is in large part dependent on their support, which primarily comes from the Cooper's ligaments, and the underlying chest on which they rest.

The breast is attached at its base to the chest wall by the deep fascia over the pectoral muscles. On its upper surface it is given some support by the covering skin where it continues on to the upper chest wall. It is this support which determines the shape of the breasts. In a small fraction of women, the frontal milk sinuses (ampulla) in the breasts are not flush with the surrounding breast tissue, which causes the sinus area to visibly bulge outward.

In discussing the support of breasts, it is helpful to draw a distinction between breasts which rest on the chest below, and those which do not. High, rounded breasts protrude almost horizontally from the chest wall. All breasts are like this in early stages of development, and such a shape is common in younger women and girls. This protruding or high breast is anchored to the chest at its base, and the weight is distributed evenly over the area of the base of the approximately dome- or cone-shaped breasts.[citation needed]

In the low breast, a proportion of the breasts weight is actually supported by the chest against which the lower breast surface comes to rest, as well as the deep anchorage at the base.

The weight is thus distributed over a larger area, which has the effect of reducing the strain. In both males and females, the thoracic cavity slopes progressively outwards from the thoracic inlet (at the top of the breastbone) above to the lowest ribs which mark its lower boundary, allowing it to support the breasts.