Breast

The term breast can refer to the upper ventral region of the human torso. Alternatively, the term is used for each of two parts of that, especially for women: the breasts are parts of the female human body that contain the organs that secrete milk used to feed infants. Males also have breasts and are born with the main milk ducts intact, but while the gland that produces milk is present in the male, it normally remains undeveloped. In some situations male breast development does occur, a condition called gynecomastia. Milk production can also occur in both men and women as a rare side-effect of some medicinal drugs (such as some antipsychotic medication). Both sexes have a large concentration of blood vessels and nerves in their nipples.

Anatomy

A woman's 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 in an 'axillary tail'. 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.

Important parts of the breasts include mammary glands, the axillary tail (tumours frequently occur here), the lobules, Cooper's ligaments, the areola and the nipple. The nipple is supplied by the T4 dermatome.

It is typical for one of a woman's breasts to be larger than the other one; statistically it is more common for the left breast to be the larger. In some rare cases, one breast may be greatly larger or smaller than the other, or fail to develop during puberty.

During puberty, sex hormones, chiefly estrogen, cause the development of a woman's breasts. This hormone has been demonstrated to cause the development of woman-like, enlarged breasts in men, a condition called gynecomastia.

As breasts are mostly composed of adipose tissue, their size can change over time if the woman gains or loses weight. It is typical for them to grow in size during pregnancy, mainly due to hypertrophy of the exocrine gland in response to prolactin.

Lymphatic drainage

As breast cancer is a common cancer, the lymphatic drainage of the breast (sites where cancer may metastasize) is important.

About 75% of lymph from the breast travels to the ipsilateral (same side) axillary lymph nodes. The rest travels to parasternal nodes, to the other breast, or abdominal lymph nodes.

The axillary nodes include the pectoral, subscapular, and humeral groups of lymph nodes. These drain to the central axillary lymph nodes, then to the apical axillary lymph nodes.

Function

It is commonly assumed by biologists that the real evolutionary purpose of women having breasts is to attract the male of the species; that, in other words, breasts are secondary sex characteristics. Some biologists believe that the shape of female breasts evolved as a frontal counterpart to that of the buttocks, the reason being that whilst other primates mate in the atypical piggy-back position, humans are more likely to successfully copulate mating face on. A secondary sexual characteristic on a woman's chest would have encouraged this in more primative incarnations of the human race, and a face on ecounter would have helped foundate a relationship between partners beyond merely a sexual one.

Others believe that the human breast evolved in order to prevent infants from suffocating while feeding. Since human infants do not have a protruding jaw like our ancestors and the other primates, the infant's nose might be blocked by a flat female chest while feeding. According to this theory, as the human jaw became recessed, so the breasts became larger to compensate.

A common misconception is that human female breasts are shaped the way that they are so that they can feed babies by producing milk. The mammary glands that secrete the milk from the breasts make up a relatively small fraction of the overall breast tissue. Most of the human female breast is actually adipose tissue (fat) and connective tissue. Breast size does not make any difference to a woman's ability to nurse a baby.

Because some cultures place a high value on symmetry of the female human form, and because women often identify their femininity and sense of self with their breasts, many women in developed countries undergo breast reconstruction after mastectomy for breast cancer.