Anatomy Breast schematic diagram (adult female human cross section) - Legend: 1. Chest wall 2. Pectoralis muscles 3. Lobules 4. Nipple 5. Areola 6. Duct 7. Fatty tissue 8. Skin
Breasts are modified sudoriferous (sweat) glands which produce milk in women, and in some rare cases, in men.[2] 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. Instead, most milk is actually in the back of the breast, and when suckling occurs, the smooth muscles of the gland push more milk forward.
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.[3]
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. (For further explanation, see anatomical terms of location.)
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,[4] 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.
Lymphatic drainage
About 75% of lymph from the breast travels to the ipsilateral 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. The lymphatic drainage of the breasts is particularly relevant to oncology, since breast cancer is a common cancer and cancer cells can break away from a tumour and spread to other parts of the body through the lymph system by metastasis.
Shape and support
Breasts vary in size, shape and position on a woman's chest, and their external appearance is not predictive of their internal anatomy or lactation potential. The natural shape of a woman's breasts is primarily dependent on the support provided by the Cooper's ligaments and the underlying chest on which they rest (the base).
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 number of women, the frontal ducts (ampullae) in the breasts are not flush with the surrounding breast tissue, which causes the sinus area to visibly bulge outward.Relatively round breasts which protrude almost horizontally.
Some breasts are high and rounded, and protrude almost horizontally from the chest wall. Such high breasts are common for girls and women in early stages of development. The protruding or high breasts are anchored to the chest at the 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.
The inframammary fold (or line, or crease) is an anatomic structure created by adherence between elements in the skin and underlying connective tissue[5] and represents the inferior extent of breast anatomy. Some teenagers may develop breasts whose skin comes into contact with the chest below the fold at an early age, and some women may never develop such breasts; both situations are perfectly normal. The relationship of the nipple position to the fold is described as ptosis, a term also applied to other body parts and which refers in general to drooping or sagging. Due to breast weight and relaxation of support structures, the nipple-areola complex and breast tissue may eventually hang below the fold, and in some cases the breasts may extend as far as, or even beyond, the navel. The length from the nipple to the sternal notch (central, upper border) in the youthful breast averages 21 cm and is a common anthropometric figure used to assess both breast symmetry and ptosis. Lengthening of both this measurement and the distance between the nipple and the fold are both characteristic of advancing grades of ptosis.
The end of the breast, which includes the nipple, may either be flat (a 180° angle) or angled (angles lower than 180°). Breast ends are rarely angled sharper than 60°. Angling of the end of the breast is caused in part by the ligaments that suspend it, such that the breast ends often have a more obtuse angle when a woman is lying on her back. Breasts exist in a range of ratios between length and base diameter, usually ranging from ½ to 1.
Development
Male with severe gynecomastiaGirls develop breasts during puberty, as a result of changing sex hormones, chiefly estrogen, which also has been demonstrated to cause the development of woman-like, enlarged breasts in men, a condition called gynecomastia.
In most cases, the breasts fold down over the chest wall during Tanner stage development, as shown in this diagram.[6] It is typical for a woman's breasts to be unequal in size particularly while the breasts are developing. Statistically it is slightly more common for the left breast to be the larger.[7] In rare cases, the breasts may be significantly different in size, or one breast may fail to develop entirely.
A large number of medical conditions are known to cause abnormal development of the breasts during puberty. Virginal breast hypertrophy is a condition which involves excessive growth of the breasts, and in some cases the continued growth beyond the usual pubescent age. Breast hypoplasia is a condition where one or both breasts fail to develop.
Changes
Breast with visible stretchmarks.
As breasts are mostly composed of adipose tissue, their size can change over time. This occurs for a number of reasons, most obviously when a girl grows during puberty and when a woman becomes pregnant. The breast size may also change if she gains (or loses) weight for any other reason. Any rapid increase in size of the breasts can result in the appearance of stretchmarks.
It is typical for a number of other changes to occur during pregnancy: in addition to becoming larger, the breasts generally become firmer, mainly due to hypertrophy of the mammary gland in response to the hormone prolactin. The size of the nipples may increase noticeably and their pigmentation may become darker. These changes may continue during breastfeeding. The breasts generally revert to approximately their previous size after pregnancy, although there may be some increased sagging and stretchmarks.
The size of a woman's breasts may fluctuate during the menstrual cycle, particularly with premenstrual water retention. An increase in breast size is a common side effect of use of the combined oral contraceptive pill.
Breasts sag if the ligaments become elongated, a natural process that can occur over time and is also influenced by the breast bouncing while exercising. Breasts can decrease in size at menopause if estrogen levels decline.
Function of Brest
Breastfeeding
An infant breastfeeding
Main article: Breastfeeding
The primary function of mammary glands is to nurture young by producing breast milk. The production of milk is called lactation. (While the mammary glands that produce milk are present in the male, they normally remain undeveloped.) The orb-like shape of breasts may help limit heat loss, as a fairly high temperature is required for the production of milk. Alternatively, one theory states that the shape of the human breast evolved in order to prevent infants from suffocating while feeding.[8] Since human infants have a small jaw (not protruding, like other primates), the infant's nose might be blocked if the mother's chest was too flat.[8] According to this theory, as the human jaw receded, the breasts became larger to compensate.[8]
Milk production unrelated to pregnancy can also occur. This condition, called galactorrhea, may be an adverse effect of some medicinal drugs (such as some antipsychotic medication), extreme physical stress or endocrine disorders. If it occurs in men it is called male lactation, and is often classified as a pathological symptom due to its strong correlation to pituitary disorders. Newborn babies are often capable of lactation because they receive the hormones prolactin and oxytocin via the mother's bloodstream, filtered through the placenta. This neonatal liquid is known colloquially as witch's milk.
Sexual role
Breasts play an important part in human sexual behavior; they are also important female secondary sex characteristics.[9] Compared to other primates, human breasts are proportionately large throughout adult females' lives and may have evolved as a visual signal of sexual maturity and fertility.[10] On sexual arousal breast size increases, venous patterns across the breasts become more visible, and nipples harden. Breasts are sensitive to touch as they have many nerve endings, and it is common to press or massage breasts with hands during sexual intercourse (as it is with other bodily areas representing feminine secondary sex characteristics as well).[citation needed] Oral stimulation of nipples and breasts is also common. Some women can achieve breast orgasms. In the ancient Indian work the Kama Sutra, marking breasts with nails and biting with teeth are explained as erotic.[11]
See also: Mammary intercourse; Toplessness; Breast fetishism.
Other suggested functions
Zoologists point out that no female mammal other than the human has breasts of comparable size, relative to the rest of the body, when not lactating and that humans are the only primate that has permanently swollen breasts. This suggests that the external form of the breasts is connected to factors other than lactation alone.[citation needed]
Some zoologists (notably Desmond Morris) believe that the shape of female breasts evolved as a frontal counterpart to that of the buttocks, the reason being that while other primates mate in the rear-entry position, humans, because of their upright posture, are more likely to successfully copulate by mating face to face, the so-called missionary position. Morris suggested in 1967 that a secondary sexual characteristic on a woman's chest would have encouraged this in more primitive incarnations of the human race, and a face on encounter may have helped found a relationship between partners beyond merely a sexual one.[12] However, this theory has since been generally disregarded due to the discovery that other primates, such as orangutans, routinely mate in the face-to-face position even though the females do not have prominent breasts.
