The<unk> : cross-section scheme of the mammary gland. In women, the<unk> s overlie the pectoralis major muscles and extend on average from the level of the second rib to the level of the sixth rib in the front of the human rib cage; thus, the<unk> s cover much of the chest area and the chest walls. At the front of the chest, the<unk>  tissue can extend from the clavicle (collarbone) to the middle of the sternum<unk> bone). At the sides of the chest, the<unk>  tissue can extend into the axilla (armpit), and can reach as far to the back as the latissimus dorsi muscle, extending from the lower back to the humerus bone (the bone of the upper arm). As a mammary gland, the<unk>  is composed of differing layers of tissue, predominantly two types: adipose tissue; and glandular tissue, which affects the lactation functions of the<unk> s. Morphologically the<unk>  is tear-shaped. The superficial tissue layer (superficial fascia) is separated from the skin by 0.5–2.5 cm of subcutaneous fat (adipose tissue). The suspensory Cooper's ligaments are fibrous-tissue prolongations that radiate from the superficial fascia to the skin envelope. The female adult<unk>  contains 14–18 irregular lactiferous lobes that converge at the nipple. The 2.0–4.5 mm milk ducts are immediately surrounded with dense connective tissue that support the glands. Milk exits the<unk>  through the nipple, which is surrounded by a pigmented area of skin called the areola. The size of the areola can vary widely among women. The areola contains modified sweat glands known as Montgomery's glands. These glands secrete oily fluid that lubricate and protect the nipple during<unk> feeding. Volatile compounds in these secretions may also serve as an olfactory stimulus for the newborn's appetite. The dimensions and weight of the<unk>  vary widely among women. A small-to-medium-sized<unk>  weighs 500 grams (1.1 pounds) or less, and a large<unk>  can weigh approximately 750 to 1,000 grams (1.7 to 2.2 pounds) or more. The tissue composition ratios of the<unk>  also vary among women. Some women's<unk> s have a higher proportion of glandular tissue than of adipose or connective tissues. The fat-to-connective-tissue ratio determines the density or firmness of the<unk> . During a woman's life, her<unk> s change size, shape, and weight due to hormonal changes during puberty, the menstrual cycle, pregnancy,<unk> feeding, and menopause. Glandular structure The<unk>  is an apocrine gland that produces the milk used to feed an infant. The nipple of the<unk>  is surrounded by the areola (nipple-areola complex). The areola has many sebaceous glands, and the skin color varies from pink to dark brown. The basic units of the<unk>  are the terminal duct lobular units (TDLUs), which produce the fatty<unk>  milk. They give the<unk>  its offspring-feeding functions as a mammary gland. They are distributed throughout the body of the<unk> . Approximately two-thirds of the lactiferous tissue is within 30 mm of the base of the nipple. The terminal lactiferous ducts drain the milk from TDLUs into 4–18 lactiferous ducts, which drain to the nipple. The milk-glands-to-fat ratio is 2:1 in a lactating woman, and 1:1 in a non-lactating woman. In addition to the milk glands, the<unk>  is also composed of connective tissues (collagen, elastin), white fat, and the suspensory Cooper's ligaments. Sensation in the<unk>  is provided by the peripheral nervous system innervation by means of the front (anterior) and side (lateral) cutaneous branches of the fourth-, fifth-, and sixth intercostal nerves. The T-4 nerve (Thoracic spinal nerve 4), which innervates the dermatomic area, supplies sensation to the nipple-areola complex. Lymphatic drainage Approximately 75% of the lymph from the<unk>  travels to the axillary lymph nodes on the same side of the body, whilst 25% of the lymph travels to the parasternal nodes (beside the sternum bone). A small amount of remaining lymph travels to the other<unk>  and to the abdominal lymph nodes. The subareolar region has a lymphatic plexus known as the "subareolar plexus of Sappey". The axillary lymph nodes include the pectoral (chest), subscapular (under the scapula), and humeral (humerus-bone area) lymph-node groups, which drain to the central axillary lymph nodes and to the apical axillary lymph nodes. The lymphatic drainage of the<unk> s is especially relevant to oncology because<unk>  cancer is common to the mammary gland, and cancer cells can metastasize (break away) from a tumour and be dispersed to other parts of the body by means of the lymphatic system. Shape, texture, and support The morphologic variations in the size, shape, volume, tissue density, pectoral locale, and spacing of the<unk> s determine their natural shape, appearance, and position on a woman's chest.<unk>  size and other characteristics do not predict the fat-to-milk-gland ratio or the potential for the woman to nurse an infant. The size and the shape of the<unk> s are influenced by normal-life hormonal changes (thelarche, menstruation, pregnancy, menopause) and medical conditions (e.g. virginal<unk>  hypertrophy). The shape of the<unk> s is naturally determined by the support of the suspensory Cooper's ligaments, the underlying muscle and bone structures of the chest, and by the skin envelope. The suspensory ligaments sustain the<unk>  from the clavicle (collarbone) and the clavico-pectoral fascia (collarbone and chest) by traversing and encompassing the fat and milk-gland tissues. The<unk>  is positioned, affixed to, and supported upon the chest wall, while its shape is established and maintained by the skin envelope. In most women, one<unk>  is slightly larger than the other. More obvious and persistent asymmetry in<unk>  size occurs in up to 25% of women. While it is a common belief that<unk> feeding causes<unk> s to sag, researchers have found that a woman's<unk> s sag due to four key factors: cigarette smoking, number of pregnancies, gravity, and weight loss or gain. The base of each<unk>  is attached to the chest by the deep fascia over the pectoralis major muscles. The space between the<unk>  and the pectoralis major muscle, called retromammary space, gives mobility to the<unk> . The chest (thoracic cavity) progressively slopes outwards from the thoracic inlet (atop the<unk> bone) and above to the lowest ribs that support the<unk> s. The inframammary fold, where the lower portion of the<unk>  meets the chest, is an anatomic feature created by the adherence of the<unk>  skin and the underlying connective tissues of the chest; the IMF is the lower-most extent of the anatomic<unk> . Normal<unk>  tissue typically has a texture that feels nodular or granular, to an extent that varies considerably from woman to woman.
Breast
Kidney
Lung
Liver
Skin
Heart
Brain
Skull