The breast or mammary gland is a modified apocrine sweat gland. Less than 1% of breast cancers occur in males. The male breast throughout life and the immature female breast resemble each other. In both the nipple is small but the areola is fully formed. The breast tissue does not extend beyond the margin of the areola; it consists of a few ducts embedded in fibrous tissue. In a histological section alveoli are very sparse indeed. The lymph drainage of the immature breast is identical with that of the fully formed organ. At puberty the female nipple and breast both enlarge and thereafter retain the female form throughout life. The female form is very variable indeed, but the size of the base of the breast is fairly constant. It extends from the second to the sixth rib in the midclavicular line; it lies over the pectoralis major and extends beyond the border of that muscle to lie on the serratus anterior and external oblique. An axillary tail is sometimes present. From this circular base the breast protrudes or depends to a degree that varies within very wide limits. Asymmetry of the two breasts is not uncommon.
Beneath the breast is a condensation of superficial fascia, the continuation upwards of the membranous layer of superficial abdominal fascia (fascia of Scarpa) and forming a posterior capsule for the breast. Between this fascia and the deep fascia over pectoralis major is a submammary space in which the lymphatics run. It is the space into which breast prostheses are inserted, and it is a relatively bloodless plane. The axillary tail when present lies in the medial wall of the axilla and may be a discrete mass very poorly connected with the duct system. Usually it lies in the subcutaneous fat, which is condensed around it — very rarely it may penetrate the deep fascia of the floor of the axilla.
The resting (non-lactating) breast consists mainly of fibrous tissue. Glandular tissue is very sparse and consists almost entirely of ducts; alveoli are difficult to find in a histological section. Prior to lactation new alveoli bud off from the ducts into the fibrous tissue and the organ usually enlarges significantly. The main ducts number about 15; they open separately on the summit of the nipple. Each is dilated into an ampulla beneath the areola. Each main duct drains a lobe of the breast; the organ is divided by fibrous tissue septa that radiate out from the centre. Each lobe is irregularly lobulated. The whole breast is embedded in the subcutaneous fat, which usually obscures the lobules from sight and touch. There is no fat beneath the nipple and areola.
The young breast is protuberant, the older breast pendulous. The former is supported by fibrous tissue strands (ligaments of Cooper) connecting the deep fascia with the overlying skin (dermis). When atrophic they allow the organ to droop; when contracted from the fibrosis around a carcinoma they cause pitting of the skin.
This is derived mainly from the lateral thoracic artery by branches that curl around the border of pectoralis major and by other branches that pierce the muscle. The internal thoracic artery also sends branches through the intercostal spaces beside the sternum; those of the second and third spaces are the largest. Similar but small perforating branches arise from the intercostal arteries; as mentioned above, the space superficial to pectoralis major is relatively bloodless. Pectoral branches of the thoracoacromial artery supply the upper part of the breast. Venous return simply follows the above-mentioned arteries. Note that the venous blood, like that from the thyroid gland, is received into the large veins that receive blood also from the vertebrae and thoracic cage — spread of malignancy by veins can thus involve these bones.
This is of the utmost importance in connexion with the spread of malignant disease.
As elsewhere in the body, so in the breast, the lymph capillaries make a richly anastomosing network continuous with the lymph capillaries of neighbouring structures — in this case those of the opposite side and of the abdominal wall. Lymph from the breast may thus radiate away to any point of the compass according to local pressures of clothing or of the examining surgeon’s hand. Only when the lymph leaves the lymphatic capillaries and enters valved lymphatic vessels is its subsequent course irrevocable. Most of the lymph of the breast drains, in fact, to the axilla.
From numerous lymphatic capillaries in the breast substance and the overlying skin, lymph from the lateral part of the breast (upper and lower outer quadrants) drains to axillary and infraclavicular nodes (p. 70), while from the medial part (upper and lower inner quadrants) it drains through the intercostal spaces into internal thoracic (parasternal) nodes. This is what might be expected — the lateral part draining laterally and the medial part medially, but it is important to note that there can be lymph flow between the lateral and medial parts of the breast and vice versa. The old concept that the skin and subareolar regions drained separately from the breast parenchyma is no longer tenable. But other pathways become possible, especially when the more usual channels have become obstructed, and lymph may then pass to the opposite breast, to cervical nodes, to the peritoneal cavity and liver through the diaphragm or through the rectus sheath, or even to inguinal nodes via the anterior abdominal wall.