of the subclavian and the thoracic aorta, the phrenic, mediastinal, and intercostal branches of the internal mammary, and the thoracic branches of the axillary.
The nerves are the intercostals and phrenics.
Mammary Glands.—On the outside of the chest walls, lodged in the fascia of the pectoral muscles, are the mammary glands, accessory organs of the generative system. They exist in both sexes but are only rudimentary in the male. In the female they are small before puberty but enlarge as the generative organs become more completely developed, forming two hemispherical eminences, one on either side, between the third and seventh ribs. During pregnancy they increase once more in size preparatory to the secretion of the milk, and in old age they atrophy. From the middle projects a small pinkish-brown conical eminence, the nipple, surrounded by a paler area, the areola. After the second month of pregnancy both nipple and areola become darker in color, a point of great diagnostic value in early pregnancy.
The mammary glands themselves consist of lobules of gland tissue with a central lactiferous tubule, the lobules being gathered into lobes with fatty tissue between. From the juncture of these tubules result fifteen or twenty excretory ducts, the tubuli lactiferi, which converge toward the areola. Beneath the nipple they dilate, forming the ampullæ, and then contract again to pass out through the nipple as straight tubes.
Breast abscess occurs most commonly in nursing mothers, as where a part is most active there is most danger of abscess. Many benign tumors of the breast, as the fibrous tumors, occur and are especially common in young women. If a fibrous tumor is allowed to develop it may become cancerous. Cancer, however, generally occurs after the age of forty and is usually due to some irritation, as to a blow from a ball.
The arteries of the breasts are the thoracic branches of the axillary, the intercostal, and the internal mammary.
The nerves are from the thoracic cutaneous.
