=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. CHAPTER VIII. THE HEART AND CIRCULATION. =The Heart.=--Shielded within the chest are, as has been said, the heart and lungs. The heart lies on the left side behind the sternum and the cartilages of the fourth to seventh ribs in a closed, conical, membranous sac, the _pericardium_, which is attached by its base to the central tendon of the diaphragm, and whose point extends up between the pleuræ of the lungs. This sac has an external fibrous layer and an internal serous layer that is reflected back over the heart itself, forming a closed sac, within which a thin fluid is secreted that serves to reduce friction during the movements of the heart, the two inner surfaces sliding over each other with every beat. [Illustration: FIG. 39.--The heart. (Stoney.)]