The secretory apparatus of the breasts is divided into fifteen or twenty lobes, these in turn being divided into clusters of lobules. The lobules in turn are composed of tiny, secreting cells, called _acini_, in which the milk is elaborated from the blood. The acini are minute globules lined by a single layer of cells and enveloped by a very delicate membrane. Tiny ducts carry the milk from the acini to the main duct of the lobule, around which the acini cluster. These ducts empty the milk into the larger duct of the lobe, which runs straight to the nipple and opens upon the surface. Just before reaching the surface, each of these lactiferous sinuses expands into an _ampulla_, a minute reservoir for collecting the milk, which is secreted during the periods between nursings. These clusters of acini uniting to form lobules with tiny ducts leading into the main duct of each lobule, closely resemble a bunch of grapes. The separate grapes correspond to the acini, their small stems correspond to the tiny ducts of the glands which lead to a larger one, and the central stem of the grape cluster, to the milk duct that opens upon the nipple. The secretory tissue really constitutes a small part of the breasts until they begin to function. But during lactation the acini become enormously developed and enlarged. After lactation ceases, the acini assume a more or less tubal form, many of them undergoing atrophic changes. CHAPTER II PHYSIOLOGY =Puberty= is that period during which childhood develops into sexual maturity, and the individual becomes capable of reproduction. The age at which puberty occurs varies with climate, race, occupation and with individuals of the same status. But the average age for girls, in temperate climates, is from the twelfth to the sixteenth year; for boys from the fourteenth to the seventeenth year. Girls in southern climates sometimes mature as early as the eighth or ninth year, while in colder regions puberty may be delayed until the eighteenth or twentieth year. At this time there are many physical and psychical manifestations of the maturing changes in the internal female generative organs. The undeveloped girl grows rapidly at this stage. Her entire body rounds out and assumes a more graceful contour; her breasts increase in size; her hips broaden; the external genitalia enlarge and hair appears over the pubis and on other parts of the body.