Although the mammary glands do not come into use until adult life, nevertheless, they are the first of all the glands which arise from the epidermis to appear during the development of the embryo. In a 6-weeks-old human embryo an ectodermal ridge, known as the milk line or primary ridge, is noted. This thickening extends along the body wall on either side from the axilla to the groin. In the human it atrophies, but a small portion remains in each pectoral region, which becomes the mammary gland. Since these ridges consist of tissue which is potentially mammary, failure of their normal disappearance will result in the persistence of accessory breast tissue. Such tissue remains along the line and can appear anywhere from the axilla to the inner aspect of Scarpa’s triangle (Fig. 197). The rare presence of accessory breast tissue in locations such as the gluteal region and the shoulder can be explained only by an ectopic placement of the milk ridge. Normally, that portion of the ridge which is located in the pectoral region starts to grow inward in the shape of buds, which form slender tubes and from them the ducts and the secreting tissues of the breast eventually develop. The nipple is either flat or depressed at birth and only later does it evert so that it projects above the surrounding skin (Fig. 198).
KeywordsMammary Gland Latissimus Dorsi Pectoralis Major Muscle Axillary Vein Pectoralis Minor Muscle
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