DEVELOPMENT OF THE MAMJVLE. 1339 



that while the majority of the lymph vessels first reach the lymph glands lying on the lateral 

 border of the pectoralis major, some free vessels may "short circuit" to glands (1) in the floor of 

 the axilla, (2) along the circumflex vessels, (3) even to glands along the axillary vein. Yet other 

 lymph vessels of the breast may reach first the glands in the costo-coracoid space. Some vessels 

 from the medial part of the breast, following the course pursued by the perforating arteries, may 

 join the lymph glands situated along the course of the internal mammary artery, but these 

 vessels are fortunately often absent. It is also to be remembered that a few, probably irregular, com- 

 munications exist across the middle line with the lymph vessels of the opposite breast ; and 

 further that lymph vessels from the infero-medial area of the breast regularly join the fascial 

 lymph vessels of the upper part of the sheath of the rectus abdominis, and through these make 

 communication with the lymph vessels of (1) the round ligament of the liver, (2) the peritoneum 

 generally, and ultimately with the abdominal lymph glands. The surgical importance of the 

 facts regarding the lymphatic drainage of the breast cannot be exaggerated. 



The nerve-supply of the gland is derived from the intercostal nerves of the fourth, fifth, and 

 sixth intercostal spaces. Along the course of these nerves sympathetic filaments reach the breast 

 from the thoracic part of the sympathetic trunk. 



DEVELOPMENT OF THE MAMMAE. 



The mammae are developed as downgrowths of the ectoderm into the underlying 

 mesodermic tissue. In the human embryo a thickened raised area of the ectoderm 

 can be recognised in the region of the future mamma at the end of the fourth 

 week. This thickened ectoderm becomes depressed in the underlying mesoderm, and thus 

 the mammary area soon becomes flat, and finally sunk below the level of the surrounding 

 epidermis. The mesoderm, where it is in contact with this downgrowth of the ectoderm, 

 is compressed, and its elements become arranged in concentric layers, which, at a later 

 stage, give rise to the connective-tissue stroma of the gland. The depressed mass of ecto- 

 derm cells soon becomes somewhat flask-shaped, and grows out into the surrounding 

 mesoderm as a number of solid processes, which represent the future ducts of the gland. 

 These processes, by dividing and branching, give rise to the future lobes and lobules, and 

 much later to the alveoli. The mammary area becomes gradually raised again in its 

 central part to form the nipple. A lumen is formed in the different parts of this branch- 

 ing system of cellular processes only at birth, and with its establishment is associated 

 the secretion of a fluid resembling milk, which often takes place at this time. The 

 ampullae appear as thickenings on the developing ducts before birth. 



In those animals which possess a number of mammary glands such as the cat, pig, 

 etc. the thickening of the ectoderm, which is the first indication of the development of 

 these structures, takes the form of a pair of ridges extending from the level of the fore- 

 limb towards the inguinal region. These converge posteriorly, and at their terminations 

 lie not far from the middle line. By the absorption of the intermediate portions the 

 ridges become divided up into a number of isolated areas, in connexion with which the 

 future glands arise. Somewhat similar linear thickenings of the ectoderm have also been 

 recognised in the human embryo, and the usual positions assumed by the accessory glands 

 when present, leads us to suspect that in all probability the ancestors of man possessed 

 numerous mammae arranged, as in lower animals, in lines converging towards the 

 inguinal region. 



