2032 HUMAN ANATOMY. 



of a few trunks that follow the perforating arteries and become afferents of the lymph- 

 nodes lying along the internal mammary artery, all the lymphatics of the breast join 

 to form two or three large trunks that pass from the lower and lateral border of the 

 organ through the subcutaneous tissue towards the axilla to empty, sometimes united 

 into a single stem, into the lymph-node that lies upon the serratus magnus over the 

 third rib. 



The nerves supplying the glandular tissue are from the fourth, fifth, and sixth 

 intercostals, the accompanying sympathetic fibres passing by way of the rami com- 

 municantes from the thoracic portion of the gangliated cord. Their ultimate distri- 

 bution may be traced to the plexuses upon the basement membrane surrounding the 

 alveoli and, according to Arnstein, even between the secretory cells. The cutaneous 

 nerves are derived from both the supraclavicular branches of the cervical plexus and 

 the anterior and lateral cutaneous branches of the second to the fifth intercostals. 



Development. — The arrangement of the several pairs of mammary glands 

 possessed by a majority of the lower animals in two longitudinal rows is foreshadowed 

 in the earliest stage of the development of these organs, so characteristic of the highest 

 class of vertebrates (mammalia). A linear thickening of the ectoblast, known as the 

 milk-ridge, appears as a low elevation that extends obliquely from the base of the 

 forelimb to the inguinal region. Along this ridge a series of enlargements, later sepa- 

 rated by absorption of the intervening portions of the ridge, indicates the anlage for 

 a corresponding number of mammae. The occurrence of a definite milk-ridge in 

 the human embryo is uncertain, although its presence has been observed (Kallius), 

 and the position of supernumerary mammae suggests its influence. 



In man a knob-like thickening of the ectoblast appears during the second month 

 of foetal life. This thickening sinks into the underlying mesoblastic tissue, which 

 undergoes proliferation and condensation and forms an investm.ent for the growing 

 epithelial mass. From this envelope the fibrous and muscular tissue of the areola 

 and nipple are derived, while the subjacent mesoblast produces the connective-tissue 

 stroma. The ectoblastic ingrowth represents a sunken area of integument that in 

 principle corresponds to the marsupial pouch of the lowest mammals {'>nonotre7nes). 

 Solid epithelial sprouts grow out from the sides of the conical or flask-shaped 

 epidermal plug and are the first anlages of the true mammary gland, later becoming 

 the excretory ducts. Subsequently the central part of the ectoblastic ingrowth 

 undergoes degeneration and destruction, and what at first was an elevation now 

 becomes a depression of the surface. From the middle of this depressed area there 

 appears, shortly before or immediately succeeding (Basch) birth, an elevation that 

 later becomes the nipple. Meanwhile, the epithelial duct-outgrowths penetrate the 

 surrounding condensed mesoblastic stroma, increase in length, subdivide, and acquire 

 a lumen at their expanded distal ends, thus giving rise to the system of ducts and 

 the lobules of immature gland-tissue. With the further development of the latter, 

 the surrounding mesoblastic stroma is broken up into the interlobular septa and 

 fibrous framework of the corpus mammae. 



At birth the gland is represented by the lactiferous ducts with their ampullae, the 

 smaller ducts, and the immature alveoli. Quite commonly the mammary glands in 

 both sexes are the seat of temporary activity during the first few days after birth, the 

 breasts yielding a secretion resembling colostrum, popularly known as "witch-milk." 



The mammae remain rudimentary during childhood until the approach of sexual 

 maturity, when thev increase in size and rotundity in consequence chiefly of the 

 deposition of fat. The full development of the true gland is deferred until the occur- 

 rence of pregnancy, when active proliferation and increase in the gland-tissue take 

 place in preparation for its functional activity as a milk-producing organ. After lacta- 

 tion has ended, the mammae undergo regression or involution, the glandular tissue being 

 reduced in amount and returning to a condition resembling that existing before 

 pregnancy. With the recurrence of the latter, the gland again enters upon a period 

 of renewed growth and preparation, to be followed in time by return to the resting 

 condition, in which the amount of glandular tissue is inconspicuous. After cessation 

 of menstruation the mammary gland gradually decreases in size, and in advanced 

 years the corpus mammae may be reduced to a fibrous disc in which gland-tissue is 

 almost entirely wanting. 



