20J2 HUMAN ANATOMY. 



o, a ,, 



lhird f t ->pl vine the glandular tissue are from the fourth fifth, and sixth 



I 



3 from both the supiadavicular branches of the cervical plexus and 



fo and lat,r.d cutaneous branches of the second to the fifth intercostal* 



Dcvclopmcnt.-The arrangement of the several pairs of mammary glands 



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



^SHariiS stage of the development of these organs, so characteristic of the highest 



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



k-ridee appears as a low elevation that extends obliquely from the base of the 



r/reVnbfo thermal region. Alongthis ridge a series of enlargements, later sepa- 



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



a corresponding number of mamm*. The occurrence of a definite milk-ndge in 



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



and the position of supernumerary mammae suggests its influence. 



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

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

 undergoes proliferation and condensation and forms an investment for the growing 

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

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

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

 principle corresponds to the marsupial pouch of the lowest mammals (monotremes) . 

 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 

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

 a lumen at tlu-ir 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 mamm.-r remain rudimentary during childhood until the approach of sexual 



:ien thev increase in size and rotundity in consequence chiefly of the 



deposit i The full development of the true gland is deferred until the occur- 



101 of pi. -ive proliferation and increase in the gland-tissue take 



pi Miation f..r its functional activity as a milk-producing organ. After lacta- 



tion has ended, the mamm.i undergo regression or in volution, the glandular tissue being 



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



pregnai th the recurrence ,,[ the latter, the gland again enters upon a period 



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



coi. wln.h the amount of glandular tissue is inconspicuous. After cessation 



of men the mammary inland gradually decreases in size, and in advanced 



irs the corpus mamma? may be reduced to a fibrous disc in which gland-tissue is 



almost entirely wanting. 



