1148 UROGENITAL SYSTEM. 
nipple only is present it is usually the left. The presence of supernumerary glands or nipples is 
not so uncommon, and a very large number of examples are recorded. The term polymasty has 
been applied to cases in which more than the normal number of mammze are present, and poly- 
thely to those in which additional glands, in a rudimentary condition, are represented by accessory 
nipples. Usually the accessory glands, or nipples, are present on the ventral aspect of the thorax, 
and in most instances they occur below and a little to the inner side of the normal site. When 
the abnormal glands are found above the normal site they generally le further from the middle 
line. Much more rarely accessory glands have been found on the abdomen, in the axilla, or in 
some other situation, including even the dorsal aspect of the trunk. As many as three extra pairs 
of mamme have been found in the same individual, and cases in which the probable representa- 
tives of mammary glands were even more numerous have been recorded. Asymmetry isvery common 
in these abnormal structures. It is interesting to note that examples of polymasty and polythely 
occur in the male almost as frequently as in the female. In some women the accessory breasts 
have yielded milk during lactation ; in other cases the abnormal organs have been very rudi- 
mentary, and represented only by a minute nipple or pigmented areola. These cases of poly- 
masty and polythely are supposed to represent a reversion to an ancestral condition, in which 
more than two mammary glands were normally present, and in which probably many young were 
produced at each birth. In this connexion it is interesting to observe that usually the accessory 
glands occur in positions normally occupied by mamme in lower animals. In the course of 
development of the mammary glands in man, specialised areas of the epidermis, similar to those 
which give origin to the mammary glands, have been observed both above and below the region 
in which the adult mammary glands are developed. These areas appear to be present normally, 
but in most cases they disappear at an early stage in the history of the embryo. In some other 
mammals rudimentary mammary glands may occur, as, for instance, in lemurs and in some cows. 
A slight functional activity of the mammary glands of the male at birth and about the time 
of puberty is not a very uncommon occurrence. 
Vessels and Nerves of the Mamma.—The breast receives its arterial supply from the per- 
forating branches of the internal mammary artery and from the external mammary branch of the 
long thoracic. Additional supply is sometimes derived from some of the intercostal vessels. The 
veins coming from the gland pour their blood into the axillary and internal mammary veins. 
Some small superficial veins from the breast join tributaries of the external jugular. The 
lymphatics of the breast are very numerous, and form extensive lymph spaces round alveoli of 
the gland. The lymphatic vessels coming from the mamma for the most part join the lymphatic 
glands of the axilla, but some vessels from the inner part of the breast, following the course pur- 
sued by the perforating arteries, join the lymphatic glands situated along the course of the internal 
mammary artery. 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 dorsal part of the sympathetic cord. 
DEVELOPMENT OF THE MAMM. 
The mammary glands are developed as downgrowths of the epidermis into the under- 
lying mesoblastic tissue. In the human embryo a thickened raised area of the epidermis 
is to be recognised in the region of the future mammary gland at the end of the fourth 
week. This thickened epiblast becomes depressed in the underlying mesoblast, and thus 
the mammary area soon becomes flat, and finally sunk below the level of the surrounding 
epidermis. The mesoblast, where it is in contact with this downgrowth of the epiblast, 
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 epi- 
blastic cells soon becomes somewhat flask-shaped, and grows out into the surrounding 
mesoblast 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 only formed in the different parts of this 
branching system of cellular processes at birth, and with its establishment is associated 
the secretion of a fluid resembling milk, which often takes place at this time. The 
ampulle appear as thickenings on the developing ducts before birth. 
In those animals which possess a number of mammary glands—such as the cat, pig, 
ete.—the thickening of the epiblast, 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 connection with which the 
future glands arise. Such linear thickenings of the epiblast have not yet been shown to 
occur in the human embryo, but the usual positions assumed by the accessory glands when 
present, leads one to suspect that in all probability the ancestors of man_ possessed 
numerous mammary glands arranged, as in lower animals, in lines converging when traced 
towards the inguinal region. 
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