_a_, Body of gland; _b_, lactiferous sinus; _c_, cavity of teat; _d_,
duct of teat; _e_, intermammary groove; _f_, septum between glands;
_g_, supramammary fat.
]
The size and form of the glands vary greatly; in the highly
specialized dairy breeds (_e. g._, Holsteins) they often attain
enormous dimensions. The size of the udder is not a certain index of
its productivity, since some so-called “fleshy” udders contain a very
large amount of interstitial tissue and relatively little parenchyma.
Small accessory teats often occur posteriorly; commonly these are
rudimentary and imperforate (polythelia), but sometimes they drain a
small amount of gland substance (polymastia). Sanson described a case
in which there were seven teats, all giving milk.
Each teat has a single duct which widens superiorly and opens freely
into a roomy =lactiferous sinus= or milk cistern (Sinus lactiferus). The
=lactiferous duct= or teat canal is lined by a glandless mucous membrane
which is covered with stratified squamous epithelium; the lower part
(ca. 1 cm.) of the canal is narrow, and is closed by a =sphincter= of
unstriped muscle. The =mucous membrane= of the sinus forms numerous
folds which render the cavity multilocular.
=Vessels and Nerves.=—The blood-supply is derived from the =external
pudic arteries=. The =veins= form a circle at the base of the udder,
from which the blood is drained by three trunks, viz., the very large
=subcutaneous abdominal= (“milk vein”), the =external pudic=, and the
=perineal vein=. The =lymphatics= are numerous, and pass to the
supramammary glands chiefly. The nerves are derived from the =inguinal
nerves= and the =posterior mesenteric plexus= of the sympathetic.