The =mammary glands= (Glandulæ lactiferæ) are modified cutaneous glands
which are so closely associated functionally with the genital organs as
to be considered accessory to them.
In the mare they are two in number, and are placed on either side of the
median plane in the inguinal region. Each gland has the form of a short
cone, much compressed transversely, and having a flat inner surface. It
consists of the glandular mass or body of the gland (Corpus mammæ) and
the =teat= or =nipple= (Papilla mammæ). The base is related to the
abdominal wall, to which it is attached by areolar tissue, which
contains a venous plexus, the superficial inguinal lymph glands, and a
variable amount of fat. The apex is constituted by the teat, which is
also flattened transversely and varies in length from one to two inches
(ca. 2.5 to 5 cm.). Between the bases of the teats is the intermammary
groove. On the apex of each teat two or three small orifices are placed
close together; these are the openings of the lactiferous ducts.
=Structure.=—The =skin= over the glands is thin, pigmented, chiefly
hairless, and supplied with numerous large sebaceous and sweat glands.
Under this are two layers of =fascia= except on the teats. The
superficial fascia presents no special features. The deep fascia
consists of two laminæ detached from the abdominal tunic which descend
on either side of the median plane, forming a =septum= between the two
glands, and constituting their ligamentum suspensorium.
These laminæ are separated by a layer of areolar tissue, so that it is
possible to remove one gland if diseased by careful dissection between
the layers of the septum.
The =gland substance= or =parenchyma= is pinkish gray in color, and of
firmer consistence than the fat which is found around and within the
gland. It is inclosed by a fibro-elastic capsule which sends inward
numerous trabeculæ; these form the =interstitial tissue=, and divide the
gland into =lobes= and =lobules=. In the latter are the secretory
=tubules= and =alveoli=, which unite to form the larger =ducts=. Each
lobe has a duct, which opens at the base of the teat into a space called
the =lactiferous sinus= (Sinus lactiferus); this is lined with mucous
membrane, and from it two (or three) =lactiferous ducts= (Ductus
lactiferi) pass through the extremity of the nipple. These ducts are
lined with a non-glandular mucous membrane, which is covered with
stratified squamous epithelium. They are surrounded by unstriped
muscular tissue, the bulk of the fibers being arranged in a circular
manner to form a =sphincter=.
The size and form of the mammary glands are subject to much variation.
In the young subject, before pregnancy, they are small and contain
little gland tissue. During the latter part of gestation, and
especially during lactation, they increase greatly in size, and the
gland tissue is highly developed. After lactation the secretory
structures undergo marked involution, and the gland is much reduced in
size. The relative amounts of gland substance and interstitial tissue
vary greatly; in some cases a gland of considerable size contains
little secretory tissue and is consequently functionally deficient.
=Vessels and Nerves.=—The =arteries= are derived from the =external
pudic artery=, which enters the gland at the posterior part of its base.
The =veins= form a plexus on either side of the base of the gland, which
is drained by the =external pudic vein= chiefly. The =lymphatics= are
numerous and pass chiefly to the superficial inguinal (or supramammary)
lymph glands. The =nerves= are derived from the =inguinal nerves= and
the =posterior mesenteric plexus= of the sympathetic system.