Page 517 - Anatomy and Physiology of Farm Animals, 8th Edition

502 / Anatomy and Physiology of Farm Animals

he mammary glands (also called These extra teats are usually small and
Tmammae) are modified sudoriferous
VetBooks.ir (sweat) glands that produce milk for the not associated with a well‐developed
gland; because they can interfere with
nourishment of offspring. They develop
from bilateral thickenings of ventrolateral milking, they are usually removed from
the udders of cows and does.
ectoderm of the embryo, the so‐called milk Male embryos develop mammary ridges
lines, which are more correctly referred to as well, although normally these do not pro-
as mammary ridges. In carnivores and the gress in development toward the formation
sow, the mammary glands develop through- of functioning glands. Amongst ungulates,
out the axillary to inguinal extent of the nonfunctional teats are normally present in
ridges, as is appropriate for species that boars; they are sometimes seen adjacent to
typically deliver multiple fetuses. However, the scrotum of bulls and rams; and they
in most other domestic animals, only the occur only rarely on the prepuce of stallions.
inguinal mammary glands develop, usually The mammary glands of domestic
the most caudal pair (e.g., mares, ewes, and species have a great deal in common, but
does) or two pairs (e.g., cows). In the inasmuch as the udder of the dairy cow has
anthropoid apes and the elephant, only the been so dramatically developed to produce
two pectoral mammary glands develop. milk far beyond that necessary to nourish
Each gland is composed of a system of the cow’s offspring, it is discussed in detail.
ducts connecting aggregates of secretory
epithelium surrounded by connective
tissue and fat and supported in a fibroelas- Mammary Glands of the Cow
tic capsule. The proportion of secretory
parenchyma to connective tissue is hormo- The udder of the cow comprises four indi-
nally dictated; during lactation, the mam- vidual glands, referred to as quarters. The
mary gland’s secretory tissues increase in skin of the udder is covered with fine hair;
volume. After the end of lactation (when however, the teat is completely hairless.
the dam is “dry”), the secretory tissues The right and left halves of the udder each
regress, and connective tissue constitutes a consist of a cranial (front) quarter and a
greater percentage of the gland. caudal (hind) quarter. Each side of the
In ruminants and horses, individual udder is almost completely independent of
glands are associated so closely to one the other insofar as blood supply, nerve
another that they are commonly referred supply, and suspensory apparatus are con-
to as a single udder. Even so, the individual cerned (discussed later).
nature of glands of the udder is readily Ventrally, the two halves of the udder
appreciated by the presence of a single teat are demarcated by a longitudinal furrow,
(papilla) for each gland. A single (as in the intermammary groove, which corre-
ruminants) or multiple (as in the mare and sponds to a median septum of connective
sow) duct system may discharge milk at the tissue dividing left and right halves.
tip of each teat. Because of the relative isolation of each
Embryonic ectoderm invaginates side, half of the udder can be removed sur-
along the mammary ridge to become the gically without damaging the other half, as
duct system of individual mammary might be done to treat an aggressive tumor.
glands. These invaginations (mammary The glandular tissue and duct systems of
buds) will ultimately be associated with the quarters in each half of the udder are
an individual teat. In all species more entirely separate from one another. Thus,
buds initially develop than will persist, all the milk from one teat is produced by
and extra buds normally regress the glandular tissue of that quarter. The
promptly after appearing. Not uncom- vasculature, nerve supply, and lymphatic
monly, though, some extra buds persist drainage, however, are common to both
and produce supernumerary teats. quarters of a given half.