Page 521 - Anatomy and Physiology of Farm Animals, 8th Edition

506 / Anatomy and Physiology of Farm Animals

trunk. The external pudendal artery passes Lymphatic Vessels
downward through the inguinal canal in a
VetBooks.ir more or less tortuous manner and divides The lymphatic vessels draining the udder

into cranial and caudal branches that supply
the front‐ and hindquarters of the udder on show up rather well superficially just under
the skin, particularly in high‐producing
the same side as the artery. Additionally, a cattle. They drain from the entire udder,
small artery that may be single or paired (as including the teat, to the superficial ingui-
determined by chance), the ventral perineal nal (mammary or supramammary)
artery, continues from the internal pudendal lymph nodes near the superficial (exter-
artery and passes downward from the vulva nal) inguinal ring above the caudal part of
just deep to the skin on the median line. The the base of the udder.
perineal artery usually supplies a small
amount of blood to the caudal part of both
halves of the udder. Microscopic Anatomy
The venous drainage from the udder is of the Mammary Gland
largely by way of a venous circle at the
base of the udder, where it attaches to the The mammary gland is classified as a com-
abdominal wall. This venous circle is pound tubuloalveolar gland. It consists of
formed from the main veins that drain the a connective tissue interstitium, paren-
udder. The external pudendal vein of chyma (secretory epithelium), ducts, vessels,
each side receives blood from both the and nerves. In the dry (not lactating) state,
cranial and caudal quarters of the same the gland has proportionately more stroma
side. Cranially, each external pudendal than parenchyma (Fig. 29‐4); during lacta-
vein is continuous with the caudal super- tion, the parenchyma undergoes marked
ficial epigastric vein and caudally growth and constitutes the bulk of the
with the perineal vein. An anastomosis gland (Fig. 29‐5).
between the two caudal superficial epi- The surface of the bovine and porcine
gastric veins just at or in front of the udder teat is covered with glabrous (hairless),
completes the venous circle. The caudal aglandular stratified squamous epithelium.
superficial epigastric vein passes forward The teats of other domestic species have
in a sagittal plane lateral to the midline on more typical haired and glandular skin.
the ventral abdominal wall and joins with This is continuous with the stratified squa-
the cranial superficial epigastric vein, mous epithelium that lines the streak canal.
which ultimately drains to the internal The duct is encircled by smooth muscle
thoracic veins and then to the cranial vena fibers that act as a sphincter.
cava. Before the heifer comes into milk, At the junction of the streak canal and
the connection between cranial and cau- the teat cistern, the epithelial lining
dal superficial epigastric veins is poorly changes abruptly from stratified squamous
developed. During first pregnancy, when to stratified columnar epithelium that is
the udder undergoes a marked increase in usually two cells thick. This stratified
size and, consequently, blood supply, the columnar epithelium lines the teat and
two veins develop a functional anastomo- gland cisterns and the larger lactiferous
sis, after which they collectively consti- ducts. As the ducts branch and become
tute the subcutaneous abdominal vein smaller, the epithelial lining changes first
or the milk vein. In high‐producing dairy to simple columnar and then to secretory
cows the subcutaneous abdominal vein is epithelium in the alveoli. The height of the
large and tortuous. It passes through a alveolar epithelium varies considerably
foramen in the rectus abdominis muscle with the level of activity of the gland.
(the milk well), joins the internal thoracic The mammary gland differs from most
vein, and ultimately drains to the cranial other exocrine glands in that the secretory
vena cava. portion is not limited to the terminations