Page 293 - Anatomy and Physiology of Farm Animals, 8th Edition

278 / Anatomy and Physiology of Farm Animals

most commonly found on the face, around ceous glands, thought to be marking glands,
Certain animals have specialized seba-
lips and eyes. These hairs are particularly
VetBooks.ir well supplied with sensory nerve endings which are characteristic of their species
(Fig. 14‐4). Sheep have several cutaneous
that are sensitive to the movement of
the hair. pouches that are lined with sebaceous
When a hair is ready to shed, the epithe- glands. These are the: (1) infraorbital
lial cells over the papilla stop multiplying pouches, found at the medial canthus of the
and become cornified. The papilla atro- eye and larger in rams than in ewes; (2)
phies, and the hair may fall out, be pulled interdigital pouches on the midline above
out, or be pushed out by a new hair that the hoofs of all four feet; and (3) inguinal
develops from epithelial sheath cells in a pouches near the base of the udder or scro-
manner similar to the hair formation just tum. Goats have sebaceous horn glands
described. A seasonal shedding of the hair caudal to the base of the horn (or where the
coat from the light coat of summer to the horn would be in polled animals); secretion
heavy coat of winter and back again is in these glands is increased during breed-
characteristic of most domestic species ing season and is especially pungent in
and is largely triggered by changes in the bucks. In pigs, sebaceous carpal glands are
photoperiod. present on the mediopalmar aspect of the
The arrector pili muscle (plural: arrec- carpus in both boars and sows.
tores pilorum muscles) is a tiny bundle of Sudoriferous glands or sweat glands
smooth muscle fibers that extends from (tubular skin glands) can be found over the
the deep portion of the hair follicle at an entire body of farm animals, including the
angle toward the epidermis (Fig. 14‐3). horse, cow, sheep, and pig, although in this
Contraction of the muscle will straighten last species they are sparse. The horse is the
the hair toward 90°. This orientation only farm animal that sweats readily.
increases the insulating properties of the Tubular glands occur on the planum nasola-
hair coat during exposure to cold and is biale of the cow, planum nasale of the sheep,
used by some animals during fight‐or‐ and planum nasale of the pig (all hairless
flight reactions, presumably as a means of areas of the nose), where they moisten these
increasing the apparent size of the animal. surfaces but play little role in cooling. Many
The arrectores pilorum muscles are inner- modified epithelial structures, including
vated by sympathetic nerves. hoofs and horns, lack sweat glands.
Horses are distinguished among
domestic animals by their abundant
Glands production of sweat as a mechanism for
cooling. Equine sweat glands, unlike
Sebaceous glands are classified as holo- those of most other species, are sensitive
crine glands because their oily secretory to circulating epinephrine, which is why
product, sebum, is produced by disintegra- a nervous horse breaks into a sweat in
tion of epithelial cells within the glands. the absence of physical exertion. Equine
Most of these oil‐producing glands are sweat, moreover, is especially rich in
derived from the external epithelial root protein; this albuminous sweat will foam
sheath and empty their secretion into the when agitated by working muscles. For
hair follicle (Figs. 14‐2 and 14‐3). this reason, a hard‐worked horse will
Contraction of the arrector pili muscle lather up on the neck and shoulders and
compresses the glands and aids in empty- between the pelvic limbs.
ing them. Sebaceous glands that open The mammary gland is thought to be a
directly onto the skin surface include those modification of tubular sweat glands. Its
in the ear canal, around the anus, and in unique importance warrants its own chap-
the penis, prepuce, and vulva, along with ter (Chapter 29), and therefore the mam-
the tarsal glands of the eyelid. mary gland is not considered here.