Page 26 - Anatomy and Physiology of Farm Animals, 8th Edition

Introduction to Anatomy and Physiology / 11

discharges to the lumen of the gland to irregular connective tissue. This forms a
strong covering that resists tearing and yet
constitute the secretion. Sebaceous glands
VetBooks.ir associated with hair follicles of the skin is flexible enough to move with the epider-
mis, or surface of the body.
are the most common holocrine glands.
An intermediate form of secretion is con- Areolar (loose) connective tissue
ducted by apocrine glands, in which a (Fig. 1‐7) is found throughout the body
small amount of cytoplasm and cell mem- wherever protective cushioning and flexi-
brane is lost with the secretion. This type of bility are needed. For example, blood vessels
secretion is sometimes described for the are surrounded by a sheath of areolar
prostate gland and some sweat glands. connective tissue, which permits the vessels
to move, and yet protects them.
Beneath the dermis is a layer of loosely
Connective Tissues arranged areolar connective tissue fibers
that attaches the skin to underlying mus-
Connective tissues, as the name implies, cles. This attachment is flexible enough to
serve to connect other tissues. They give permit movement of the skin. It also per-
form and strength to many organs and mits the formation of a thick layer of fat
often provide protection and leverage. between the skin and underlying muscles.
Connective tissues include elastic tissue, Whenever the skin is adherent to bony
collagenous (white fibrous) tissue, reticular prominences because of a lack of areolar
(netlike) tissue, adipose (fat) tissue, carti- tissue, the skin will not move, and no layer
lage, and bone. of fat can form. This feature is seen in beef
Elastic tissue contains kinked fibers cattle that have ties; in this case, the skin
that tend to regain their original shape over the back shows large dimples where
after being stretched. This tissue is found fat cannot fill in because the skin is adher-
in the ligamentum nuchae, a strong band ent to the vertebrae.
that helps to support the head, particularly Reticular connective tissue consists of
in horses and cattle. Elastic tissue also fine reticular fibrils made by fibroblasts,
is found in the abdominal tunic, in the which forms a scaffolding for other cells.
ligamenta flava of the spinal canal, in elastic Reticular tissue makes up part of the
arteries, and mixed with other tissues framework of endocrine and lymphatic
wherever elasticity is needed. organs.
Collagenous (white fibrous) tissue is Adipose tissue (fat) tissue (Fig. 1‐8)
found throughout the body in various forms when connective tissue cells called
forms. Individual cells (fibroblasts) produce adipocytes store fat as inclusions within
long proteinaceous fibers of collagen, which the cytoplasm of the cell. As more fat is
have remarkable tensile strength. These stored, the cell eventually becomes so
fibers may be arranged in regular repeating filled with fat that the nucleus is pushed to
units, or laid down in a more random, one side of the cell, which, as a result,
irregular arrangement. becomes spherical. Most fat in the animal
In dense regular connective tissue body is white, although it may have a yellow
(Fig. 1-7), the fibers are arranged in paral- tinge in horses and some breeds of dairy
lel bundles, forming cords or bands of cattle because of carotenoids in the feed.
considerable strength. These are the ten- In contrast to this white fat, a small
dons, which connect muscles to bones, amount of brown fat may be found in
and the ligaments, which connect bones domestic mammals, hibernating mammals,
to bones. The fibers of dense irregular rodents, and human infants. The brown
connective tissue are arranged in a thick fat is found between the scapulae, in the
mat, with fibers running in all directions. axillae, in the mediastinum, and in asso-
The dermis of the skin, which may be ciation with mesenteries in the abdomen.
tanned to make leather, consists of dense Brown fat is able to generate heat to protect