Page 111 - Anatomy and Physiology of Farm Animals, 8th Edition

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African group of hunter‐gatherers, com- subjected to stress but return to its original
shape when the stress is removed. Mature
VetBooks.ir monly called Pygmies, whose average bone is relatively inelastic. A rod of bone
height is unusually short as a result of
can be elongated only about 0.5% of its
low IGF levels despite normal growth
hormone. length before breaking. However, even this
Androgens, such as the sex hormone much deformity is not perfectly elastic; the
testosterone, and estrogens have a variety deformity is permanent, and the bone will
of complex effects on the rates of bone not return completely to its original length
growth. The well‐recognized growth spurt if stretched near its breaking point. This
associated with puberty is thought to be characteristic of deforming under stress
due to stimulatory effects of androgens without returning to the original shape
and estrogens that increase in circulation is exaggerated in bone diseases such as
at this time. Androgens appear to have a rickets.
greater stimulatory effect than estrogens, In addition to tension (stretching), bone
and this difference is responsible, in part, may be subjected to stresses of compres-
for differences in body size between males sion, shearing, bending, and torsion (twist-
and females. The stimulatory effects of ing). A bone will support considerably
androgens on growth are in part due to more weight in a static situation (support-
their ability to increase the secretion of ing weight without moving) than under a
growth hormone. While sex hormones are dynamic load. A dynamic load results from
capable of stimulating the rate of growth, impact between the bone and another
they also bring about epiphyseal closure, object. For example, the leg bones of a
which ultimately limits body size. The horse bear a static load when the horse is
mechanisms responsible for this effect are standing quietly, but will bear a dynamic
not completely understood, but differences load when the horse is running, jumping,
in the magnitude of their stimulatory effect or kicking. Compression, bending, and
on ossification versus their stimulatory shearing of the leg bones are all stresses
effect on cartilage production may be produced by this type of activity. When a
responsible. horse or other animal pivots with one or
more feet bearing weight, torsion or twist-
ing is added to the other stresses. This is
Intramembranous Ossification seen particularly well in the action of cut-
ting horses. Muscles and tendons that run
Many of the flat bones, such as bones of parallel to a bone tend to act like guy wires
the skull, are preformed in a fibrous mem- and reduce stresses, particularly bending
brane, or matrix, which is infiltrated with and shearing stresses.
osteoid tissue. The osteoid tissue calcifies Bone, even in a fresh carcass, appears
to form true bone. The layers of perios- hard, dense, inelastic, and almost lifeless.
teum on either side of the bone then form Actually, bone is quite a dynamic tissue,
additional bone. Like long bones of the and all bone is constantly being formed
limbs, large flat bones in mature animals and resorbed. The continuous turnover of
consist of compact bone surrounding a bone in mature animals is termed remod-
cancellous bone core. eling. Through remodeling, bone can
shrink (atrophy), increase in size (hyper-
trophy), repair breaks, and rearrange its
Physiology of Bone internal structure to best resist stresses
and strains. In both normal and pathologic
Bone Mechanics and Remodeling conditions, bone can reshape itself accord-
ing to good engineering principles to sus-
Elasticity is the characteristic of a sub- tain a maximum of stress with a minimum
stance that enables it to change shape when of bone tissue. Atrophy of bone occurs