Page 179 - Anatomy and Physiology of Farm Animals, 8th Edition

164 / Anatomy and Physiology of Farm Animals

of the stay apparatus are also the elements ribs. This muscle is characterized by a
thick, tendinous layer that is capable of
of the limb that permit the horse to stand
VetBooks.ir with minimal physical effort. supporting the trunk without contraction
To understand the function of the stay
of the muscle fibers. The weight of the
apparatus, it is necessary to keep several trunk is thereby supported effortlessly by a
concepts central: slinglike structure composed of right and
left mm. serrati ventrales (see Fig. 7‐8); by
1. Ligaments, while elastic, do not stretch this construct, much of the weight of the
nearly as much as muscle. Therefore horse is transferred to the thoracic limbs.
tendons, because they connect with When bearing weight, the shoulder
muscles, offer much less resistance to joint tends to flex, and the fetlock, pastern,
stretch than do ligaments.
2. When joints are loaded (i.e., bearing and coffin joints tend to hyperextend
beyond their normal physiologic angles,
weight), they tend to collapse.
3. To keep joints from collapsing with while the carpus and elbow are relatively
stable when loaded in the extended posi-
minimal muscular effort, ligamentous tion. Components of the stay apparatus
straps must cross the joint to counteract cross all of these joints, counteracting their
the tendency to fold during weight bear- tendency to collapse under load (Fig. 8‐12).
ing (Fig 8‐12).
The extensor surface of the shoulder is
crossed by the tendon that is the origin of
Thoracic Limb the m. biceps brachii, a very broad, partly
cartilaginous tendon that is continuous
The thoracic limb is in part affixed to the with a very dense fibrous band running
trunk by the fan‐shaped m. serratus ven- through the length of the muscle belly.
tralis, extending from the scapula to the This fibrous band blends distad with the

(A) (B) (C)
Load Load

Loaded
shoulder
flexes

Ligamentous
band prevents
flexion

Elbow and carpus
relatively stable
in extension

Ligamentous
Loaded bands prevent
fetlock and digit hyperextension
hyperextends

Figure 8-12. When the appendicular skeleton is loaded with weight (A), the joints tend to collapse (B). The
stay apparatus is a series of ligamentous bands that cross the joints and passively prevent this collapse (C).