Page 139 - Anatomy and Physiology of Farm Animals, 8th Edition

124 / Anatomy and Physiology of Farm Animals

are attached to the periosteum of the bone brachii is an example of a muscle with
multiple heads, in this case three.
by a short, strong connection made by the
VetBooks.ir connective tissues associated with endo‐, Functional Grouping of Muscles
peri‐, and epimysium of the muscle.
Tendons, fibrous bands connecting
muscles to bone, are composed of dense, If a muscle is on the side of a joint toward
regular collagen in parallel bundles. Most which it bends (decreasing the angle
tendons are cords or bands that attach described by the joint), it is a flexor of that
spindle‐shaped or pennate muscles to joint. A muscle on the opposite side is an
bones. Other tendons are flat sheets known extensor (Fig. 7‐2). The m. biceps brachii,
as aponeuroses (singular, aponeurosis), on the cranial side of the limb, flexes the
usually attached to somewhat flattened elbow. The m. triceps brachii, on the caudal
muscles. The heavy fibrous sheets that aspect of the limb, takes origin from the
cover the muscles of the loin are good scapula and humerus and inserts on the
examples of aponeuroses. ulna. Thus, the triceps is an extensor of the
Most muscles have attachments to two elbow (Fig. 7‐3).
or more bones. Some muscles are also/ Muscles that tend to pull a limb toward
instead attached to soft tissue, such as the the median plane are adductors, and those
skin, or to other muscles (e.g., m. tensor that tend to move the limb away from the
fasciae antebrachii and its attachment to median plane are abductors. Muscles that
the m. latissimus dorsi). Traditionally, the pass over more than one joint often have
more proximal attachment is called the different descriptions of their functions,
muscle’s origin, and the more distal depending on the joint on which they are
attachment is its insertion. This arrange­ acting. The deep digital flexor muscle of the
ment is reasonably clear in the limbs, but hindlimb is a flexor of the digital joints and
less so elsewhere. In these other locations simultaneously an extensor of the hock.
(e.g., the trunk and neck), the less mobile Muscles that encircle an opening, whether
attachment is usually designated as the they are striated or smooth, are sphincters.
origin and the more movable point is its The smooth muscle surrounding the open­
insertion. Since the only thing a muscle ing between the stomach and the intestine
can actively do is contract, it nearly always forms the pyloric sphincter, which controls
tends to bring its origin and insertion passage of food from the stomach. The
closer together, causing one or both of the m. orbicularis oculi is composed of striated
attachment points to move. muscle fibers in the eyelids, and its contrac­
It is important to note that distinguishing tion squeezes the eyelids shut.
between origin and insertion for some Cutaneous muscles occur in the super­
muscles is particularly difficult, as rela­ ficial fascia (a layer of connective tissue)
tive motion may change. For example, between the skin and the deep fascia
contraction of the m. brachiocephalicus, covering the skeletal muscles. These cuta­
attached to the cervical vertebrae and skull neous muscles attach to the skin and are
cranially and to the humerus caudally, may responsible for movement of the skin.
advance the thoracic limb when the foot When a fly rests on a horse, the cutaneous
is off the ground and free to swing; or muscles enable the horse to shake the skin
it may flex the neck to the side if the foot to dislodge the fly.
is bearing weight. In such cases, the dis­ The muscles involved in a specific
tinction between origin and insertion is action, such as extension of the elbow, may
primarily a semantic one, without any also be classified according to the part each
real anatomical importance. plays in the action. The agonists are the
Some muscles have distinctive divisions, muscles directly responsible for producing
called heads, which have separate origins the desired action. The antagonists are
but a common insertion. The m. triceps muscles that oppose that action; they have