Page 145 - Anatomy and Physiology of Farm Animals, 8th Edition

130 / Anatomy and Physiology of Farm Animals

Extrinsic Muscles of the Thoracic Limb the scapula; it inserts on the deep (medial)
face of the scapular cartilage.
VetBooks.ir Extrinsic muscles are those with one attach­ and most important muscle attaching the
The m. serratus ventralis is the largest
ment on the neck or trunk and another
on the appendicular skeleton. For the most thoracic limb to the trunk. It is a large,
part, this attachment to the thoracic limb fan‐shaped muscle. The origin of the
occurs on the scapula or humerus, which m. serratus ventralis is the widest part of
means that these extrinsic muscle either the muscle and extends from the trans­
act on the limb as a whole (e.g., drawing it verse processes of the cervical and thoracic
forward or backward relative to the trunk, vertebrae and from the ribs along a curved
Fig. 7‐6) or by producing movement at the line just dorsal to the sternum as far caudal
shoulder joint. Unlike primates in whom as the tenth costal cartilage. The insertion
the shoulder joint is capable of a wide range is on the medial side of the dorsal portion
of motion, in domestic animals the chief of the scapula. The m. serratus ventralis on
movement of the proximal part of the tho­ each side of the body together form a signifi­
racic limb is a pendulous swing forward cant portion of the sling that supports the
and backward. trunk between the thoracic limbs (Fig. 7‐7).
Superficially, the m. trapezius is a trian­ The cervical portion, on contraction, tends
gular flat muscle that takes origin along the to rotate the distal part of the scapula
dorsal midline from the head to the lumbar backward, while the thoracic portion
vertebrae. The muscle fibers of the m. trape- rotates it forward. The muscle’s cervical
zius converge to insert on the spine of the attachments allow it to lift the neck as well.
scapula. The portion originating cranial to The m. omotransversarius takes origin
the scapula helps swing the scapula forward; from the transverse processes of the more
the one attaching behind draws it back. cranial cervical vertebrae and inserts on
The m. rhomboideus is a heavier the distal part of the spine of the scapula
muscle just deep to the trapezius. The (on the fascia associated with the shoulder
m. rhomboideus also originates from the in the horse). With these attachments, the
dorsal midline both cranial and caudal to m. omotransversarius usually pulls the

(A) (B)

b b
a c c
a
d d

Figure 7-6. Examples of muscles that draw the entire thoracic limb forward (A) or backward (B). a,
m. brachiocephalicus; b, cervical part of m. trapezius; c, thoracic part of m. trapezius; d, m. latissimus
dorsi. a and b advance the entire limb while c and d draw it caudad.