Page 146 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Muscular System / 131

(upper arm) and is therefore also an extrinsic
muscle of the forelimb. It originates on the
VetBooks.ir a occipital bone of the skull and transverse

processes of the cervical vertebrae, and it
inserts on the lateral side of the proximal
part of the humerus proximal to the deltoid
tuberosity. The m. brachiocephalicus covers
the cranial aspect of the point of the
shoulder. It raises and advances the shoulder.
The m. brachiocephalicus is also the
principal extensor of the shoulder and acts
as a lateral flexor of the neck when the limb
is weight bearing.
In primates and birds, the m. brachioce-
phalicus is fully divided into cranial and
caudal parts by the presence of a clavicle
(collar bone), but this bone is a connective
tissue vestige (identified as the “clavicular
b
tendon”) or absent in ungulates. In these
species, the vestigial clavicle subdivides the
m. brachiocephalicus into the m. cleido-
Figure 7-7. Muscular suspension of the thorax brachialis, extending from the clavicular
between the thoracic limbs. The blue arrow repre­ tendon to the humerus, and the m. cleido-
sents the weight of the trunk, neck, and head. a, cephalicus, extending from the clavicular
m. serratus ventralis; b, pectoral mm. Other muscles tendon to the head and neck. In species
not depicted contribute to this muscular support other than the horse, the m. cleidocephalicus
as well.
may be further subdivided into a mastoid
part attaching to the mastoid process of the
distal end of the scapula forward, although temporal bone and either an occipital part
with the limb in weight‐bearing position, (in ruminants and pigs) or a cervical part
it instead assists lateral flexion of the neck. (in carnivores). In horses, the sole attach­
The m. latissimus dorsi is a wide, tri­ ment of the m. cleidocephalicus is to the mas­
angular muscle that originates from the toid process, and in this species the muscle
spinous processes of the thoracic and is often called the m. cleidomastoideus.
lumbar vertebrae by means of a wide, thick The m. supraspinatus originates from
aponeurosis, the thoracolumbar fascia. the supraspinous fossa of the scapula cranial
It inserts with the m. teres major on the to the spine. It inserts on the greater tuber­
medial side of the humerus where its con­ cle of the humerus. The m. supraspinatus
traction can pull the thoracic limb caudad may assist in extending the shoulder but
or, if the limb is fixed, advance the trunk. acts chiefly as a stabilizing muscle of the
shoulder joint. This is one of the muscles
Muscles Acting on the Shoulder Joint that atrophies (shrinks) in sweeny in horses,
a condition that results from damage to its
motor innervation, the suprascapular nerve.
The shoulder, being a ball‐and‐socket joint,
has the potential to create a wide range of
motions. In the quadruped, however, its Flexors of the Shoulder. The m. teres
major originates from the dorsal part of
chief actions are extension and flexion.
the caudal border of the scapula and inserts
Extensors of the Shoulder. The m. on the teres major tuberosity on the medial
brac hi ocephalicus, as the name implies, side of the shaft of the humerus. It is a
extends from the head to the brachium strong flexor of the shoulder joint.