Page 65 - Adams and Stashak's Lameness in Horses, 7th Edition

Functional Anatomy of the Equine Musculoskeletal System 31

from the superficial surface of the dorsal part of the the scapula craniad; the thoracic part draws the scapula
dorsoscapular ligament. Both parts of the rhomboideus caudad. When the limb is fixed, the cervical portion lifts
VetBooks.ir (Figure 1.27). This muscle is innervated by the sixth and racic nerve and branches from the fifth to the eighth
the neck dorsad and/or pulls it laterad. The long tho­
muscle insert on the medial side of the scapular cartilage
cervical nerves supply this muscle.
seventh cervical nerves and dorsal branches of nerves
adjacent to the rhomboideus thoracis. The rhomboideus Pectoral muscles attach to the sternum. There are two
draws the scapula dorsocraniad, and, when the limb is superficial pectoral muscles: (1) the descending pectoral
stationary, the cervical part helps to raise the neck. muscle passing from the cartilage of the manubrium of
The latissimus dorsi muscle has a triangular shape the sternum to the deltoid tuberosity and the crest of the
with the origin arising through a broad aponeurosis humerus and brachial fascia and (2) the transverse pec­
from the thoracolumbar fascia. Thin near its aponeu­ toral muscle extending from the ventral part of the ster­
rotic origin, the muscle becomes thicker as it approaches num to the superficial fascia on the medial aspect of the
its insertion, passing medial to the long head of the tri­ antebrachium and to the humeral crest. The largest pec­
ceps brachii to converge on a flat, common tendon of toral muscle, the deep pectoral (ascending pectoral) mus­
insertion with the teres major muscle on the teres major cle (Figure 1.29), passes from its attachments on the
tuberosity of the humerus (Figure 1.27). xiphoid cartilage, the ventral part of the sternum, the
From deep to superficial, the muscles contributing fourth to ninth costal cartilages, and the abdominal tunic
most substantially to the attachment of the thoracic to the cranial parts of the lesser and greater humeral
limb to the trunk and neck are the serratus ventralis, tubercles. The subclavius has been traditionally grouped
pectoral muscles (including subclavius), brachiocephali­ with the pectorals. It arises from the first four costal car­
cus, and omotransversarius. The cervical part of serratus tilages and the cranial half of the sternum and ends in an
ventralis extends from the transverse processes of aponeurosis over the dorsal part of the supraspinatus
the cervical vertebrae C4–C7 to the serrated face of the muscle and the scapular fascia (Figure 1.29).
medial scapula and scapular cartilage; the thoracic part The superficial pectoral muscles adduct the limb. The
of serratus ventralis converges from the lateral surfaces deep pectoral and subclavius are also adductors, and, if
of the first eight or nine ribs to the serrated face of the limb is fixed in the advanced position, they pull the
the scapula and the scapular cartilage. Elastic sheets trunk craniad. Cranial and caudal pectoral nerves (with
from the ventral part of the dorsoscapular ligament musculocutaneous and intercostal nerves contributing
(Figure 1.31) are interspersed through the attachments to the cranial pectoral nerves) supply these muscles.
of the serratus ventralis on the scapula. The right and The brachiocephalicus is traditionally divided into
left serratus ventralis muscles form a muscular sling sus­ two parts based on the embryological location (not
pending the thorax between the thoracic limbs. When apparent in the fully formed muscle) of the clavicular
both muscles contract, they elevate the thorax; acting intersection. The cleidobrachialis part of the brachioce­
independently, each serratus ventralis shifts the trunk’s phalicus muscle extends from the clavicular intersection
weight to the ipsilateral limb. During locomotion the to the arm and is innervated by the axillary nerve. The
cervical part of the muscle draws the dorsal border of mastoid part of the muscle (a.k.a. cleidomastoideus) lies
between the intersection and its attachments to the mas­
toid process and nuchal crest of the skull, partly over­
Nuchal ligament lapping the omotransversarius muscle dorsally. The
omotransversarius originates from the wing of the atlas
Supraspinous bursa
and the transverse processes of the second, third, and
Trapezius m. fourth cervical vertebrae and inserts on the humeral
crest and fascia of the shoulder and arm. The cleidobra­
Rhomboideus m. chialis is innervated by the accessory nerve (cranial
nerve XI), and the omotransversarius is innervated seg­
Dorsoscapular ligament mentally by cervical spinal nerves.

dorSoScapular lIgaMent
Scapula
Further attachment of the limb to the trunk is
afforded by a thickened specialization of the thora­
columbar fascia, the dorsoscapular ligament
(Figure 1.31). Its collagenous portion attaches to the
Serratus ventralis m. third, fourth, and fifth thoracic spines under the nuchal
ligament close to the supraspinous bursa. This part of
15
the dorsoscapular ligament passes ventrad, attaching
to the medial surface of the rhomboideus thoracis
muscle. As it curves ventral to the muscle, the collagen­
ous part transitions to elastic tissue. A horizontal lam­
ina of this elastic part forms the ventral sheath of the
rhomboideus thoracis muscle. Vertical laminae project
Figure 1.31. Cross section of right dorsoscapular ligament. The from the ventral aspect of the horizontal lamina,
ligament covers the medial and ventral aspects of the rhomboideus investing bundles of the serratus ventralis muscle that
and gives rise to sheets of elastic tissue that interdigitate with the insert on the scapula (Figure 1.31).
fascicles of the serratus ventralis.