The Thoracic Limb of the Horse 1: Attachment of the thoracic limb to the trunk;
anatomy and function.
Different from ourselves, in the horse the attachment of the thoracic limb (uniformly
designated the “upper” limb in humans) to the trunk is only by muscle and fascia. The
clavicle is lacking and a well-defined rudiment of the clavicle is likewise absent.
Where the brachiocephalicus muscle of the horse passes in front of the shoulder joint,
there is often a short fibrous intersection, which is undoubtedly a clavicular rudiment.
The intersection does not always extend the entire width of the muscle as in the dog
and cat and there is no cartilage or bone embedded in it. The brachiocephalicus and
a number of other muscles attach the thoracic limb to the trunk and are listed here.
With their associated fascia, these muscles, attach the axial skeleton to the scapula,
humerus, and forearm, allowing movement of the limb in relation to the trunk, support
of the trunk by the limb, and respiratory movements made possible by attachment of
forelimb muscles to the ribs and sternum.
Trapezius.
Origin: ligamentum nuchae (cervical part) and summits of spinous
processes of thoracic vertebrae at the withers (thoracic part).
Insertion: spine of the scapula; chiefly its tuber spinae.
Rhomboideus.
Origin: ligamentum nuchae (cervical part) and summits of the spinous
processes of thoracic vertebrae at the withers (thoracic part).
Insertion: medial aspect of the scapular cartilage.
Brachiocephalicus.
Origin: mastoid process of the temporal bone (cleidomastoideus); nuchal
crest of the occipital bone (cleidooccipitalis).
Insertion: clavicular rudiment, humeral crest and deltoid tuberosity.
Note: From its attachment at the mastoid process and nuchal crest
to the point of the shoulder and the clavicular rudiment, the muscle is homologous to
the human cleidomastoideus muscle. From the point of the shoulder to the
attachment at the deltoid tuberosity and humeral crest, the muscle is homologous to
the clavicular deltoid of human anatomy.
Omotransversarius.
Origin: lateral margin of the wing of the atlas and the transverse
processes of cervical vertebrae C2 – C4.
Insertion: lateral fascia of the shoulder and brachium.
Note: This muscle lies beside the brachiocephalicus and used to be
described as a part of that muscle.
Latissimus dorsi.
Origin: spinous processes of thoracic vertebrae at the height of the withers
(T3, T4) and extending caudally to the cranial lumbar spines.
Insertion: teres major tuberosity of the humerus.
Omohyoideus:
Origin: basihyoid bone by means of a tendinous plate that passes from the
bone ventrally and attaches the omohyoideus and sternohyoideus muscles on its
caudal aspect, the geniohyoideus muscle on its rostral face.
Insertion: scapula by means of the medial scapular fascia.
Note: Functionally, the omohyoid-geniohyoid combination appears
to be a means of assisting in drawing the shoulder forward as the head and neck are
extended when the horse is running. The omohyoid will also draw the hyoid apparatus
caudally in swallowing; but the size of the muscle and its superficial fusion with the
cleidomastoideus suggest that its function is much involved with its action on the limb.
Fig. 1. Muscular horse, lateral view. After Handbuch der Anatomie fur Künstler, W.
Ellenberger, H. Baum, and H. Dittrich; 1901. Except for the caudal part of the
cutaneus colli, all cutaneous muscles are removed in this figure.
rhomboideus
trapezius
latissimus dorsi
subclavius
brachiocephalicus
omotransversarius
supf. pectoral,
descending
part
serratus ventralis,
thoracic part
deep pectoral
Redrawn from the
original by David
Stewart Geary
Serratus ventralis.
Origin: transverse processes of the fourth to seventh cervical vertebrae
(cervical part) and the lateral aspect of ribs 1 – 8 (thoracic part).
Insertion: facies serrata of the scapula.
Superficial pectoral.
Descending part:
Origin: manubrium of the sternum.
Insertion: humeral crest and deltoid tuberosity. Its tendon joins the tendon
of insertion of the brachiocephalicus.
Transverse part:
Origin: ventral margin of the sternum from the manubrium to the level of
th
the 6 costal cartilage and sternebra.
Insertion: medial antebrachial fascia.
Fig. 2. Lateral view of the shoulder. Only the cutaneous muscles have been
removed. The dashed line shows the approximate division of
brachiocephalicus and omotransversarius muscles.
trapezius
cervical part
thoracic part
latissimus dorsi
sternocephalicus
brachiocephalicus
omotransversarius
supf pectoral,
descending part
serratus ventralis,
thoracic part
deltoideus
Fig. 3. Deeper dissection of the shoulder, lateral view.
Trapezius, brachiocephalicus, omotransversarius, latissimus
dorsi and deltoideus muscles removed.
rhomboideus
serratus ventralis
cervical part
external
jugular
vein
omohyoideus
supf pectoral
descending part
serratus ventralis
thoracic part
deep pectoral
Drawn by David
Stewart Geary
Subclavius:
Origin: lateral side of the sternum caudal to the manubrium, and the
cartilages of the first four ribs.
Insertion: cranial margin of the scapula and the upper part of the
tendinous sheet covering the supraspinatus muscle.
Deep pectoral:
Origin: lateral side of the caudal part of the sternum and costal cartilages
of ribs 4 – 9; abdominal tunic.
Insertion: medial side of the proximal humerus below the proximal margin
of the cranial part of the medial tubercle.
Fig. 4. Craniolateral view of the shoulder. The sternal origin
of the cutaneus colli and all of the cutaneus trunci are
removed. Drawn by David S. Geary.
trapezius
external
jugular vein
cutaneus colli
(cut away in part to
expose the
external jugular
vein, which passes
deep to it; its thick
sternal part has
also been removed
to expose the
origin of the
sternocephalicus
from the
manubrium)
sternocephalicus
subclavius
supf pectoral
descending part
transverse part
deltoideus
triceps brachii
long head
lateral head
omotransversarius
brachiocephalicus
cephalic vein
Note: The omotransversarius was at first regarded as a part of the
brachiocephalicus. It is distinguished from that muscle by its origin from the atlas and
cervical vertebrae. The brachiocephalicus arises from the skull. See Fig, 1.
Note: The external jugular vein lies deep to the thin cutaneus colli muscle within
the jugular furrow . The furrow is the sulcus between the sternocephalicus muscle
ventrally and the brachiocephalicus muscle dorsally. The furrow ends at the jugular
fossa where the vein passes beneath the thicker, sternal, attachment of the cutaneus
colli. When excited, a horse will often contract the cutaneus colli, which erases
the furrow. Venepuncture is not made more difficult as the large vein’s flow beneath
the thinner part of the muscle is easily detected by palpation.
Fig. 6. Deep dissection showing in particular the relations of the subclavius and deep
pectoral at the shoulder. All of the cutaneus colli is removed.
Drawn by David Stewart Geary.
omohyoideus
serratus vent.
rhomboideus
sternocephalicus
supf. cerv. lln
ext. jugular v. (cut)
cephalic v. (cut)
subclavius
deep pectoral
deep pectoral
biceps brachii
(sheathed by deep
These muscles which attach the
limb to the trunk are designated extrinsic muscles.
fascia)
All of the muscles named function in moving the limb with respect to the trunk. Cranial
and caudal movement of the scapula determines the length of the stride. To advance
the limb maximally, the scapula must be as far forward as possible as extension of
the limb joints then advanes the limb maximally. Simultaneus ventral movement of the
scapula, which the animal obtains by contracting the cervical serratus ventralis,
extends the limb even farther. In the same manner, muscles that draw the scapula
caudally and ventrally provide maximum length of the stride.
Muscles can only shorten the distance between their attachments so that all of the
muscles listed also act to move the trunk if the limb attachment is taken as the fixed
point. In walking, running, jumping, the limb is advanced and the thoracic parts of the
trapezius and rhomboideus, the latissimus dorsi, thoracic serratus ventralis,
subclavius, and pectoral muscles then serve to draw the trunk forward. In the normal
standing attitude, the brachiocephalicus and omotransversarius move the neck but
draw the limb forward in the racing animal. The omohyoideus draws the hyoid
apparatus caudally at the completion of swallowing; but, in the running animal,
cooperates with the brachiocephalicus and omotransversarius in drawing the limb
forward.
The thoracic limbs bear most of the horse’s weight. Of the extrinsic muscles, the
serratus ventralis muscle, and the fascia associated with it, is most responsible for
this weight-bearing function. Sisson has a diagram illustrating the thoracic fascial
attachment:
Fig. 7.
rhomboideus
omohyoideus
Drawn by David
Stewart Geary
sternocephalicus
serratus ventralis
cervical part
subclavius
serratus ventralis
thoracic part
supf pectoral
deep pectoral
When an animal is in severe respiratory distress, it will hold its head and neck stiffly
extended and stand with its thoracic limbs far forward. Human beings do the same
thing, grasping the headpost of the bed for maximum inspiration. This is to take
advantage of the costal attachment of the thoracic serratus ventralis in a maximal
inspiratory effort and to draw the sternum forward (upward in humans) by contraction
of the sternocephalicus. The sternal ribs increase in length from the first to the last so
that, when the sternum is drawn forward, each rib is replaced by a longer,
succeeding, one and the dorsoventral diameter of the thorax is increased. This is
undoubtedly very slight in the horse. An animal which demonstrates the distress
posture very well is the cat with pulmonary edema.
Fig. 7 emphasizes the serratus ventralis and pectoral muscles. The subclavius is
drawn tightly against the supraspinatus in this illustration and, different from usual, in
this lateral view shows only as a thin sliver of striated muscle. As Figure 6 shows, it is
a substantial, fleshy, muscle and it is only its disposition here that makes it appear so
slim.