Text and photos: Elizabeth Farrell and Nicholas Zolkowski
The shoulder bones and musculature of the red fox are similar in pattern to that of most quadrupedal mammals, reflecting the generalized locomotion used throughout Family Canidae. Canids do, however, have a more flattened and rectangular scapula, as well as a reduced or vestigial clavicle, as compared to other mammals. This reduced clavicle allows an increased stride length, as does the modified scapula. Both of these adaptations further contribute to the creation of a propulsive, forward-moving stride, with little lateral sway [1]. The elongation of the limbs and the strong attachments of the ligaments also contribute to highly restricted range of motion. Non-cursorial mammals, such as the tree shrew and the squirrel, exhibit a wider range of motion in the shoulder joint.
The musculature of the pectoral limb is divided into two regions, extrinsic and intrinsic. Extrinsic muscles are those muscles that attach the limb to the axial skeleton while the intrinsic muscles are those muscles that connect different bones on the limb itself. The extrinsic muscles of the scapular region generally provide motion of the trunk forward and backward relative to the limb, while the intrinsic muscles of the scapular region are generally used in flexion/extension and adduction/abduction of the shoulder joint. Extrinsic muscles include the deep and superficial pectoral muscles, the rhomboideus muscle and the trapezius muscle. Intrinsic muscles can be further divided into: lateral intrinsic muscles, such as the deltoideus, infraspinatus, supraspinatus, and teres minor muscles; and medial intrinsic muscles, such as the subscapularis and teres major muscles. [1] For references during this lab, we compared the musculature of the fox to that of a dog.
While the prior removal of the cutaneous layer made this dissection somewhat easier, the most difficult part was distinguishing between fascia and muscle. We carefully removed the layer of deep fascia, which had a large amount of adipose tissue associated with it. Using the probe, the fascia was scraped until the muscles became visible. At this point, we could trace the various muscles by observing their borders and fiber directions. The most obvious superficial muscles are the latissimus dorsi and the trapezius. The clavicle is present, but is reduced to a small, rectangular bone. The first muscle we observed was the trapezius on the dorsal side of the scapula. The trapezius runs from the cervical spine to the thoracic and lumbar spine, and functions to elevate the arm. It also has lateral attachments on the spine of the scapula. We cut and reflected the trapezius from the scapula to observe the deep muscles.
The first muscle superficial to the scapula we located was the deltoideus muscle,
which follows the cranial and distal borders of the scapula, and, along with
the teres major and teres minor muscles, flex the shoulder joint. Dorsal to
the deltoideus and superior to the scapular spine is the supraspinatus muscle,
which extends the shoulder joint. We cut and reflected the deltoideus cranially
to reveal the teres minor muscle and the infraspinatus muscle deep to it. Inferior
to the scapular spine is the infraspinatus muscle, which abducts the shoulder.
Distal to the infraspinatus muscle is the much smaller teres minor muscle. The
teres minor originates from the infraspinatus, and does not differentiate significantly
from it during development.
On the medial view, the rhomboid muscles, which act to elevate the arm, are attached to the vertebral border of the scapula. Also in this area is the teres major muscle, which runs along the caudal border of the scapula. The limb is restricted in its movement because of these muscle attachments, so we cut the latissimus dorsi muscle along its ventral border and insertion point and reflected it. We abducted the limb to observe the deep aspect of the scapula. Here we observed the subscapularis muscle, which abducts the shoulder joint, and coracobrachialis muscle, which extends the shoulder.
| Muscle | Origin | Insertion | Function |
| Latissimus dorsi | spinous processes of lumbar and last 7 thoracic vertebra, last 2 or 3 ribs | teres tuberosity of the scapula, medial fascia of arm | draw trunk cranially when limb is fixed, draw limb caudally when limb is free |
| Trapezius | supraspinous ligament from 3rd/4th cervical vertebra and 9th/10th thoracic vertebra | spine of scapula at distal end | elevate the forelimb |
| Rhomboids | median fibrous raphe of neck, nuchal crest of occipital bone | vertebral border of scapula | elevate the forelimb |
| Infraspinatus | infraspinous fossa | area distal to greater tubercle of humerus | abduct shoulder |
| Subscapularis | subscapular fossa | lesser tubercle of humerus | adduct shoulder joint |
| Deltoideus | spine and acromial process of scapula | deltoid tuberosity | flex shoulder joint |
| Teres minor | caudal border of scapula | anconeal crest proximal to deltoid tuberosity of humerus | flex shoulder joint |
| Teres major | caudal border and angle of scapula and caudal surface of subscapularis | teres tuberosity of humerus | flex shoulder joint |
| Coracobrachialis | coracoid process of scapula | crest of lesser tubercle of humerus proximal to teres tuberosity | extend shoulder joint |
| Supraspinatus | supraspinous fossa | greater tubercle of humerus | extend shoulder joint |
Adapted from Miller (1952) [2]
1. Alderton, David. 1994. Foxes, Wolves, and Wild Dogs of the World. Facts on File, New York.
2. Miller, Malcolm E. 1952. Guide to the Dissection of the Dog. (3rd Ed.).
Edward Brothers Inc., New York.