Section III—Musculature
As the flight and wing musculature are among the more interesting features of the pigeon, they are the focus of this dissection. To study the musculature, the skin must be removed.
Begin by brushing aside the feathers from the midventral surface of the thoracic region. Note that there are no feathers attached at the midline. Make a longitudinal incision through the skin, but work carefully, as it is quite thin. Extend the incision from the cloaca anteriorly to the anterior part of the breast, and then spread the skin, separating it from the underlying tissue using a blunt probe. In most places the skin will come off readily, but in some spots muscular slips attach to the skin, and must be cut through. You will reveal the pectoralis (Figure 8.10), the largest and most superficial of the flight muscles. At the anterior part of the pectoralis lies the thin-walled crop, a sac-like specialization of the esophagus (see below) that is used to store food. Carefully continue the incision anteriorly, being careful not to injure the crop, to just below the bill. Skin the neck, but do not damage vessels.Several folds of skin form the wing surface. The large fold between the shoulder and carpus, forming the leading edge of the wing, is the propatagium. Several delicate muscles and tendons lie within it. A large postpatagium projects posteriorly from the ulna. With the wing outstretched, gently push back on the leading edge of the propatagium. The tension you feel is due to the long tendon (see below). Begin skinning the wing ventrally, near the central part of the brachium, and work your way anteriorly. As you approach the leading edge, be watchful of the tendon, a thin, whitish strand, and uncover it. Proceed to skin the rest of the wing up to the carpus. Then continue skinning the dorsal surface of the wing and body.
The dominance of the pectoralis can now be appreciated.
It arises from the sternum and, being the main depressor of the wing, inserts mainly on the humerus. The shoulder is covered mainly by several parts of the deltoideus muscle (Figures 8.10 and 8.11). Anteriorly, the deltoid is represented by the pars propatagialis, a single hypertrophied belly consisting of long and
FIGURE 8.9 Extended left wing of the pigeon, in dorsal view, showing arrangement of feathers.
FIGURE 8.10 Pectoral and wing musculature of the pigeon in ventral view. Left-side pectoralis muscle cut away to show underlying structures.
short heads. The long head is the smaller, anterior, sheetlike portion. The more posterior short head is thicker. The long head is just anterior to a small, triangular slip of the pectoralis that inserts on the common belly of the pars propatagialis. The tendon of the long head, the long tendon noted above, extends laterally toward the carpus and divides into two separate tendons, but these may be difficult to discern. Distally, the short head forms a tendinous sheet that mainly spreads to cover the muscles on the antebrachium. The biceps brachii muscle covers the anteromedial part of the brachium. The pars propatagialis is a slip of the biceps and arises from its anterior edge. Anteriorly, the pars propatagialis fuses with the tendon of the long tendon. Its distal end gives rise to a tendon that angles distally to fuse with one of the divisions of the long tendon. The wing muscles described in this paragraph, except the biceps brachii, serve mainly to alter tension of the wing surface. They are commonly referred to as tensors and play a significant role in flight aerodynamics.
Several other wing muscles may be noted. Just posterior to the short head of the pars propatagialis of the deltoideus is the deltoideus major, which pulls the humerus medially and posteriorly. The triceps brachii has three portions, but only two of them, the scapulotriceps and humerotriceps, can be readily identified.
The third, the coracotriceps, is a small muscle arising from the tendon of the expansor secundariorum. This tendon extends
FIGURE 8.11 Musculature of the pigeon in dorsal view.
from the axilla, or armpit, along the posterior edge of the triceps brachii. Carefully dissect the margin of the postpatagium to find the small and delicate muscular portion, which fans onto the quills of the secondary remiges. The expansor secundariorum acts to spread the secondaries.
The main elevator of the wing is the Supracoracoideus, which lies deep to and is covered entirely by the pectoralis. To expose it, make a cut through the pectoralis near its center and at right angles to the fiber direction, but do so only a few millimeters at a time. Spread the incision as you cut. This will help avoid damaging the
FIGURE 8.12 Pectoral musculature of the pigeon in left ventrolateral view. (a) Left-side pectoralis muscle cut away. (b) Sternum and appendicular skeleton isolated to show supracoracoideus muscle and the course of its tendon through the triosseal canal.
underlying Supracoracoideus. Once you have cut through the pectoralis and identified the Supracora- coideus, cut out a large portion of the pectoralis to expose the supracoracoideus (Figures 8.10 and 8.12). Its fibers converge toward the middle of the muscle and extend to a stout anterolaterally directed tendon that passes through the triosseal canal onto the dorsal surface of the humerus. This acts as a pulley system, through which the wing is elevated mainly by a muscle, the supracoracoideus, that lies ventral to the wing. This arrangement enhances stability for flight by maintaining the center of gravity below the wings. To see the tendon of the supracoracoideus (Figures 8.10-8.12), cut the deltoideus major and the pars propatagialis of the deltoideus near their center and reflect them.
You will uncover the deltoideus minor, which contributes to elevation of the wing. The tendon passes just posterior to the posterior margin of the deltoideus minor.Other musculature that may be noted without much further effort includes the rhomboideus superficialis and rhomboideus profundus, which extend between vertebral spines and the scapula. The superficialis covers all but the anteriormost portion of the profundus. The latissimus dorsi is a relatively small muscle extending laterally from the middorsal line. These muscles form part of the complex of muscles that help support and stabilize the shoulder. Although there are various other back muscles, an interesting condition in birds is the relative lack or smaller size of the back musculature as compared, for example, with that of the cat. Strength and stability of the trunk is not compromised, however, as the skeletal elements are firmly fused together (see above), and this in turn allows the musculature to be reduced to minimize weight.
Key Terms: Musculature
biceps brachii
crop
deltoideus
deltoideus major deltoideus minor esophagus expansor secundariorum humerotriceps latissimus dorsi long tendon of long head, pars propatagialis of deltoideus
pars propatagialis of biceps brachii (biceps
slip, biceps
propatagialis, patagial accessory)
pars propatagialis, long and short heads of deltoideus
pectoralis postpatagium propatagium rhomboideus
profundus rhomboideus
superficialis scapulotriceps supracoracoideus