Myology and Serology of the Avian Family Fringillidae: A Taxonomic Study

by William B. Stallcup

Musculus flexor perforatus digiti II (Figs.[ 3],[ 9]).—This is a slender muscle which lies on the lateral side of the crus beneath the pars externa of the m. gastrocnemius and is intimately connected anteromedially with the m. flexor digitorum longus and posteromedially with the m. flexor hallucis longus. The origin is by a strong tendon from the lateral surface of the external condyle of the femur at the point of origin of the m. flexor perforans et perforatus digiti II. This tendon serves also as the origin of the anterior head of the m. flexor hallucis longus. The tendon connects also by a broad tendinous band with the distal arm of the loop for the m. biceps femoris and by a similar band with the lateral edge of the fibula immediately distal to the head. The tendon of insertion passes distally, perforates the tibial cartilage near its lateral edge, traverses the middle medial canal of the hypotarsus (Fig.[ 6]), and passes distally to the foot. At the distal end of the tarsometatarsus the tendon is held against the medial surface of the first metatarsal by a straplike sheath. The tendon then passes over a sesamoid bone between the first metatarsal and the base of the second digit and is bound to this bone by a sheath. The tendon inserts mainly along the posteromedial edge of the proximal end of the first phalanx of the second digit, although the termination is sheathlike and covers the entire posterior surface of this phalanx. This sheathlike termination is perforated by the tendons of the m. flexor perforans et perforatus digiti II and the branch of the m. flexor digitorum longus that inserts on the second digit.

Action.—Flexes second digit.

Comparison.—In Vireo this muscle is larger and more deeply situated than it is in the other species examined and has no connection with the m. flexor hallucis longus.

Musculus flexor perforatus digiti III (Fig.[ 5]).—Long and flattened, this muscle lies on the posteromedial side of the crus beneath the m. gastrocnemius. The belly is tightly fused laterally with the belly of the m. flexor hallucis longus and posteriorly with the belly of the m. flexor perforatus digiti IV. The origin is by a long, strong tendon from a small tubercle just medial to, and at the proximal end of, the external condyle of the femur. Below the middle of the crus this muscle terminates in a strong tendon which perforates the tibial cartilage near its lateral edge. In this region the tendon is sheathlike and wrapped around the tendon of the m. flexor perforatus digiti IV. These two tendons together pass through the posterolateral canal of the hypotarsus (Fig.[ 6]). Immediately distal to the hypotarsus the two tendons separate, and the tendon of the m. flexor perforatus digiti III receives a branch of the tendon of the m. peroneus longus. The tendon passes distally over the surface of the second trochlea, and its insertion is sheathlike on the posterior surface of the first phalanx, and on the proximal end of the second. In the area of insertion this tendon is perforated by that of the m. flexor perforans et perforatus digiti III and by that of the m. flexor digitorum longus to the third digit.

Action.—Flexes digit III.

Comparison.—In Passer, Estrilda, Poephila, Hesperiphona, Carpodacus, Pinicola, Leucosticte, Spinus, and Loxia the edges of the sheathlike tendon are thickened at the points of insertion, so that the tendon appears to have two branches which insert along the posterolateral edges of the first phalanx and are connected medially by a fascia.

Musculus flexor perforatus digiti IV (Fig.[ 3]).—Extending along the posterior edge of the crus, this slender muscle lies beneath the m. gastrocnemius. The belly is fused with those of the m. flexor hallucis longus and m. flexor perforatus digiti III. Its origin is fleshy from the intercondyloid region of the distal end of the femur and has a few fibers arising from the tendon of origin of the m. flexor perforatus digiti III. Near the distal end of the crus the muscle gives rise to the strong tendon of insertion which perforates the tibial cartilage near its lateral edge and in this region is ensheathed by the tendon of the m. flexor perforatus digiti III. The two tendons pass together through the posterolateral canal of the hypotarsus (Fig.[ 6]). The tendon continues distally along the tarsometatarsus and the posterior surface of digit IV. The tendon bifurcates at approximately the middle of the first phalanx. A short lateral branch inserts on the posterolateral edge of the proximal end of the second phalanx. The long medial branch is perforated by a branch of the m. flexor digitorum longus; the distal end is flattened, has thickened edges, and inserts over the posterior surfaces of the distal end of the second phalanx, and over the proximal end of the third phalanx.

Action.—Flexes digit IV.

Comparison.—No significant differences noted among the species studied.

Musculus flexor perforans et perforatus digiti II (Figs.[ 2],[ 9]).—Small and spindle-shaped, this muscle lies on the posterolateral side of the crus immediately beneath the pars externa of the m. gastrocnemius. The origin is fleshy and arises in company with the m. flexor perforans et perforatus digiti III from a point on the posterolateral surface of the distal end of the femur between the point of origin of the pars externa of the m. gastrocnemius and the fibular condyle. The belly extends approximately one-fourth of the way down the crus and gives rise to the tendon of insertion which passes distally and superficially through the posterior edge of the tibial cartilage. The tendon traverses the posteromedial canal of the hypotarsus (Fig.[ 6]) and continues along the posterior surface of the tarsometatarsus. Between the first metatarsal and the base of the second digit the tendon is enclosed by the medial surface of a sesamoid bone. This tendon then perforates that of the m. flexor perforatus digiti II at the level of the first phalanx and in turn is perforated by the tendon of the m. flexor digitorum longus at the proximal end of the second phalanx. The insertion is on the posterior surface of the second phalanx.