Part 5

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

+Musculus peroneus brevis+ (Figs. 2, 3).--Lying along the anterolateral surface of the tibia, this slender, pinnate muscle arises from a fleshy origin along this surface and along the anterior surface of the fibula from a point immediately proximal to the insertion of the m. biceps femoris to a point approximately two-thirds of the way down the crus. Near the distal end of the tibia the muscle gives rise to the tendon of insertion that passes through a groove on the anterolateral edge of the tibia just above the external condyle. Here the tendon is held in place by a broad fibrous loop and passes under the anterior branch of the tendon of insertion of the m. peroneus longus and inserts on a prominence on the lateral edge of the proximal end of the tarsometatarsus.

Action.--Extends tarsometatarsus and may abduct it slightly.

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

+Musculus gastrocnemius+ (Figs. 1, 4).--The largest muscle of the pelvic appendage, it covers superficially all of the posterior surface, most of the medial surface, and half of the lateral surface of the crus. The muscle originates by three distinct heads.

The pars externa covers the posterolateral surface of the crus, is intermediate in size between the other two heads, and arises by a short, strong tendon from a small bony protuberance on the posterolateral side of the distal end of the femur immediately proximal to the fibular condyle. The tendon is intimately connected with the distal arm of the loop for the m. biceps femoris.

The pars media is the smallest of the three heads and lies on the medial surface of the crus. The head of the pars media is separated from the pars interna by the tendon of insertion of the m. semimembranosus and originates by a short, strong tendon from the posteromedial surface of the proximal end of the internal condyle of the femur. The proximal portion of the pars media has tendinous connections with the tendon of the m. semitendinosus and with the pars posticus of the m. adductor longus et brevis.

The pars interna is the largest of the three heads and covers most of the medial surface of the crus. This head in its proximal portion is distinctly divided into anterior and posterior parts, the former overlapping the latter medially. The origin of the posterior part is fleshy from the anterior half of the tibial head. Some of the fibers of the anterior part arise directly from the inner cnemial crest while its remaining fibers arise from the patellar tendon (Fig. 1) and form a band that extends around the anterior surface of the knee, covering the insertion of the m. sartorius.

Approximately half way down the crus, the three heads give rise to the tendon of insertion, the tendo achillis, which passes over and is tightly bound to the posterior surface of the tibial cartilage. The insertion is tendinous on the posterior surface of the hypotarsus and along the posterolateral ridge of the tarsometatarsus. This tendon seems to be continuous with a fascia which forms a sheath around the posterior surface of the tarsometatarsus holding the other tendons of this region firmly in the posterior sulcus.

Action.--Extends tarsometatarsus.

Comparison.--Study of the pars externa and pars media reveals no significant differences among the species dissected. The pars interna, however, is subject to some variation which is described below.

Pars interna bipartite

Vireo Chlorura Seiurus Pipilo Icterus Calamospiza Molothrus Chondestes Piranga Junco Richmondena Spizella Guiraca Zonotrichia Passerina Passerella Spiza Calcarius

The two parts of the m. gastrocnemius are most distinct in Vireo. Icterus, Molothrus, Richmondena, Guiraca, and Passerina lack the fibrous band that passes around the front of the knee. In Spiza this band of fibers is smaller than in the other species.

Pars interna undivided

Passer Pinicola Estrilda Leucosticte Poephila Spinus Hesperiphona Loxia Carpodacus

In Leucosticte, although the pars interna is undivided, there is a band of fibers which extends around the front of the knee (see discussion, p. 183).

+Musculus plantaris+ (Fig. 5).--Small and slender, this muscle lies on the posteromedial surface of the crus, beneath the pars interna of the m. gastrocnemius and originates by fleshy fibers from the posteromedial surface of the proximal end of the tibia immediately distal to the internal articular surface. The belly extends approximately one-sixth of the way down the crus and gives rise to a long, slender tendon that inserts on the proximomedial edge of the tibial cartilage.

Action.--Extends tarsometatarsus.

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

+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.