The _pars posticus_ originates by a broad, flat tendon on a line across the posterior half of the membrane that covers the ischiopubic fenestra. The insertion is at the point of origin of the _pars media_ of the _m. gastrocnemius_ on the posteromedial surface of the proximal end of the internal condyle of the femur. There is a broad tendinous connection with the proximal end of the _pars media_ of the _m.

gastrocnemius_. The anterior edge of the _pars posticus_ is overlapped medially by the posterior edge of the _pars anticus_.

Action.--Flexes thigh; may flex crus also and may extend tarsometatarsus.

Comparison.--In _Vireo olivaceous_, the origin of this muscle does not extend the length of the ischiopubic fenestra. The origin, furthermore, is along the dorsal edge of the ischiopubic fenestra and not from the membrane covering the fenestra. Finally, in this species, the origin of the _pars posticus_ is fleshy.

_+Musculus tibialis anticus+_ (Figs. 2, 5).--Lying along the anterior edge of the crus, a part of this muscle is covered by the _m. peroneus longus_. The origin is by two distinct heads, each of which is pinnate. The anterior head arises directly from the edges of the outer and inner cnemial crests. The posterior head arises by a short, strong tendon from a small pit on the anterodistal edge of the external condyle of the femur. This tendon and the proximal end of the muscle pa.s.s between the head of the fibula and the outer cnemial crest. The two heads of the muscle fuse at a place slightly more than one-half of the distance down the crus. At the distal end of the crus this muscle gives rise to a strong tendon which pa.s.ses under a fibrous loop immediately proximal to the external condyle in company with the _m.

extensor digitorum longus_ and which pa.s.ses between the condyles of the tibia and inserts on a tubercle on the anteromedial edge of the proximal end of the tarsometatarsus.

Action.--Flexes tarsometatarsus.

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

_+Musculus extensor digitorum longus+_ (Figs. 3, 5, 8).--Slender and pinnate, this muscle lies along the anteromedial surface of the tibia.

The origin is fleshy from most of the region between the cnemial crests and from a line along the anterior surface of the proximal fourth of the tibia. Approximately two-thirds of the distance down the crus the muscle gives rise to the tendon of insertion which pa.s.ses through the fibrous loop near the distal end of the tibia in company with the _m. tibialis anticus_. The tendon then pa.s.ses along beneath the supratendinal bridge at the distal end of the tibia, traverses the anterior intercondylar fossa, and pa.s.ses beneath a bony bridge on the anteromedial surface of the proximal end of the tarsometatarsus. The tendon continues along the anterior surface of the tarsometatarsus to a point immediately above the bases of the toes and there gives rise to three branches, one to the anterior surface of each foretoe. The insertions of each branch are on the anterior surfaces of the phalanges as shown in Fig. 8.

Action.--Extends foretoes.

Comparison.--This muscle is weakly developed in _Leucosticte_ and _Calvarius_; the belly is slender and extends only half way down the crus before giving rise to the tendon of insertion. The functional significance of this variation is difficult to understand. The convergence in muscle pattern shown by these two genera, however, is in all probability the result of similarities in behavior patterns.

These birds perch less frequently than do the other birds studied.

Thus, the toes are neither flexed nor extended as often; the smaller size of the _m. extensor digitorum longus_ may have resulted in part from this lessened activity. Except for the variations just noted, there are no significant differences among the species studied; even the rather complex patterns of insertion are identical.

_+Musculus peroneus longus+_ (Fig. 1).--Relatively thin and straplike, this muscle lies on the anterolateral surface of the crus and is intimately attached to the underlying muscles. The part of the origin from the proximal edges of the inner and outer cnemial crests is semitendinous but the part of the origin from the lateral edge of the shaft of the fibula is tendinous. Approximately two-thirds the distance down the crus the muscle gives rise to the tendon of insertion. Immediately above the external condyle of the tibiotarsus this tendon divides. The posterior branch inserts on the proximal end of the lateral edge of the tibial cartilage. The anterior branch pa.s.ses over the lateral surface of the external condyle to the posterior surface of the tarsometatarsus and there unites with the tendon of the _m. flexor perforatus digiti III_.

Action.--Extends tarsometatarsus and flexes third digit.

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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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_ bipart.i.te

_Vireo_ _Chlorura_ _Seiurus_ _Pipilo_ _Icterus_ _Calamospiza_ _Molothrus_ _Chondestes_ _Piranga_ _Junco_ _Richmondena_ _Spizella_ _Guiraca_ _Zonotrichia_ _Pa.s.serina_ _Pa.s.serella_ _Spiza_ _Calcarius_

The two parts of the _m. gastrocnemius_ are most distinct in _Vireo_.

_Icterus_, _Molothrus_, _Richmondena_, _Guiraca_, and _Pa.s.serina_ lack the fibrous band that pa.s.ses around the front of the knee. In _Spiza_ this band of fibers is smaller than in the other species.

_Pars interna_ undivided

_Pa.s.ser_ _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 pa.s.ses distally, perforates the tibial cartilage near its lateral edge, traverses the middle medial ca.n.a.l of the hypotarsus (Fig. 6), and pa.s.ses 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 pa.s.ses 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 pa.s.s through the posterolateral ca.n.a.l 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 pa.s.ses 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 _Pa.s.ser_, _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 pa.s.s together through the posterolateral ca.n.a.l 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.