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 pa.s.ses distally and superficially through the posterior edge of the tibial cartilage. The tendon traverses the posteromedial ca.n.a.l 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.

Action.--Flexes digit II.

Comparison.--In _Pa.s.ser_, _Estrilda_, _Poephila_, _Hesperiphona_, _Carpodacus_, _Pinicola_, _Leucosticte_, _Spinus_, and _Loxia_ the proximal portion of this muscle is more intimately connected with the posterior edge of the _m. flexor perforans et perforatus digiti III_ than it is in the other species examined.

_+Musculus flexor perforans et perforatus digiti III+_ (Fig. 2).--Long and pinnate, this muscle lies on the lateral surface of the crus beneath the _m. peroneus longus_ and _pars externa_ of the _m.

gastrocnemius_. There are two distinct heads. The origin of the anterior head is fleshy from the proximal edge of the outer cnemial crest and from the internal edge of the distal end of the patellar tendon. The posterior head arises by a tendon from the femur in company with the _m. flexor perforans et perforatus digiti II_, is connected also with the tendon of origin of the _m. flexor perforatus digiti II_, and is loosely attached to the head of the fibula. Fibers from the belly of the muscle attach throughout its length to the lateral edge of the fibula, and the muscle is tightly fused also with adjacent muscles. The tendon of insertion is formed approximately one-half the way down the crus. The tendon perforates the posterior surface of the tibial cartilage and pa.s.ses through the posteromedial ca.n.a.l of the hypotarsus (Fig. 6). At the base of the third digit the tendon ensheathes that of the _m. flexor digitorum longus_ and the two together perforate the tendon of the _m. flexor perforatus digiti III_. Immediately distal to this perforation the tendon of the _m.

flexor perforans et perforatus digiti III_ ceases to ensheath that of the _m. flexor digitorum longus_. The latter pa.s.ses beneath that of the former. Near the distal end of the second phalanx the tendon of the _m. flexor digitorum longus_ perforates that of the _m. flexor perforans et perforatus digiti III_. The latter inserts on the posterior surface of the distal end of the second phalanx and the proximal end of the third.

Action.--Flexes digit III.

Comparison.--In _Pa.s.ser_, _Estrilda_, and _Poephila_, and in all the cardueline finches examined the proximal portion of this muscle is more intimately connected with the anterior edge of the _m. flexor perforans et perforatus digiti II_ than it is in the other species examined.

_+Musculus flexor digitorum longus+_ (Figs. 3, 5).--This strong, pinnate muscle is deeply situated along the posterior surfaces of the tibia and fibula. There are two distinct heads of origin. The lateral head arises by means of fleshy fibers from the posterior edge of the head of the fibula. The medial head arises by means of fleshy fibers from the region under the ledgelike external and internal articular surfaces of the proximal end of the tibia. Neither head has any connection with the femur in contrast to the condition, described by Hudson (1937: 46-47) in the crow, _Corvus brachyrhynchos_, and in the raven, _Corvus corax_. Near the point of insertion of the _m. biceps femoris_ the two heads fuse. The common belly is attached by fleshy fibers to the posterior surface of the tibia and fibula for two-thirds of the distance down the crus. Near the distal end of the crus the muscle terminates in a strong tendon which pa.s.ses deeply through the tibial cartilage and traverses the anteromedial ca.n.a.l of the hypotarsus (Fig. 6). About midway down the tarsometatarsus this tendon becomes ossified. Immediately above the bases of the toes it gives rise to three branches, one to the posterior surface of each of the foretoes. These branches perforate the other flexor muscles of the toes as described in the accounts of those muscles and insert as follows: The branch to digit II inserts on the base of the ungual phalanx and by a stout, tendinous slip on the distal end of the second phalanx (Fig. 9). The branch to digit III inserts on the base of the distal end of the third phalanx and a stronger slip to the distal end of the second or proximal end of the third. The branch to digit IV inserts on the base of the ungual phalanx, with one tendinous slip to the distal end of the third phalanx and another to the distal end of the fourth.

Action.--Flexes foretoes.

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

_+Musculus flexor hallucis longus+_ (Fig. 3).--Situated immediately posterior to the _m. flexor digitorum longus_, the belly of this large, pinnate muscle is intimately connected anteriorly to that of the _m. flexor perforatus digiti II_. The _m. flexor hallucis longus_ arises by two heads which are separated by the tendon of insertion of the _m. biceps femoris_. The smaller anterior head arises from the same tendon as does the _m. flexor perforatus digiti II_. The larger posterior head arises by means of fleshy fibers from the intercondyloid region of the posterior surface of the femur along with the _m. flexor perforatus digiti III_ and _IV_. The two heads join just distal to the point of insertion of the _m. biceps femoris_.

There is no trace of a tendinous band connecting the two heads as there is in the crow and in the raven (Hudson, 1937:49). Near the distal end of the shank the muscle gives rise to a strong tendon which perforates the tibial cartilage along its lateral edge and pa.s.ses through the anterolateral ca.n.a.l of the hypotarsus (Fig. 6). The tendon crosses over to the medial surface of the tarsometatarsus, pa.s.ses distally, and perforates the sheathlike tendon of the _m. flexor hallucis brevis_ between the first metatarsal and the trochlea for digit II. The tendon continues along the posterior surface of the hallux and has a double insertion; the main tendon attaches to the base of the ungual phalanx and a smaller branch inserts on the distal end of the proximal phalanx.

Action.--Flexes hallux.

Comparison.--In _Vireo_ this muscle has only the posterior head of origin and is not connected with the _m. flexor perforatus digiti II_.

The muscle is proportionately smaller and weaker than in any of the other species studied.

_+Musculus extensor hallucis longus+_ (Fig. 4).--One of the smallest muscles of the leg, the origin is fleshy from the anteromedial edge of the proximal end of the tarsometatarsus. The belly is long and slender and terminates distally in a slender tendon which pa.s.ses distally along the posterior surfaces of the first metatarsal and the first digit. The insertion is on the base of the ungual phalanx. Near the distal end of the proximal phalanx, the tendon pa.s.ses between two thick bands of fibro-elastic tissue which insert also on the ungual phalanx. These bands of tissue function as automatic extensors of the claw.

Action.--Extends hallux; action must be slight.

Comparison.--In _Vireo_ this muscle is proportionately larger and better developed than it is in any of the other species examined.

_+Musculus flexor hallucis brevis+_ (Fig. 4).--This minute muscle has a fleshy origin from the medial surface of the hypotarsus. The short belly terminates in a weak, slender tendon which pa.s.ses down the posteromedial surface of the tarsometatarsus and into the s.p.a.ce between the first metatarsal and the trochlea for digit II. In this region the tendon envelops the tendon of the _m. flexor hallucis longus_ and inserts on the distal end of the first metatarsal and on the proximal end of the first phalanx of the first digit.

Action.--Flexes hallux; action must be slight.

Comparison.--The small size of this muscle makes it exceedingly difficult to study. The muscle is larger in _Vireo_ than in any of the other species examined. This may be correlated with the smaller size of the _m. flexor hallucis longus_ in this species. The muscle does not seem to be so well developed in the cardueline finches as it is in the other species.

_+Musculus abductor digiti IV+_ (Fig. 2).--Extremely small, delicate and difficult to demonstrate, this muscle arises in a fleshy origin immediately from underneath the posterior edge of the external cotyla of the tarsometatarsus. The tendon of insertion is long and slender and inserts along the lateral edge of the first phalanx of digit IV.

Action.--Abducts digit IV.

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

_+Musculus lumbricalis.+_--Semitendinous throughout its length, this muscle arises from the ossified tendon of the _m. flexor digitorum longus_ at a point immediately proximal to the branching of this tendon. The insertion is on the joint pulleys and capsules at the base of the third and fourth digits.

Action.--Hudson (1937:57) states that: "Meckel (_vide_ Gadow--1891, p.

204) considered this muscle as serving to draw the joint pulley behind in order to protect it from pinching during the bending of the toes.

It perhaps also tends to flex the third and fourth digits."

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

Discussion of the Myological Investigations

Simpson (1944:12) and others have emphasized that different parts of organisms evolve at different rates. Beecher (1951b:275) in stating that "... the hind limb is very similar in muscle pattern throughout the Order Pa.s.seriformes and seems to have become relatively static after attaining a high level of general efficiency ..." implies that the muscle pattern of the leg must be one of long standing and slow change. This concept was emphasized by Hudson (1937) who found but little variation in muscle pattern among members of the several families of pa.s.serine birds. The concept is further confirmed by the present investigation. The intricate patterns of origin and of insertion seem to remain almost the same throughout the order in spite of adaptive radiation which has occurred.

Two major differences in patterns of leg-musculature, however, were found among the species studied, and these differences are significant since they are consistent between subfamilies. The muscles involved are the _m. obturator externus_ and the _pars interna_ of the _m.

gastrocnemius_.

The _m. obturator externus_ is bipart.i.te, consisting of dorsal and ventral parts, in the pa.s.serine species studied by Hudson (1937) and in all of the species examined by me except the ploceids and the cardueline finches. In the ploceids and cardueline finches this muscle is undivided and resembles in its position, origin, and insertion only the ventral portion of the muscle found in the other birds studied. It is difficult to imagine what advantage or disadvantage might be a.s.sociated with the bipart.i.te or with the undivided condition. The action of this muscle is to rotate the femur (right femur clockwise, left femur counterclockwise), and certainly the greater ma.s.s of the bipart.i.te muscle could lend greater strength to such action. The possible significance of this is discussed below.

List of Abbreviations Used in Figures

Abd. dig. IV _M. abductor digiti IV_ Acc. _M. accessorius semitendinosi_ Add. long. _M. adductor longus et brevis_ Anterolat. can. Anterolateral ca.n.a.l of hypotarsus Anteromed. can. Anteromedial ca.n.a.l of hypotarsus Bic. fem. _M. biceps femoris_ Bic. loop Loop for _m. biceps femoris_ Ext. cot. External cotyla Ext. dig. l. _M. extensor digitorum longus_ Ext. hal. l. _M. extensor hallucis longus_ Fem. tib. ext. _M. femorotibialis externus_ Fem. tib. int. _M. femorotibialis internus_ Fem. tib. med. _M. femorotibialis medius_ F. dig. l. _M. flexor digitorum longus_ F. hal. brev. _M. flexor hallucis brevis_ F. hal. l. _M. flexor hallucis longus_ F. p. et p. d. II _M. flexor perforans et perforatus digiti II_ F. p. et p. d. III _M. flexor perforans et perforatus digiti III_ F. per. d. II _M. flexor perforatus digiti II_ F. per. d. III _M. flexor perforatus digiti III_ F. per. d. IV _M. flexor perforatus digiti IV_ Gas. _M. gastrocnemius_ Iliacus _M. iliacus_ Il. tib. _M. iliotibialis_ Il. troc. ant. _M. iliotrochantericus anticus_ Il. troc. med. _M. iliotrochantericus medius_ Il. troc. post. _M. iliotrochantericus posticus_ Int. cot. Internal cotyla Isch. fem. _M. ischiofemoralis_ Midmed. can. Midmedial ca.n.a.l of hypotarsus Obt. ext. _M. obturator externus_ Obt. int. _M. obturator internus_ P. ant. _Pars anticus_ P. ext. _Pars externa_ P. int. _Pars interna_ P. med. _Pars media_ P. post. _Pars posticus_ Per. brev. _M. peroneus brevis_ Per. long. _M. peroneus longus_ Pirif. _M. piriformis_ Plan. _M. plantaris_ Posterolat. can. Posterolateral ca.n.a.l of hypotarsus Posteromed. can. Posteromedial ca.n.a.l of hypotarsus Sar. _M. sartorius_ Semim. _M. semimembranosus_ Semit. _M. semitendinosus_ Tib. ant. _M. tibialis anticus_ Tib. cart. Tibial cartilage

[Ill.u.s.tration: FIG. 1. _Pipilo erythrophthalmus._ Lateral view of the superficial muscles of the left leg, 1.5.]

[Ill.u.s.tration: FIG. 2. _Pipilo erythrophthalmus._ Lateral view of the left leg showing a deeper set of muscles. The superficial muscles _iliotibialis_, _sartorius_, _gastrocnemius_ and _peroneus longus_ have been removed, 1.5.]

[Ill.u.s.tration: FIG. 3. _Pipilo erythrophthalmus._ Lateral view of the left leg showing the still deeper muscles. In addition to those listed for figure 2, the following muscles have been wholly or partly removed: _iliotrochantericus posticus_, _femorotibialis externus_, _femorotibialis medius_, _biceps femoris_, _semitendinosus_, _tibialis anticus_, _flexor perforans et perforatus digiti II_, and _flexor perforans et perforatus digiti III_, 1.5.]

[Ill.u.s.tration: FIG. 4. _Pipilo erythrophthalmus._ Medial view of the superficial muscles of the left leg, 1.5.]

[Ill.u.s.tration: FIG. 5. _Pipilo erythrophthalmus._ Medial view of the left leg showing a deeper set of muscles than those seen in figure 4. The following superficial muscles have been removed: _iliotibialis_, _sartorius_, _femorotibialis internus_, _obturator internus_, _adductor longus (pars posticus)_, _gastrocnemius_, and _peroneus longus_, 1.5.]

[Ill.u.s.tration: FIG. 6. _Pipilo erythrophthalmus._ Proximal end of left tarsometatarsus and the hypotarsus, 4.]

[Ill.u.s.tration: FIG. 7. _Pipilo erythrophthalmus._ Lateral view of proximal end of left femur and a portion of the pelvis, 3.5.]

[Ill.u.s.tration: FIG. 8. _Pipilo erythrophthalmus._ Upper surfaces of the phalanges of the foretoes of the left foot showing insertions of the _M. extensor digitorum longus_, 3.]

[Ill.u.s.tration: FIG. 9. _Pipilo erythrophthalmus._ Medial view of the second digit of the left foot, showing insertions of the flexor muscles, 3.]

The division of the _pars interna_ of the _m. gastrocnemius_ into anterior and posterior parts has not been reported by previous authors yet the division is quite distinct in those birds in which it occurs.

Hudson (1937:36) points out that in some non-pa.s.serine birds the _pars interna_ is double, but that in these species the _m. semimembranosus_ inserts between the two parts. This is not the condition in those species studied by me. Only the ploceids and the cardueline finches in the present investigation fail to show such a division. The undivided muscle in these birds resembles, in its origin and position, the posterior portion of the muscle found in those species showing the bipart.i.te condition. The greater ma.s.s of the bipart.i.te muscle probably makes possible a stronger extension of the tarsometatarsus.