Part 6

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.

Action.--Flexes digit II.

Comparison.--In Passer, 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 passes through the posteromedial canal 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 passes 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 Passer, 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 passes deeply through the tibial cartilage and traverses the anteromedial canal 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 passes through the anterolateral canal of the hypotarsus (Fig. 6). The tendon crosses over to the medial surface of the tarsometatarsus, passes 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 passes 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 passes 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 passes down the posteromedial surface of the tarsometatarsus and into the space 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 Passeriformes 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 passerine 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.