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