(100)  Mm. iliotibiales.  According to developmental studies in  Gallus (Schroeter and Tosney, ms.) the iliotibialis group of muscles consists of M. iliotibialis cranialis and lateralis, M. ambiens (Annot. 103), and M. iliofibularis (Annot. 102).  M. iliotibialis medialis is most probably a derivative from the same group.


(101)  M. iliotibialis cranialis.  Synonymy:  M. sartorius (Hudson, 1937); M. extensor iliotibialis anterior (Fisher, 1946).  There is considerable variation in the proximal attachments of this muscle to the caudal thoracic vertebrae, synsacrum and preacetabular ilium (Figs. 6.14).  More than one head may be present (see review in Berger,1966; Vanden Berge, 1975, and others).

M. iliotibialis lateralis.  The extent to which this muscle attaches along the lateral margin of Ala preacetabularis and Ala postacetabularis of the ilium is the basis for recognizing two subdivisions, Pars preacetabularis and Pars postacetabularis,  respectively (Fig. 6.14).   The two parts may be essentially continuous and very extensive along both the preacetabular and postacetabular alae of the ilium, or there may be an intermediate "acetabular gap," Hiatus acetabularis.  The latter is a derived state resulting from the loss of the central or acetabular portion of the muscle (Raikow, 1987).  M. iliotibialis medialisA separate muscle, lying deep to the preacetabular part of the iliotibialis lateralis, has been described in the flamingos (Vanden Berge,1970), and in the Australian Banded Stilt, Cladorhynchus leucocephalus  (Olson and Feduccia,1980).


All three muscles terminate distally with Mm. femorotibiales (Annot. 107) in a common tendon (Lig. patellae, Arthr. Annot. 158; Osteo. Annot. 265; PNS Annot. 46) on the patellar crest.


(102)  M. iliofibularis.  Synonymy:  M. biceps femoralis.  M. iliofibularis, together with Mm. flexor cruris lateralis and flexor cruris medialis (Annot. 108, 109), have been considered homologs of the mammalian "hamstring" muscles.  Which of these three, if any, is the counterpart of which mammalian muscle is anything but clear (for synonymy, see Vanden Berge, 1975).  M. iliofibularis is apparently derived from the iliotibialis muscle group, independent from the other two muscles (Schroeter and Tosney, ms.).   The tendon of insertion typically passes through a fibrous loop (Ansa m. iliofibularis, Arthr. Annot. 186), with a branch of the tibial nerve (N. paraperoneus) and N. peroneus (PNS Annot. 51), and inserts on Tuberculum m. iliofibularis, Corpus fibulae.  See Figs. 6.15,16.


(103)  M. ambiens attaches proximally on Tuberculum preacetabulare (Osteo. Annot. 244) on the body of the ilium.  The distal tendon merely grooves or sometimes perforates the patellar ligament or the patella itself (Sulcus [Canalis] m. ambientis,  Osteo. Annot. 265), before terminating on the proximal aponeurosis of the digital flexor muscles attaching in the popliteal fossa.  See Fig. 6.17.


(104)  Mm. iliotrochanterici.  Synonymy: Gadow and Selenka (1891) considered this muscle group to be peculiar to birds, without homology in mammals, but others, following Fisher (1946), refer to M. iliotrochantericus caudalis as "gluteus profundus"; M. iliotrochantericus cranialis as "iliacus",  and retain the name  M. iliotrochantericus medius.  Recently, Rowe (1986) has suggested homology with a similar set of muscles in reptiles but with differing views in the associated terminology. 

The three muscles derive from a deep proximal group of thigh muscles, including also M. iliofemoralis externus (Annot. 105) and M. iliofemoralis internus (Annot. 106) (Schroeter and Tosney, ms.).  All three muscles originate on the preacetabular ilium and insert on the lateral aspect of Trochanter femoris  (Osteo. Annot. 257; details in Ballmann,1969b).  According to Vanden Berge (1982), statements about the presence/absence of M. iliotrochantericus medius, as a distinct muscle element in the avian thigh, should also characterize the passage of a neurovascular bundle (N. coxalis cranialis; A. coxae cranialis, see Art. Annot. 70) proximally between Mm.iliotrochantericus medialis and cranialis (Rosser et al., 1982, and Schulin, 1987).  See Figs. 6.15,16.. 


(105)  M. iliofemoralis externus.  Synonymy:  M. gluteus medius et minimus (Hudson,1937); M. piriformis (Fisher,1946). The original term, M. iliofemoralis externus (Gadow and Selenka,1891), is descriptive of the origin on a caudal prolongation of the Fossa iliaca dorsalis of the preacetabular ilium (Osteo. Annot. 232) and insertion on the lateral surface of the Trochanter femoris. The degree of separation (or independence) of this muscle from M. iliotrochantericus caudalis (Annot. 104) is quite variable (Fig. 6.15); the muscle is absent in some taxa but reappears as a developmental anomaly or even as a re-established feature (Raikow et al.,1979). See Rowe (1986) for possible homology with reptilian thigh muscles.  


(106)  M. iliofemoralis internus.  Synonymy:  M. iliacus; M. psoas.  M. iliofemoralis internus is the name originally proposed by Gadow and Selenka (1891) and is descriptive of the origin on the ilium, deep (medial) to the origin of the iliotrochantericus medius, with insertion on the caudomedial surface of the proximal end of the femur (Fig. 6.17).


(107)  Mm. femorotibiales.  According to Berger (1966), chief variations in M. femorotibialis lateralis [externus] pertain to the origin of Pars proximalis and the presence or absence of a discrete Pars distalis.  The proximal part inserts on Lig. patellae (Arthr. Annot. 158); the distal part, on Crista cnemialis lateralis on the proximal tibia.  The three subdivisions, Mm. femorotibialis lateralis [externus], intermedius, and medialis [internus], form a common tendon in which the patella develops (Osteo. 265, 269) and on which also inserts Mm. iliotibiales (Annot. 101).


(108)  M. flexor cruris lateralis.  Synonymy:  M. caudilioflexorius (Gadow,1891); M. semitendinosus (Hudson,1937); M. flexor cruris lateralis is the name proposed by Fisher (1946). The muscle is derived with M. flexor cruris medialis (Annot. 109) from a "flexor group" (Schroeter and Tosney, ms.).  Two parts have been recognized for many years: Pars pelvica, the main belly which is attached to the caudal end of the pelvis (ilium and ischium) and Membrana iliocaudalis (Arthr. Annot. 185), and Pars accessoria (pars femoralis, or "accessory semitendinosus") which is attached to the distal femur.  The latter, however, is part of the insertion, not a second head of origin; it joins with the pelvic part in a distal raphe with additional tendinous variations in certain groups (McKitrick, 1986; Raikow, 1987).  See Fig. 6.16.


Variation occurs among avian groups.  Pars pelvica  may be present without pars accessoria, but the latter apparently is never present alone.  In some major avian taxa, both parts are absent (e.g., in accipiter hawks and owls; Berger, 1966; Fig. 6.19, from Hoff, 1966).


(109)  M. flexor cruris medialis.  Synonymy:  M. ischioflexorius; M. semimembranosus; M. flexor cruris medialis is the name proposed by Fisher (1946). M. flexor cruris medialis is deep to M. flexor cruris lateralis and the origin is typically on the lateral surface of the ischium. The tendon of insertion may attach directly on the proximal portion of the tibia, or it may form a common aponeurosis with that of M. flexor cruris lateralis (Annot. 108) and the proximal aponeurosis of M. gastrocnemius, Pars intermedia (Annot. 118; examples in Goodge,1972; Swierczewski and Raikow,1981; Schulin,1987).  See Figs. 6.17.


(110)  M. caudofemoralis.  Synonymy:  M. piriformis;  according to Romer (1927), a homolog of the coccygeofemorales muscle system in reptiles. Differentiation of the muscle during development, at least in Gallus (Schroeter and Tosney, ms.), suggests that M. caudofemoralis should be described as a single muscle having two parts, Pars caudalis and Pars pelvica, rather than two independent muscles (NAA,1979; Vanden Berge, 1982).  Pars caudalis is the element most frequently described and is apparently only rarely absent in the presence of Pars pelvica.  However, both parts may be absent (see Table IX.2 in Berger,1966).  See Fig. 6.16.


In the course of avian evolution, the relative length of the skeleton of the tail became reduced; Pars caudalis (formerly M. caudofemoralis, NAA,1979) is retained as the ancestral muscle. An independent functional role in movement of the leg has been suggested but this seems unlikely since attachments to the tail (Aponeurosis cruciata, Annot. 65 and 66) indicate a more probable functional role in movements of the tail. The muscle does have an accessory function in air exchange within the abdominal air sacs (Baumel et al., 1990).

Pars pelvica (formerly M. iliofemoralis, NAA,1979) is a secondarily derived belly whereby the muscle attaches chiefly to the ilium, but also ischium in some taxa. Pars caudalis and Pars pelvica may have a common tendon of insertion on the femur, without continuity of their respective contractile tissue, but in some taxa (e.g., ratites), their respective femoral attachments are separate.


(111)  Mm. obturatorius lateralis and medialis.  Lateralis is sometimes known as "externus;" medialis, as "internus."  Such alternate names suggest homology with muscles of the same name in mammals, but according to Romer (1927), the obturator muscles differentiate from a common primordium which appears to be equivalent to the obturatorius externus in mammals, not obturator internus.  Substitution of the terms lateralis and medialis are consistent with the positional relationships between them. 


(112)   M. obturatorius lateralis.  Pars dorsalis and Pars ventralis are clearly defined parts of this muscle in passerines (Raikow, 1976; 1978, Fig. 5; 1987), and in some other avian taxa (Berger, 1966, 1969).


(113)  M. pubo-ischio-femoralis.  The synonym "M. adductor," with several qualifying terms ("longus et brevis," "superficialis et profundus," "longus et magnus") has been in common use.  However, as Cracraft (1971) has demonstrated, the muscle probably has no significant functional role in adduction of the femur,  but is more likely a postural muscle (Helmi and Cracraft, 1977).  The name M. pubo-ischio-femoralis (Gadow and Selenka, 1891) is descriptive in terms of the origin (pubis and/or ischium) and insertion (femur) of this muscle.  In nonpasserine birds, the two subdivisions are Pars lateralis  and Pars medialis; in passerines, Pars cranialis is equivalent to the lateral subdivision and  Pars caudalis, to the medial subdivision (Raikow, 1976).  The two main subdivisions are separable by the orientation of their respective fasciculi and/or by the passage of the motor nerve (R. muscularis, N. obturatorius) between them.  In the colies or mousebirds (Coliiformes), a third portion, Pars accessoria, attaching distally on the tibiotarsus, has been described (Berman and Raikow, 1982, Fig. 2); it may be unique to this group of birds.  See Figs. 6.16, 17.


(114)  M. tibialis cranialis.  In owls (Strigidae), Caput femorale and Caput tibiale may remain independent,  forming two bellies and two tendons of insertion (Hoff, 1966). The proximal tendon of Caput femorale takes origin on Condylus lateralis of the femur (Fovea tendinea); perforates or merely grooves Meniscus lateralis (Arthr. Annot. 159), and passes distally across Incisura tibialis  (Osteo. Annot. 273).  As the tendon crosses the knee, it may be restrained by a retinaculum in some birds (Berman and Raikow,1982); its tendon of insertion is restrained by a fibrous arch (Retinaculum extensorium tibiotarsi,  Arthr. Annot. 187) and inserts on the tarsometatarsus (Tuberositas m. tibialis cranialis; see Osteo. Annot. 291).  In some taxa, a metatarsal neurovascular bundle passes between the bifurcation of the tendon at its insertion (Vanden Berge,1970; for detailed information, Midtgard,1982).  See Fig. 6.15; Arthr. Fig. 5.08.


(115)  M. extensor digitorum longus.  The proximal attachment is in Sulcus intercnemialis of the tibiotarsus (Osteo. Annot. 272);  the distal tendon is restrained, in common with that of M. tibialis cranialis (Annot. 114),  by a fibrous arch (Retinaculum extensorium tibiotarsi,  Arthr. Annot. 187).  Distally, the tendon enters Canalis extensorius (Osteo. Annot. 278) and passes beneath the ossified "supratendinal bridge" (Pons supratendineus,  Osteo. Annot. 277), crosses the intertarsal joint, and then passes beneath the fibrous Retinaculum extensorium tarsometatarsi (Arthr. Annot. 188) which may sometimes be ossified (Osteo. Annot. 287).  The tendon typically gives rise to branchings to the second, third, and fourth toes, and rarely to the hallux (for exceptions, see Berman and Raikow,1982; Fig. 6.27, from Berman,1984).


(116)  M. fibularis [peroneus] longus.  Variation in the Mm. fibularis longus and brevis in many taxa was described on the basis of five principal morphological characters (Mitchell (1913).  Hudson (1937) and Berger (1966) did not consider the wide variation in relative development of the two muscles significant in terms of avian systematics; for a differing opinion, see Kurochkin (1968). Typically, the tendon of insertion attaches to the tibial cartilage by an aponeurosis,  grooves the proximal end of the Tarsometatarsus (Sulcus m. fibularis [peronei] longus) between the insertion of M. fibularis brevis (Annot. 117; Tuberculum retinaculi m. fibularis, Osteo. Annot. 282) and the attachment of Lig. collaterale laterale (Arthr. Annot. 172), and terminates on the tendon of M. flexor perforatus digiti III (Annot. 121).  These relationships are subject to some variation however.  See Fig. 6.16 and Arthr. Fig. 5.08.


(117)  M. fibularis [peroneus] brevis.  This muscle is variably developed among avian taxa.  In New World nine-primaried oscines (Passeriformes), a derivative head, Caput tibiale, distinct from the primary head,Caput fibulare, is said to be an important taxonomic character (Kurochkin,1968;Raikow,1976,1978).  The tendon lies in a sulcus on the distal end of the tibiotarsus where it is restrained by a retinaculum (Tuberculum retinaculi m. fibularis, Osteo. Annot. 282 ).  Insertion is on the proximal end of the tarsometatarsus (Tuberculum m. fibularis brevis), ventral (plantar) to the sulcus for M. fibularis longus (Annot. 116); see Arthr. Fig. 5.08.


(118)  M. gastrocnemius.  Typically consists of three parts identified as Pars lateralis (=externa), Pars intermedia, and  Pars medialis (=interna).  In addition, a separate fourth part,  Pars supramedialis, has been described in some species of the suboscine passerine families Eurylaimidae and Philepittidae (Raikow, 1987).  These subdivisions are best recognized by the term "Pars" rather than "Caput" since they are not merely separate heads of one muscle, but may be entirely separate bellies, sharing in the formation of a common tendon of insertion.  The presence or absence of accessory heads associated with Pars lateralis (Vanden Berge, 1970) or Pars medialis (Raikow, 1970, 1978, 1987) may serve as taxonomic characters among avian taxa.


(119)  M. plantaris [tibialis caudalis, Ghetie,1976].   No definitive homology with the mammalian muscle of this name has been established.  Presence or absence of it was once a part of the technical taxonomic diagnosis of avian taxa. Raikow (pers. comm.) indicated that the tendon of insertion in dendrocolaptid passerines is regularly in common with that of M. gastrocnemius, Pars intermedia.  A similar type of insertion has been described in other birds (Vanden Berge, 1970); see Arthr. Fig. 5.08.


(120)  M. popliteus.  Homology with the mammalian muscle of the same name is not established.  The muscle is attached in Fossa flexoria (Osteo. Annot. 274), proximal to the attachment of M. flexor digitorum longus (Annot. 121, 123), on the proximal end of the tibia, in a position equivalent to the popliteus muscle in mammals.  Insertion is on the caudal surface of the head of the fibula.


(121) Mm. flexores perforantes et perforati digiti II et III.  Synonymy: M. flexor digitorum medius (Frewein,1967).  Mm. flexores perforati digiti II, III, et IV.  Synonymy: M. flexor digitorum superficialis (Frewein,1967).  M. flexor hallucis longus; M. flexor digitorum longus. Synonymy: M. flexor digitorum profundus (Frewein,1967)See Fig. 6.16.

 A "perforans" [perforating] tendon is one which passes between the decussating medial and lateral phalangeal attachments of the inserting "perforatus" [perforated] tendon at metatarsophalangeal and/or interphalangeal joints.   The "sleeve" formed by the so-called "bifurcation" of each perforated tendon around the corresponding perforating tendon,  is known as  Manica flexoria (ICVGAN,1983).  In the avian foot, the second and third toes typically have three sets of flexor tendons, one of which is both perforating as well as perforated.  Therefore, these two toes each have one "proximal" flexor tendon (M. flexor perforatus digiti II or III) perforated by two others, one of which ("intermediate" M. flexor perforans et perforatus digiti II or III) is itself perforated by the respective "distal" deep digital flexor tendon (M. flexor digitorum longus) of that toe.

            In some taxa,  M. flexor perforatus digiti II attaches on both the medial and lateral side of Phalanx proximalis and is perforated by the tendons of M. flexor perforans et perforatus digiti II ("M. flexor digitorum medius") and M. flexor digitorum longus (see Schreiweis, 1982; Vanden Berge, 1970, and Zusi and Bentz, 1984, for examples).  In many other taxa, the same tendon has been described as "not perforated," but the accompanying descriptions do not necessarily indicate that the inserting tendon is inserting only on the lateral, or only on the medial aspect of the phalanx in a primary, i.e., "non-perforated" state. See Berman and Raikow, 1982; Schreisweis,1982; McKitrick, 1985, and Raikow, 1987, for clear descriptions of this relationship between these tendons.

            The hallux typically has a "superficial" flexor tendon (M. flexor hallucis brevis, Annot. 126) which is perforated by the tendon of the deep flexor, M. flexor hallucis longus.  The fourth toe has a "superficial" tendon (M. flexor perforatus digiti IV) which is perforated by the corresponding deep digital flexor tendon of M. flexor digitorum longus.  An "intermediate" M. flexor perforans et perforatus digiti IV has never been described in the avian fourth toe.

            All of these long flexor tendons also pass through Cartilago tibialis (Arthr. Annot. 164, and Fig. 5.08) and Hypotarsus (Osteo. Annot. 288) to enter Sulcus flexorius on the plantar aspect of the foot.  The position of these long tendons, and their positional relationships to one another, are usually definite for different taxa (see Hudson,1937, and Vanden Berge,1970, for examples).  The tendons also pass through Canalis flexorius plantae (Arthr. Annot. 178) and  sulci on the plantar surfaces of the Ligg. plantare (Arthr. Annot. 182, and Fig. 5.09) of the metatarsophalangeal and interphalangeal articulations on each toe, with the exception of the terminal interphalangeal joint.

M. flexores perforantes et perforati digiti II et III are considered "intermediate" flexors in the sense that their tendons "perforate" those of the "superficial" flexors (Mm. flexores perforati digiti II et III), but are, in turn, perforated by corresponding digital tendons of M. flexor digitorum longus.

M. flexores perforati digiti II, III, et IV.   One or more of these three muscles may have two proximal attachments or "heads."  The flexors of the second and fourth toes arise on the aponeurosis of M. flexor perforatus digiti III (see Cracraft,1971 and Vanden Berge, ms.) which attaches in Fossa poplitea of the femur.  A second, lateral, or "fibular" attachment may be continuous with the tendon of M. ambiens (Annot. 103) when the latter is present.  Raikow (1987) described variations in the heads and tendon(s) of M. flexor perforatus digiti IV in several passerines.


(122)  Vinculum tendinum flexorum.  The fibroelastic band (Fig. 6.19) which connects the tendons of M. flexor perforans et perforatus digiti III and M. flexor perforatus digiti III in the foot is highly variable among birds generally (Hudson, 1937).

A second fibroelastic band unites the tendons of Mm. flexor hallucis longus and flexor digitorum longus in some birds.  The tendons of these muscles, however, may be totally independent in some birds, partially or completely fused in others.  Raikow (1985a,1987) has reviewed the various degrees and patterns of fusion and interconnections between the deep digital flexors in association with different functional and adaptive specializations of the



(123)  M. flexor hallucis longus; M. flexor digitorum longus.   Origins of M. flexor hallucis longus are distinctly separate from those of M. flexor digitorum longus. There may be multiple heads of origin, one or more of which may have a variable relationship to the tendon of insertion of M. iliofibularis (Annot. 102; Raikow and Cracraft,1983; McKitrick,1985, Raikow,1987).  The relationship between these two deep flexor muscles and their tendons, distally, has been classified into at least eight different types, including one or more subtypes in certain groups.   Raikow (1985). discusses and illustrates these types, and presents in tabulated form a concise summary of the distribution of each type among avian groups, related also to the type of foot and the relative development of the hallux.  The tendons terminate on Tuberculum flexorium of each ungual phalanx.


(124)  Area tuberculata tendinis, Plicae tenosynoviales, Lig. elasticum tendinis flexoris, Lig. elasticum extensorium unguis.  These terms are more fully described and well illustrated in Quinn and Baumel (1990); see Arthr. Annot. 183.

            Area tuberculata tendinis.  Roughened patches on the digital plantar surface of tendons of Mm. flexor hallucis longus and flexor digitorum longus opposite transverse semicircular folds (Plicae tenosynoviales) on the apposing surface of the tendon sheath.  The sheath plicae are pressed against the tuberculate areas to form a locking mechanism holding the distal interphalangeal joints in the flexed position.  First thought to be an adaptation peculiar to "perching" birds ( Schaffer,1903), this  "tendon-locking mechanism" occurs in birds of many foot types.  It  appears in ovo (fetal to late fetal: 15d, Gallus) concurrently with initiation of limb movement, and not as a result of post-hatch mechanical pressures.


(125)  M. extensor hallucis longus.  Pars proximalis and Pars distalis are well defined in some taxa (Hudson, Lanzillotti and Edwards, 1959), but the entire muscle is one of the more variable of the group of short muscles of the toes (Berger, 1966; Raikow, 1976; Berman and Raikow, 1982; Berman, 1984).  Since there is but one short extensor of the hallux, the use of the qualifier, "longus," may seem unnecessary.  However, Pars proximalis may represent the "long" extensor and Pars distalis, the short ("brevis").  In  addition, there may be an additional head present (pars accessoria, Berman and Raikow, 1982). 


(126)  M. flexor hallucis brevis. The muscle is attached on the shaft of the tarsometatarsus (Fossa parahypotarsalis medialis) and is separated from Sulcus flexorius (Osteo. Annot. 294) by a ligamentous intermuscular septum extending distally from Crista medianoplantaris (Osteo. Annot. 290) in some taxa (e.g., Larus). Distally, the tendon is perforated by the tendon of M. flexor hallucis longus (Annot. 121).


(127)  M. abductor digiti II; M. adductor digiti II.   M. abductor digiti II (functionally, probably an extensor: Cracraft,1971) attaches medially on the distal half of the tarsometatarsus and inserts on the base of the proximal phalanx (Fig. 6.18).   M. adductor digiti II (Fig. 6.26) attaches in the Sulcus flexorius, deep to the flexor tendons, and on the lateral face of the base of the first phalanx (adduct to the third toe); it is absent in some taxa (e.g.,Tetraonidae; Hudson et al.,1959; Berger, 1966).


(128)  M. extensor proprius digiti III.  In most taxa of birds, this muscle is of rare and irregular occurrence; see Fig. 6.18.  Hudson, Schreiweis and ChenWang (1972) state that it has been seen in five living genera of ratites and in the tinamous (Tinamiformes).  Berman (1984) describes accessory muscle slips in Amazona.  See Holmes (1962) for further information.


(129)  M. extensor proprius digiti IV; M. extensor brevis digiti IV.  The tendon of insertion of  M. extensor brevis digiti IV passes through Canalis interosseus tendineus (Osteo. Annot. 295, 298).  The presence of a second short toe muscle,  M. extensor proprius digiti IV, is described and illustrated by Berman and Raikow (1982, Fig. 8A).  See also Trochlea accessoria, Osteo. Annot. 297.


(130)  M. abductor digiti IV.  Present in most groups of birds (Berger, 1966), but often minute and identified only by staining techniques (Bock and Shear, 1972).


 M. adductor digiti IV has also been described by Gadow and Selenka (1891), and by Hudson (1937), but it is apparently insignificant and extremely rare in occurrence.