Skeleton

The majority of the osteological terms used in the present paper are those used by Howard (1929); however, many skeletal features are not named by Howard. Since names for most of these parts were not found in the other literature examined, it was necessary for me to propose terms for them. Most of this new terminology pertains to the pelvis. All of the osteological terms used in the present paper, whether used by Howard or not, are briefly defined below. Those of the pelvis are illustrated in Fig. 1. Most of the remaining terms are illustrated by Howard (1929).

Pelvis

The median dorsal ridge is the blunt ridge in the midline of the anterior part of the synsacrum formed by the neural spines of the vertebrae. The antitrochanter, on the posterodorsal rim of the acetabulum, is a pyramid-shaped projection that articulates with the proximal end of the femur. The anterior iliac crest is a ridge along the dorsomedial border of the ilium, beginning almost at the anterior end of that bone; the crest curves laterally as it extends posteriorly and (for purposes of the present definition) ends at the level of the posterior edge of the antitrochanter, where the crest is continuous with the lateral iliac process. The lateral iliac process is a pronounced, laterally or ventrolaterally, projecting ridge on the ventrolateral surface of the ilium posterior to the level of the antitrochanter; the process does not extend as far as the posterior end of the ilium. The lateral ischiatic ridge is a relatively slight ridge continuous with the posterior end of the lateral iliac process and curves posteroventrally across the lateral surface of the posterior part of the ischium; the ridge extends to the ventral edge of the ischium in some individuals and not in others. The dorsolateral iliac ridge begins at the lateral edge of the ilium near the posterior end of the lateral iliac process and curves posteromedially and somewhat dorsally, extending to the posterior edge of the ilium. The lateral iliac fossa is the concavity below the overhanging lateral iliac process. The ilio-ischiatic fenestra is a large oblong opening behind the acetabulum between the ilium and the ischium. The obturator foramen is a small oval opening posteroventral to the acetabulum between the ischium and the pubis. The ventral ischiatic tubercle is the angle formed by the ventrally projecting ischium at the point (near its midlength) where the ischium overlaps and lies lateral to (and fused to) the pubis. The pectineal process is an anterolaterally directed projection of the ventrolateral edge of the ilium anteroventral to the acetabulum. The femoral notch of the ilium is a shallow notch in the ventrolateral edge of the ilium approximately halfway between the last rib and the pectineal process. The oblique iliac crest is a pronounced blunt ridge on the ventral surface of the ilium and extends from the posterolateral corner of the last synsacro-thoraco-lumbar vertebra to near the anteroventral border of the ilio-ischiatic fenestra. The internal ilio-ischiatic crest is more or less continuous with the oblique iliac crest and extends posteriorly along the dorsal border of the ischium (forming the ventral border of the ilio-ischiatic fenestra), and then curves sharply dorsomedially onto the ventral surface of the ilium. The iliac recess is a concavity dorsolateral to the sharply curving posterior end of the internal ilio-ischiatic crest.

The terminology applied to the synsacral vertebrae by different authors varies. The terminology proposed by DuToit (1913) is employed in the present account. See my Fig. 1B. This terminology differs considerably from that used by Howard (1929). DuToit divides the fused synsacral vertebrae into the following five groups, listed in anteroposterior sequence: (1) synsacro-thoracic, which bear movable ribs; (2) synsacro-thoraco-lumbar, which lack movable ribs but possess well developed laterally directed parapophyses, in addition to the more dorsally directed diapophyses; (3) synsacro-lumbar, which lack parapophyses, although possessing inconspicuous diapophyses; these vertebrae are shortened anteroposteriorly and are so firmly fused together that often the number present can be determined only by counting the intervertebral foramina; (4) synsacro-sacral, which have much more pronounced transverse processes than do the synsacro-lumbar vertebrae; these transverse processes are expanded distally where they fuse with the ilium and represent both parapophyses and diapophyses partly or completely fused together plus sacral ribs (detectable only in the embryo); there are considered to be two of these vertebrae; they are situated at approximately the level of the acetabulum; (5) synsacro-caudal, which include the remainder of the fused vertebrae; no marked gross morphological features differentiate the synsacro-sacral and the synsacro-caudal groups of vertebrae. The boundaries between all but the last two groups of vertebrae are usually, but not always, easily determined. It may be difficult to determine whether a vertebra with rudimentary parapophyses belongs to the synsacro-thoraco-lumbar or the synsacro-lumbar group. Sometimes a parapophysis will be better developed on one side of a vertebra than on the other.

Femur

The trochanter is a large squarish tuberosity on the lateral surface of the proximal end of the femur. The trochanteric ridge is a sharp, longitudinal (relative to the femur) ridge forming the anterior edge of the trochanter. The obturator ridge is a short, blunt, longitudinal ridge forming the posterior edge of the trochanter. The anterior intermuscular line is a slight ridge extending distally from the trochanteric ridge. The posterolateral intermuscular line is a slight ridge extending distally from the obturator ridge. The posterior intermuscular line is a slight, longitudinal ridge on the mid-posterior surface of the femur. The internal condyle is a large rounded articular prominence on the medial side of the distal end of the femur. On the lateral side of the distal end of the femur are two articular prominences—the lateralmost, smaller one is the fibular condyle, separated by the fibular groove (visible from posterior aspect only) from the larger and more medial external condyle. The popliteal area is a depression on the posterior surface of the distal part of the femur immediately proximal to the condyles.

Tibiotarsus and Fibula

The inner cnemial crest is pronounced and directed anteriorly on the anterior surface of the proximal end of the tibiotarsus. The outer cnemial crest is pronounced and directed anterolaterally on the anterolateral surface of the proximal end of the tibiotarsus. The rotular crest is transverse and forms the anterior border of the proximal end of the tibiotarsus; the crest extends between the dorsal ends of the two cnemial crests and also extends medial to the inner cnemial crest. The fibular crest is longitudinal on the lateral surface of the tibiotarsus and fuses with the middle part of the fibula. The fibular tubercle is small and on the lateral surface of the fibula near the level of the middle of the fibular crest. The anteromedial intermuscular line is a slight ridge extending from the inner cnemial crest down the anteromedial surface of the tibiotarsus. The anterolateral intermuscular line is a slight ridge extending from the fibular crest down the anterolateral surface of the tibiotarsus. The supratendinal bridge is a transverse bony arch over a longitudinal groove near the distal end of the anterior surface of the tibiotarsus.

Tarsometatarsus

The hypotarsus is a large, pronounced, squarish protuberance on the posterior surface of the proximal end of the tarsometatarsus and contains grooves and canals for the passage of the flexor tendons. The longitudinal ridges forming the lateral and medial edges of the posterior surface of the hypotarsus are termed calcaneal ridges. The posterior metatarsal crest is long and sharp; it is continuous with the medial calcaneal ridge that extends most of the way down the posterior surface of the tarsometatarsus medial to the midline; there is an opening between this crest and the tarsometatarsus immediately distal to the hypotarsus. The medial metatarsal depression is large; it is on the medial surface of the proximal end of the tarsometatarsus. The anterior metatarsal groove is a longitudinal groove in the midline of the proximal part of the anterior surface of the tarsometatarsus. The three trochleae are large rounded articular prominences at the distal end of the tarsometatarsus; there is one at the base of each of the digits II, III, and IV. The term distal foramen (as used by Howard) refers to a short, anteroposteriorly directed canal that perforates the tarsometatarsus a short distance proximal to the intertrochlear notch between the trochleae for digits III and IV. Beginning at the middle of this canal and extending distally at a right angle to it is the intertrochlear canal, which opens via the terminal foramen into the intertrochlear notch between the trochleae for digits III and IV.

Nerves

For ease of description I have coined terms for the major divisions of the femoral and sciatic nerves.

Muscles

My terminology follows that of Fisher (1946) and Fisher and Goodman (1955) except for Mm. femoritibialis externus, flexor cruris lateralis (accessory head), and obturator internus et externus. Fisher (1946:547) states that most of his names for the hip and thigh muscles are those of Howell (1938) and the names for the shank and foot muscles are those of Hudson (1937). Fisher deviates, without explanation, from Howell's terminology in respect to Mm. vastus medialis and femoritibialis internus, M. caudofemoralis, M. flexor cruris lateralis, and Mm. obturator internus and obturator externus. Fisher's synonymy of these muscles (1946: table 42) is in error. Fisher understandably deviates from Hudson in respect to Mm. extensor brevis digiti III and extensor proprius digiti III (see Holmes, 1962), although Fisher's synonymy is in error here. See my table 1.

I am not using Fisher and Goodman's term femoritibialis externus; this muscle is here considered as a part of M. vastus lateralis. A great deal of confusion surrounds the terminology of the muscle complex here termed Mm. vastus lateralis and vastus medialis. Hudson (1937), Hudson, et al. (1959), Fisher (1946), and Fisher and Goodman (1955) have used different terminology for this complex. Most of the confusion stems from Gadow's (1891) unclear description of this complex, which he subdivided into two units termed Mm. femori-tibialis externus and femori-tibialis medius. Many birds have three parts to this complex. It is difficult to determine how to apply Gadow's two terms to these three parts. As nearly as I can determine, the correct method is that of Hudson, et al. (1959); but because Gadow's terms have been used in different ways (even by the same worker), it seems best to abandon these terms. Berger (1956:272) believes that the muscle unit that Fisher and Goodman term M. femoritibialis externus represents a head of M. vastus lateralis; I am accepting his opinion. For the three parts of the complex under discussion, I am using the terms M. vastus medialis and M. vastus lateralis pars lateralis and pars postica.

Fisher (Fisher, 1946; Fisher and Goodman, 1955) considers the muscle here termed M. femorocruralis as an accessory head of M. flexor cruris lateralis. The two muscle units in question are closely associated; they insert broadly on opposite sides of a common tendinous raphe. Howell (1938:73) considers this to be a secondary fusion of unrelated muscles. Romer (1927:366) states that in the chick embryo M. femorocruralis is in reality a shank muscle that migrates into the thigh during development. Therefore, Fisher's usage of a single name for these two unrelated muscles is unsatisfactory. I am using Howell's terminology in which the name flexor cruris lateralis represents the main head only of Fisher's M. flexor cruris lateralis and the name femorocruralis represents Fisher's accessory head.

Gadow (1891) divides the obturator complex into two muscles (or muscle groups), which he terms M. obturator and Mm. accessorii M. obturatoris. He states that the former is homologous with the mammalian obturator internus and the latter with the obturator externus. Hudson (1937), accepting Gadow's homologies, renamed these muscles M. obturator internus and M. obturator externus. Nearly all subsequent workers have followed Hudson's terminology, with its implication that these muscles are homologous with the mammalian muscles of the same name. Howell (1938) is an exception. He points out (pp. 78, 79) that the obturator internus of Hudson is homologous with the obturator externus of mammals. His evidence is convincing: "In origin the obturator is somewhat suggestive of the mammalian obturator internus, for which it has uniformly been mistaken. That the latter interpretation is incorrect, however, is attested by the facts that it receives twigs of n. obturatorius within the pelvis, passes through the obturator foramen rather than dorsal to the border of the ischium, and it is segregated from any muscle with tibial innervation. Insertion has shifted only to a slight and unimportant degree as compared with that of the mammalian obturator externus, and beyond question it is the equivalent of that muscle. The stimulus for a longer muscle, has been the same, resulting in the extension of origin to within the pelvis of the externus in birds and the internus in mammals, but the obturator internus is an extension of a part of the gemellus mass and this does not occur in any vertebrate class but Mammalia." Howell applies the term M. obturator to the entire obturator complex.

Romer (1927), studying the development of the thigh musculature in chick embryos, concluded that the entire obturator complex is homologous with the mammalian obturator externus plus quadratus femoris. He considered the avian M. flexor ischiofemoralis to be the homologue of the mammalian obturator internus.

Gadow, in his work on the ratites (1880:34), states that M. obturator (obturator internus of Hudson) cannot be homologous to the mammalian obturator internus, but must represent the obturator externus. His reasoning is as follows: "Als M. pectineus kann man diesen Muskel nicht auffassen, da er auf der AussenflÄche des Trochanter major inserirt, ferner auch nicht als M. obturator internus der menschlichen Anatomie, da er nicht vom Plexus ischiadicus, sondern vom Plexus cruralis aus innervirt wird. Seiner Innervation und Insertion nach wÄre er nur mit dem M. obturator externus zu vergleichen, wobei er seinen Ursprung im VerhÄltniss zum Menschen nur bedeutend weiter auf das Os ischii und Os pubis distalwÄrts ausgedehnt hÄtte und so allerdings der Lage nach mit Ausnahme seines Insertionsdrittels ein 'internus' geworden wÄre."

Since Gadow gives different homologues for M. obturator in two of his works (1880 and 1891), one would suspect that he had changed his opinion in the interim; however, there is no evidence that he did so. In 1880 he gives supporting evidence (quoted above) for his view; in 1891 he does not. After describing (1891:173) how the origin of M. obturator in bird ancestors presumably migrated from a location outside the pelvis to a position inside the pelvis prior to the meeting of the pubis and ischium external to the muscle, he states: "Eine Ähnliche Entwicklung ist fÜr den Obturator internus der SÄugethiere anzunehmen, welchem der M. obturator der VÖgel entspricht." A similar development in mammals is impossible, owing to the different relationship of the muscle to the pelvic bones in this class. Gadow says nothing more about the mammalian homologue of M. obturator. In view of this discrepancy, Gadow can hardly be considered as a supporter of the idea that the avian M. obturator is homologous with the mammalian obturator internus.

The evidence is conclusive, it seems to me, that the obturator internus of Hudson is not homologous with the mammalian obturator internus. Therefore, the term obturator internus is inappropriate for the avian muscle and must be abandoned. I shall follow Howell (1938) in naming the entire obturator complex M. obturator. This term, of course, is not used in the sense in which it is used by Gadow. The use of the term obturator externus for the entire complex is avoided because it may not correspond exactly to the mammalian obturator externus. As mentioned previously, Romer considers the avian muscle to be homologous not only with the mammalian obturator externus but also with the quadratus femoris.

I am following the policy of Wilcox (1948) and Berger (1952) in latinizing the term anterior, changing it to anticus. When preceded by the feminine word pars, the feminine ending is used (antica).

In table 1 my terminology is compared with that of Fisher and Goodman (1955), Howell (1938), Hudson (1937), and Gadow (1891). The terminology of Fisher (1946) is identical with that of Fisher and Goodman (1955) except that in his earlier work Fisher did not describe or name M. femoritibialis externus, and M. lumbricales of his earlier work is not mentioned in his later work. The terminology of Hudson, et al. (1959) is identical with that of Hudson (1937) except that the manner in which the femoritibialis complex is subdivided is identical with that of Gadow (1891) and different from that in Hudson's earlier work; also the abbreviations p. ext. and p. int. are substituted in his later paper for pars anterior and pars posterior, respectively, of M. adductor longus et brevis.