Chapter I: Part 1

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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY

Volume 8, No. 2, pp. 157-211, figures 1-23, 4 tables

---------------------- November 15, 1954 ----------------------

Myology and Serology
of the Avian Family Fringillidae,
A Taxonomic Study

BY
WILLIAM B. STALLCUP

UNIVERSITY OF KANSAS
LAWRENCE
1954

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Robert W. Wilson

Volume 8, No. 2, pp. 157-211, figures 1-23, 4 tables
Published November 15, 1954

UNIVERSITY OF KANSAS
Lawrence, Kansas

PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1954
[Union Label]
25-4632

Myology and Serology
of the Avian Family Fringillidae,
A Taxonomic Study

BY
WILLIAM B. STALLCUP

CONTENTS

PAGE

INTRODUCTION 160

MYOLOGY OF THE PELVIC APPENDAGE 162
General Statement 162
Materials and Methods 163
Description of Muscles 164
Discussion of Myological Investigations 175

COMPARATIVE SEROLOGY 185
General Statement 185
Preparation of Antigens 186
Preparation of Antisera 188
Methods of Serological Testing 188
Experimental Data 190
Discussion of Serological Investigations 190

CONCLUSIONS 201

SUMMARY 208

LITERATURE CITED 210

INTRODUCTION

The relationships of many groups of birds within the Order Passeriformes are poorly understood. Most ornithologists agree that some of the passerine families of current classifications are artificial groups. These artificial groupings are the result of early work which gave chief attention to readily adaptive external structures. The size and shape of the bill, for example, have been over-emphasized in the past as taxonomic characters. It is now recognized that the bill is a highly adaptive structure and that it frequently shows convergence and parallelism.

Since studies of external morphology have failed in some cases to provide a clear understanding of the relationships of passerine birds, it seems appropriate that attention be given to other morphological features, to physiological features, and to life history studies in an attempt to find other clues to relationships at the family and subfamily levels.

This paper reports the results of a study of the relationships of some birds of the Family Fringillidae and is based on the comparative myology of the pelvic appendage and on the comparative serology of saline-soluble proteins. Where necessary for comparative purposes, birds from other families have been included in these investigations.

It has long been recognized that the Fringillidae include dissimilar groups. Recent work by Beecher (1951b, 1953) on the musculature of the jaw and by Tordoff (1954) primarily on the structure of the bony palate has emphasized the artificial nature of the assemblage although these authors disagree regarding major divisions within it (see below).

The Fringillidae have been distinguished from other families of nine-primaried oscines by only one character--a heavy and conical bill (for crushing seeds). Bills of this form have been developed independently in several other, unrelated, groups; as Tordoff (1954:7) has pointed out, _Molothrus_ of the Family Icteridae, _Psittorostra_ of the Family Drepaniidae, and most members of the Family Ploceidae have bills as heavy and conical as those of the fringillids. The ploceids are distinguished from the fringillids by a single external character: a fairly well-developed tenth primary whereas in fringillids the tenth primary is absent or vestigial. Tordoff (1954:20) points out, however, that this distinction is of limited value since in other passerine families the tenth primary may be present in some species of a genus and absent in others. The Genus _Vireo_ is an example. Furthermore, at least one ploceid (_Philetairus_) has a small, vestigial tenth primary, whereas some fringillids (_Emberizoides_, for example) possess a tenth primary which is rather large and ventrally placed (Chapin, 1917:253-254). Thus, it is obvious that studies based on other features are necessary in order to attain a better understanding of the relationships of the birds involved.

Sushkin's studies (1924, 1925) of the structure of the bony and horny palates have served as a basis for the division of the Fringillidae into as many as five subfamilies (Hellmayr, 1938:v): Richmondeninae, Geospizinae, Fringillinae, Carduelinae, and Emberizinae.

Beecher (1951b:280) points out that "the richmondenine finches arise so uninterruptedly out of the tanagers that ornithologists have had to draw the dividing line between the two groups arbitrarily." His study of pattern of jaw-musculature substantiates this. He states further that the cardueline finches arise without disjunction from the tanagers. He suggests, therefore, that the two groups of "tanager-finches" be made subfamilies of the Thraupidae and that a third subfamily be maintained for the more typical tanagers. He states that the emberizine finches are of different origin, arising from the wood warblers (1953:307). Beecher (1951a:431; 1953:309) includes the Dickcissel, _Spiza americana_, in the Family Icteridae, chiefly on the basis of jaw muscle-pattern and the horny palate.

Tordoff (1954:10-11) presents evidence that the occurrence of palato-maxillary bones in nine-primaried birds indicates relationship among the forms possessing them. He points out that all fringillids except the Carduelinae possess palato-maxillaries that are either free or more or less fused to the prepalatine bar. He points out also that in all carduelines, the prepalatine bar is flared at its juncture with the premaxilla, and that the mediopalatine processes are fused across the midline; noncardueline fringillids lack these characteristics. In addition to the above he cites differences between the carduelines and the "other" fringillids in the appendicular skeletons, in geographic distribution, in patterns of migration, and in habits. Tordoff concludes, therefore, that the carduelines are not fringillids but ploceids, their closest affinities being with the ploceid Subfamily Estrildinae. On the basis of palatal structure, the Fringillinae and Geospizinae are combined with the Emberizinae, the name Fringillinae being maintained for the subfamily. The tanagers merge with the Richmondeninae on the one hand and with the Fringillinae on the other. On this basis, Tordoff (1954:32) suggests that the Family Fringillidae be divided into subfamilies as follows: Richmondeninae, Thraupinae, and Fringillinae. The carduelines are placed as the Subfamily Carduelinae in the Family Ploceidae.

From the foregoing, it is apparent that the two most recent lines of research have given rise to conflicting theories regarding relationships within the Family Fringillidae. The purpose of my investigation, therefore, has been to gather information, from other fields, which might clarify the relationships of these birds.

Since the muscle pattern of the leg in the Order Passeriformes is thought to be one of long standing and slow change, any variation which consistently distinguishes one group of species from another could be significant. With the hope that such variation might be found, a study of the comparative myology of the legs was undertaken.

The usefulness of comparative serology as a means of determining relationship has been demonstrated in many investigations. Its use in this instance was undertaken for several reasons: comparative serology has its basis in biochemical systems which seem to evolve slowly; its methods are objective; and its use has, heretofore, resulted in the accumulation of data which seem compatible, in most instances, with data obtained from other sources.

I acknowledge with pleasure the guidance received in this study from Prof. Harrison B. Tordoff of the University of Kansas. I am indebted also to Prof. Charles A. Leone without whose direction and assistance the serological investigations would not have been possible; to Professors E. Raymond Hall and A. Byron Leonard whose suggestions and criticisms have been most helpful in the preparation of this paper; and to T. D. Burleigh of the U. S. Fish and Wildlife Service for gifts of several specimens used in this work. Assistance with certain parts of the study were received from a contract (NR163014) between the Office of Naval Research of the United States Navy and the University of Kansas.

MYOLOGY OF THE PELVIC APPENDAGE

General Statement

In an excellent paper in which the muscles of the pelvic appendage of birds are carefully and accurately described, Hudson (1937) reviewed briefly the more important literature pertaining to the musculature of the leg which had been published to that date. A review of such information here, therefore, seems unnecessary.

Myological formulae suggested by Garrod (1873, 1874) have been extensively used by taxonomists as aids in characterizing the orders of birds. Relatively few investigations, however, involving the comparative myology of the leg have been undertaken at family and subfamily levels. The works of Fisher (1946), Hudson (1948), and Berger (1952) are notable exceptions.

The terminology for the muscles used in this paper follows that of Hudson (1937), except that I have followed Berger (1952) in Latinizing all names. Homologies are not given since these are reviewed by Hudson. Osteological terms are from Howard (1929).

Materials and Methods

Specimens were preserved in a solution of one part formalin to eight parts of water. Thorough injection of all tissues was necessary for satisfactory preservation. Most of the down and contour feathers were removed to allow the preservative to reach the skin.

In preparing specimens for study, the legs and pelvic girdle were removed and washed in running water for several hours to remove much of the formalin. They were then transferred to a mixture of 50 per cent alcohol and a small amount of glycerine.

All specimens were dissected with the aid of a low power binocular microscope. Where possible, several specimens of each species were examined for individual differences. Such differences were found to be slight, involving mainly size and shape of the muscles. The size is dependent partly on the age of the bird, muscles from older birds being larger and better developed. The shape of a muscle (whether long and slender or short and thick) is due in part to the position in which the leg was preserved; that is to say, a muscle may be extended in one bird and contracted in another. For these reasons, descriptions and comparisons are based mainly on the origin and insertion of a muscle and on its position in relation to adjoining muscles.

Birds dissected in this study are listed below (in the order of the A. O. U. Check-List):

SPECIES

_Vireo olivaceus_ (Linnaeus) _Leucosticte tephrocotis_ _Seiurus motacilla_ (Vieillot) (Swainson) _Passer domesticus_ (Linnaeus) _Spinus tristis_ (Linnaeus) _Estrilda amandava_ (Linnaeus) _Loxia curvirostra_ Linnaeus _Poephila guttata_ (Reichenbach) _Chlorura chlorura_ (Audubon) _Icterus galbula_ (Linnaeus) _Pipilo erythrophthalmus_ _Molothrus ater_ (Boddaert) (Linnaeus) _Piranga rubra_ (Linnaeus) _Calamospiza melanocorys_ _Richmondena cardinalis_ (Linnaeus) Stejneger _Guiraca caerulea_ (Linnaeus) _Chondestes grammacus_ (Say) _Passerina cyanea_ (Linnaeus) _Junco hyemalis_ (Linnaeus) _Spiza americana_ (Gmelin) _Spizella arborea_ (Wilson) _Hesperiphona vespertina_ (Cooper) _Zonotrichia querula_ (Nuttall) _Carpodacus purpureus_ (Gmelin) _Passerella iliaca_ (Merrem) _Pinicola enucleator_ (Linnaeus) _Calcarius lapponicus_ (Linnaeus)

Description of Muscles

The descriptions which follow are those of the muscles in the leg of the Red-eyed Towhee, _Pipilo erythrophthalmus_. Differences between species, where present, are noted for each muscle. The term thigh is used to refer to the proximal segment of the leg; the term crus is used for that segment of the leg immediately distal to the thigh.

_+Musculus iliotrochantericus posticus+_ (Fig. 2).--The origin of this muscle is fleshy from the entire concave lateral surface of the ilium anterior to the acetabulum. The fibers converge posteriorly, and the muscle inserts by a short, broad tendon on the lateral surface of the femur immediately distal to the trochanter. It is the largest muscle which passes from the ilium to the femur.

Action.--Moves femur forward and rotates it anteriorly.

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

_+Musculus iliotrochantericus anticus+_ (Fig. 3).--Covered laterally by the _m. iliotrochantericus posticus_, this slender muscle has a fleshy origin from the anteroventral edge of the ilium between the origins of the _m. sartorius_ anteriorly and the _m. iliotrochantericus medius_ posteriorly. The _m. iliotrochantericus anticus_ is directed caudoventrally and inserts by a broad, flat tendon on the anterolateral surface of the femur between the heads of the _m. femorotibialis externus_ and _m. femorotibialis medius_ and just distal to the insertion of the _m. iliotrochantericus medius_.

Action.--Moves femur forward and rotates it anteriorly.

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

_+Musculus iliotrochantericus medius+_ (Fig. 3).--Smallest of the three _iliotrochantericus_ muscles, this bandlike muscle has a fleshy origin from the ventral edge of the ilium just posterior to the origin of the _m. iliotrochantericus anticus_. The fibers are directed caudoventrally, and the insertion is tendinous on the anterolateral surface of the femur between the insertion of the other two _iliotrochantericus_ muscles.

Action.--Moves femur forward and rotates it anteriorly.

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

_+Musculus iliacus+_ (Figs. 4, 5).--Arising from a fleshy origin on the ventral edge of the ilium just posterior to the origin of the _m. iliotrochantericus medius_, this small slender muscle passes posteroventrally to its fleshy insertion on the posteromedial surface of the femur just proximal to the origin of the _m. femorotibialis internus_.

Action.--Moves femur forward and rotates it posteriorly.

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

_+Musculus sartorius+_ (Figs. 1, 4).--A long, straplike muscle, the _sartorius_ forms the anterior edge of the thigh. The origin is fleshy, half from the anterior edge of the ilium and from the median dorsal ridge of this bone and half from the posterior one or two free dorsal vertebrae. The insertion is fleshy along a narrow line on the anteromedial edge of the head of the tibia and on the medial region of the patellar tendon.

Action.--Moves thigh forward and upward and extends shank.

Comparison.--In _Loxia_ and _Spinus_, only one-third of the origin is from the last free dorsal vertebra. In _Hesperiphona_, _Carpodacus_, _Pinicola_, and _Leucosticte_, only one-fifth of the origin is from this vertebra.

_+Musculus iliotibialis+_ (Fig. 1).--Broad and triangular, this muscle covers most of the deeper muscles of the lateral aspect of the thigh. The middle region is fused with the underlying _femorotibialis_ muscles. In the distal half of this muscle there are three distinct parts; the anterior and posterior edges are fleshy and the central part is aponeurotic. The origin is from a narrow line along the iliac crests--from the origin of the _m. sartorius_, anteriorly, to the origin of the _m. semitendinosus_ posteriorly. The origin is aponeurotic in the preacetabular region but fleshy in the postacetabular region. The distal part of the muscle is aponeurotic and joins with the _femorotibialis_ muscles in the formation of the patellar tendon. This tendon incloses the patella and inserts on a line along the proximal edges of the cnemial crests of the tibiotarsus.

Action.--Extends crus.

Comparison.--In _Vireo_ the central aponeurotic portion of this muscle is absent.

_+Musculus femorotibialis externus+_ (Fig. 2).--Covering the lateral and anterolateral surfaces of the femur, this large muscle has a fleshy origin from the lateral edge of the proximal three-fourths of the femur. The origin separates the insertion of the _m. iliotrochantericus anticus_ from that of the _m. ischiofemoralis_ and, in turn, is separated from the origin of the _m. femorotibialis medius_ by the insertions of the _m. iliotrochantericus anticus_ and _m. iliotrochantericus medius_. Approximately midway of the length of the femur this muscle fuses anteromesially with the _m. femorotibialis medius_. Distally, the _m. femorotibialis externus_ contributes to the formation of the patellar tendon which inserts on a line along the proximal edges of the cnemial crests of the tibiotarsus.

Action.--Extends crus.

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

_+Musculus femorotibialis medius+_ (Figs. 2, 4).--The origin of this muscle, which lies along the anterior edge of the femur, is fleshy from the entire length of the femur proximal to the level of attachment of the proximal arm of the biceps loop. Laterally this muscle is completely fused for most of its length with the _m. femorotibialis externus_ and contributes to the formation of the patellar tendon, which inserts on a line along the proximal edges of the cnemial crests of the tibiotarsus. Many of the fibers, nevertheless, insert on the proximal edge of the patella.

Action.--Extends crus.

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

_+Musculus femorotibialis internus+_ (Fig. 4).--One of the most superficial muscles lying on the medial surface of the thigh, this muscle is divided, especially near the distal end, into two parts, lateral and medial. The origin of the lateral part is fleshy from a line on the medial surface of the femur; the origin begins proximally at a point near the insertion of the _m. iliacus_. The medial, bulkier part of the muscle has a fleshy origin on the medial surface of the lower one-third of the femur. The two parts fuse to some extent above the points of insertion and insert on the medial edge of the head of the tibia.

Action.--Rotates tibia anteriorly.

Comparison.--Two parts of this muscle variously fused; otherwise, no significant differences in the species studied.

_+Musculus piriformis+_ (Fig. 3).--This muscle is represented by the _pars caudifemoralis_ only, the _pars iliofemoralis_ being absent in passerine birds as far as is known. The _pars caudifemoralis_ is flat, somewhat spindle-shaped, and passes anteroventrally from the pygostyle to the femur. The origin is tendinous from the anteroventral edge of the pygostyle, and the insertion is semitendinous on the posterolateral surface of the shaft of the femur about one-fourth its length from the proximal end.

Action.--Moves femur posteriorly and rotates it in this direction; moves tail laterally and depresses it.

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

_+Musculus semitendinosus+_ (Figs. 2, 3, 5).--The origin from the extreme posterior edge of the posterior iliac crest of the ilium is fleshy and is aponeurotic from the last vertebra of the synsacrum and the transverse processes of several caudal vertebrae. The straplike belly passes along the posterolateral margin of the thigh. Immediately posterior to the knee, the muscle is divided transversely by a ligament. That portion passing anteriorly from the ligament is the _m. accessorius semitendinosi_ (here considered a part of the _m. semitendinosus_) and is discussed below. The ligament continues distally in two parts; one part inserts on the medial surface of the _pars media_ of the _m. gastrocnemius_ and the other part fuses with the tendon of insertion of the _m. semimembranosus_.

The _m. accessorius semitendinosi_ extends anteriorly from the above mentioned ligament to a fleshy insertion on the posterolateral surface of the femur immediately proximal to the condyles.

Action.--Moves femur posteriorly, flexes the crus and aids in extending the tarsometatarsus.

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

_+Musculus semimembranosus+_ (Figs. 3, 4, 5).--This straplike muscle passes along the posteromedial surface of the thigh. The origin is semitendinous along a line on the ischium, from a point dorsal to the middle of the ischiopubic fenestra to the posterior end of the ischium, and from a small area of the abdominal musculature posterior to the ischium. The insertion is by means of a broad, thin tendon on a ridge on the medial surface of the tibia immediately distal to the head of this bone. The tendon of insertion passes between the head of the _pars media_ and _pars interna_ of the _m. gastrocnemius_ and is fused with the tendon of the _m. semitendinosus_.

Action.--Flexes crus.

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

_+Musculus biceps femoris+_ (Fig. 2).--Long, thin, and somewhat triangular, this muscle lies on the lateral side of the thigh just underneath the _m. iliotibialis_. Its origin is from a line along the anterior and posterior iliac crests underneath the origin of the _m. iliotibialis_. Anterior to the acetabulum the origin is aponeurotic, and the edge of this aponeurosis passes over the proximal end of the femur. The origin posterior to the acetabulum is fleshy. The most anterior point of origin is difficult to ascertain but it lies near the center of the anterior iliac crest. The most posterior point of origin is immediately dorsal to the posterior end of the ilioischiatic fenestra. Behind the knee the fibers of this muscle converge to form the strong tendon of insertion which passes through the biceps loop, under the tendon of origin of the _m. flexor perforatus digiti II_, and inserts on a small tubercle on the posterolateral edge of the fibula at the point of the tibia-fibula fusion.

The biceps loop is tendinous and the distal end attaches to a protuberance on the posterolateral edge of the femur at the proximal edge of the external condyle. The proximal end attaches to the anterolateral edge of the femur immediately proximal to the distal end of the loop, which extends posterior to the femur. The distal arm of this loop is connected with the tendon of origin of the _m. flexor perforatus digiti II_ by a strong tendon.

Action.--Flexes crus.

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

_+Musculus ischiofemoralis+_ (Fig. 3).--Short and thick, this muscle arises directly from the lateral surface of the ischium between the posterior iliac crest and the ischiopubic fenestra. The area of origin extends to the posterior edge of the ischium. The insertion is tendinous on the lateral surface of the trochanter opposite the insertion of the _m. iliotrochantericus medius_.

Action.--Moves femur posteriorly and rotates it in this direction.

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

_+Musculus obturator internus+_ (Figs. 4, 7).--Lying on the inside of the pelvis and covering the medial surface of the ischiopubic fenestra, is this flat, pinnate, leaf-shaped muscle. The origin is fleshy and is from the ischium and pubis around the edges of this fenestra; none of the fibers arises from the membrane stretched across the fenestra. Anteriorly the fibers converge and form a strong tendon that passes through the obturator foramen and inserts on the posterolateral surface of the trochanter of the femur.

Action.--Rotates femur posteriorly.

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

_+Musculus obturator externus+_ (Fig. 7).--Short and fleshy, this muscle consists of two parts which are not easily separable but which may be traced throughout its length. The parts are more nearly distinct at the origin. The dorsal part arises directly from the ischium along the dorsal edge of the obturator foramen. The larger ventral part arises directly from the anterior and ventral edges of the obturator foramen. The fibers of the dorsal part pass anteriorly, cover the tendon of the _m. obturator internus_ laterally, and insert on the trochanter around the point of insertion of the latter muscle. The fibers of the ventral part pass parallel with the tendon of the _m. obturator internus_ and insert on the trochanter immediately distal and posterior to the tendon of the latter muscle.

Action.--Rotates femur posteriorly.

Comparison.--In _Passer_, _Estrilda_, _Poephila_, _Hesperiphona_, _Carpodacus_, _Pinicola_, _Leucosticte_, _Spinus_ and _Loxia_, this muscle is undivided and, in its position, origin, and insertion, resembles the ventral part of the bipartite muscle described above. The origin is from the anterior and ventral edges of the obturator foramen and the insertion is on the trochanter of the femur immediately distal and posterior to the insertion of the _m. obturator internus_. In all other genera examined, the muscle is bipartite. In _Chlorura_ the dorsal part is larger and better developed than it is in the other genera.

_+Musculus adductor longus et brevis+_ (Figs. 3, 4, 5).--Consisting of two distinct, straplike parts, this large muscle lies on the medial surface of the thigh, posterior to the femur.

The _pars anticus_ has a semitendinous origin on a line that extends posteriorly from the posteroventral edge of the obturator foramen to a point half way across the membrane that covers the ischiopubic fenestra. The insertion is fleshy along the posterior surface of the femur from the level of the insertion of the _m. piriformis_ distally to the medial surface of the internal condyle.

The _pars posticus_ originates by a broad, flat tendon on a line across the posterior half of the membrane that covers the ischiopubic fenestra. The insertion is at the point of origin of the _pars media_ of the _m. gastrocnemius_ on the posteromedial surface of the proximal end of the internal condyle of the femur. There is a broad tendinous connection with the proximal end of the _pars media_ of the _m. gastrocnemius_. The anterior edge of the _pars posticus_ is overlapped medially by the posterior edge of the _pars anticus_.

Action.--Flexes thigh; may flex crus also and may extend tarsometatarsus.

Comparison.--In _Vireo olivaceous_, the origin of this muscle does not extend the length of the ischiopubic fenestra. The origin, furthermore, is along the dorsal edge of the ischiopubic fenestra and not from the membrane covering the fenestra. Finally, in this species, the origin of the _pars posticus_ is fleshy.

_+Musculus tibialis anticus+_ (Figs. 2, 5).--Lying along the anterior edge of the crus, a part of this muscle is covered by the _m. peroneus longus_. The origin is by two distinct heads, each of which is pinnate. The anterior head arises directly from the edges of the outer and inner cnemial crests. The posterior head arises by a short, strong tendon from a small pit on the anterodistal edge of the external condyle of the femur. This tendon and the proximal end of the muscle pass between the head of the fibula and the outer cnemial crest. The two heads of the muscle fuse at a place slightly more than one-half of the distance down the crus. At the distal end of the crus this muscle gives rise to a strong tendon which passes under a fibrous loop immediately proximal to the external condyle in company with the _m. extensor digitorum longus_ and which passes between the condyles of the tibia and inserts on a tubercle on the anteromedial edge of the proximal end of the tarsometatarsus.

Action.--Flexes tarsometatarsus.

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

_+Musculus extensor digitorum longus+_ (Figs. 3, 5, 8).--Slender and pinnate, this muscle lies along the anteromedial surface of the tibia. The origin is fleshy from most of the region between the cnemial crests and from a line along the anterior surface of the proximal fourth of the tibia. Approximately two-thirds of the distance down the crus the muscle gives rise to the tendon of insertion which passes through the fibrous loop near the distal end of the tibia in company with the _m. tibialis anticus_. The tendon then passes along beneath the supratendinal bridge at the distal end of the tibia, traverses the anterior intercondylar fossa, and passes beneath a bony bridge on the anteromedial surface of the proximal end of the tarsometatarsus. The tendon continues along the anterior surface of the tarsometatarsus to a point immediately above the bases of the toes and there gives rise to three branches, one to the anterior surface of each foretoe. The insertions of each branch are on the anterior surfaces of the phalanges as shown in Fig. 8.

Action.--Extends foretoes.

Comparison.--This muscle is weakly developed in _Leucosticte_ and _Calvarius_; the belly is slender and extends only half way down the crus before giving rise to the tendon of insertion. The functional significance of this variation is difficult to understand. The convergence in muscle pattern shown by these two genera, however, is in all probability the result of similarities in behavior patterns. These birds perch less frequently than do the other birds studied. Thus, the toes are neither flexed nor extended as often; the smaller size of the _m. extensor digitorum longus_ may have resulted in part from this lessened activity. Except for the variations just noted, there are no significant differences among the species studied; even the rather complex patterns of insertion are identical.

_+Musculus peroneus longus+_ (Fig. 1).--Relatively thin and straplike, this muscle lies on the anterolateral surface of the crus and is intimately attached to the underlying muscles. The part of the origin from the proximal edges of the inner and outer cnemial crests is semitendinous but the part of the origin from the lateral edge of the shaft of the fibula is tendinous. Approximately two-thirds the distance down the crus the muscle gives rise to the tendon of insertion. Immediately above the external condyle of the tibiotarsus this tendon divides. The posterior branch inserts on the proximal end of the lateral edge of the tibial cartilage. The anterior branch passes over the lateral surface of the external condyle to the posterior surface of the tarsometatarsus and there unites with the tendon of the _m. flexor perforatus digiti III_.

Action.--Extends tarsometatarsus and flexes third digit.

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

_+Musculus peroneus brevis+_ (Figs. 2, 3).--Lying along the anterolateral surface of the tibia, this slender, pinnate muscle arises from a fleshy origin along this surface and along the anterior surface of the fibula from a point immediately proximal to the insertion of the _m. biceps femoris_ to a point approximately two-thirds of the way down the crus. Near the distal end of the tibia the muscle gives rise to the tendon of insertion that passes through a groove on the anterolateral edge of the tibia just above the external condyle. Here the tendon is held in place by a broad fibrous loop and passes under the anterior branch of the tendon of insertion of the _m. peroneus longus_ and inserts on a prominence on the lateral edge of the proximal end of the tarsometatarsus.

Action.--Extends tarsometatarsus and may abduct it slightly.

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

_+Musculus gastrocnemius+_ (Figs. 1, 4).--The largest muscle of the pelvic appendage, it covers superficially all of the posterior surface, most of the medial surface, and half of the lateral surface of the crus. The muscle originates by three distinct heads.

The _pars externa_ covers the posterolateral surface of the crus, is intermediate in size between the other two heads, and arises by a short, strong tendon from a small bony protuberance on the posterolateral side of the distal end of the femur immediately proximal to the fibular condyle. The tendon is intimately connected with the distal arm of the loop for the _m. biceps femoris_.

The _pars media_ is the smallest of the three heads and lies on the medial surface of the crus. The head of the _pars media_ is separated from the _pars interna_ by the tendon of insertion of the _m. semimembranosus_ and originates by a short, strong tendon from the posteromedial surface of the proximal end of the internal condyle of the femur. The proximal portion of the _pars media_ has tendinous connections with the tendon of the _m. semitendinosus_ and with the _pars posticus_ of the _m. adductor longus et brevis_.

The _pars interna_ is the largest of the three heads and covers most of the medial surface of the crus. This head in its proximal portion is distinctly divided into anterior and posterior parts, the former overlapping the latter medially. The origin of the posterior part is fleshy from the anterior half of the tibial head. Some of the fibers of the anterior part arise directly from the inner cnemial crest while its remaining fibers arise from the patellar tendon (Fig. 1) and form a band that extends around the anterior surface of the knee, covering the insertion of the _m. sartorius_.

Approximately half way down the crus, the three heads give rise to the tendon of insertion, the _tendo achillis_, which passes over and is tightly bound to the posterior surface of the tibial cartilage. The insertion is tendinous on the posterior surface of the hypotarsus and along the posterolateral ridge of the tarsometatarsus. This tendon seems to be continuous with a fascia which forms a sheath around the posterior surface of the tarsometatarsus holding the other tendons of this region firmly in the posterior sulcus.

Action.--Extends tarsometatarsus.

Comparison.--Study of the _pars externa_ and _pars media_ reveals no significant differences among the species dissected. The _pars interna_, however, is subject to some variation which is described below.

_Pars interna_ bipartite

_Vireo_ _Chlorura_
_Seiurus_ _Pipilo_
_Icterus_ _Calamospiza_
_Molothrus_ _Chondestes_
_Piranga_ _Junco_
_Richmondena_ _Spizella_
_Guiraca_ _Zonotrichia_
_Passerina_ _Passerella_
_Spiza_ _Calcarius_

The two parts of the _m. gastrocnemius_ are most distinct in _Vireo_. _Icterus_, _Molothrus_, _Richmondena_, _Guiraca_, and _Passerina_ lack the fibrous band that passes around the front of the knee. In _Spiza_ this band of fibers is smaller than in the other species.

_Pars interna_ undivided

_Passer_ _Pinicola_
_Estrilda_ _Leucosticte_
_Poephila_ _Spinus_
_Hesperiphona_ _Loxia_
_Carpodacus_

In _Leucosticte_, although the _pars interna_ is undivided, there is a band of fibers which extends around the front of the knee (see discussion, p. 183).

_+Musculus plantaris+_ (Fig. 5).--Small and slender, this muscle lies on the posteromedial surface of the crus, beneath the _pars interna_ of the _m. gastrocnemius_ and originates by fleshy fibers from the posteromedial surface of the proximal end of the tibia immediately distal to the internal articular surface. The belly extends approximately one-sixth of the way down the crus and gives rise to a long, slender tendon that inserts on the proximomedial edge of the tibial cartilage.

Action.--Extends tarsometatarsus.

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

_+Musculus flexor perforatus digiti II+_ (Figs. 3, 9).--This is a slender muscle which lies on the lateral side of the crus beneath the _pars externa_ of the _m. gastrocnemius_ and is intimately connected anteromedially with the _m. flexor digitorum longus_ and posteromedially with the _m. flexor hallucis longus_. The origin is by a strong tendon from the lateral surface of the external condyle of the femur at the point of origin of the _m. flexor perforans et perforatus digiti II_. This tendon serves also as the origin of the anterior head of the _m. flexor hallucis longus_. The tendon connects also by a broad tendinous band with the distal arm of the loop for the _m. biceps femoris_ and by a similar band with the lateral edge of the fibula immediately distal to the head. The tendon of insertion passes distally, perforates the tibial cartilage near its lateral edge, traverses the middle medial canal of the hypotarsus (Fig. 6), and passes distally to the foot. At the distal end of the tarsometatarsus the tendon is held against the medial surface of the first metatarsal by a straplike sheath. The tendon then passes over a sesamoid bone between the first metatarsal and the base of the second digit and is bound to this bone by a sheath. The tendon inserts mainly along the posteromedial edge of the proximal end of the first phalanx of the second digit, although the termination is sheathlike and covers the entire posterior surface of this phalanx. This sheathlike termination is perforated by the tendons of the _m. flexor perforans et perforatus digiti II_ and the branch of the _m. flexor digitorum longus_ that inserts on the second digit.

Action.--Flexes second digit.

Comparison.--In _Vireo_ this muscle is larger and more deeply situated than it is in the other species examined and has no connection with the _m. flexor hallucis longus_.

_+Musculus flexor perforatus digiti III+_ (Fig. 5).--Long and flattened, this muscle lies on the posteromedial side of the crus beneath the _m. gastrocnemius_. The belly is tightly fused laterally with the belly of the _m. flexor hallucis longus_ and posteriorly with the belly of the _m. flexor perforatus digiti IV_. The origin is by a long, strong tendon from a small tubercle just medial to, and at the proximal end of, the external condyle of the femur. Below the middle of the crus this muscle terminates in a strong tendon which perforates the tibial cartilage near its lateral edge. In this region the tendon is sheathlike and wrapped around the tendon of the _m. flexor perforatus digiti IV_. These two tendons together pass through the posterolateral canal of the hypotarsus (Fig. 6). Immediately distal to the hypotarsus the two tendons separate, and the tendon of the _m. flexor perforatus digiti III_ receives a branch of the tendon of the _m. peroneus longus_. The tendon passes distally over the surface of the second trochlea, and its insertion is sheathlike on the posterior surface of the first phalanx, and on the proximal end of the second. In the area of insertion this tendon is perforated by that of the _m. flexor perforans et perforatus digiti III_ and by that of the _m. flexor digitorum longus_ to the third digit.

Action.--Flexes digit III.

Comparison.--In _Passer_, _Estrilda_, _Poephila_, _Hesperiphona_, _Carpodacus_, _Pinicola_, _Leucosticte_, _Spinus_, and _Loxia_ the edges of the sheathlike tendon are thickened at the points of insertion, so that the tendon appears to have two branches which insert along the posterolateral edges of the first phalanx and are connected medially by a fascia.

_+Musculus flexor perforatus digiti IV+_ (Fig. 3).--Extending along the posterior edge of the crus, this slender muscle lies beneath the _m. gastrocnemius_. The belly is fused with those of the _m. flexor hallucis longus_ and _m. flexor perforatus digiti III_. Its origin is fleshy from the intercondyloid region of the distal end of the femur and has a few fibers arising from the tendon of origin of the _m. flexor perforatus digiti III_. Near the distal end of the crus the muscle gives rise to the strong tendon of insertion which perforates the tibial cartilage near its lateral edge and in this region is ensheathed by the tendon of the _m. flexor perforatus digiti III_. The two tendons pass together through the posterolateral canal of the hypotarsus (Fig. 6). The tendon continues distally along the tarsometatarsus and the posterior surface of digit IV. The tendon bifurcates at approximately the middle of the first phalanx. A short lateral branch inserts on the posterolateral edge of the proximal end of the second phalanx. The long medial branch is perforated by a branch of the _m. flexor digitorum longus_; the distal end is flattened, has thickened edges, and inserts over the posterior surfaces of the distal end of the second phalanx, and over the proximal end of the third phalanx.

Action.--Flexes digit IV.

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_.

The _m. obturator externus_ is bipartite, consisting of dorsal and ventral parts, in the passerine 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 associated with the bipartite 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 mass of the bipartite 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 canal of hypotarsus
Anteromed. can. Anteromedial canal 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 canal 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 canal of hypotarsus
Posteromed. can. Posteromedial canal of hypotarsus
Sar. _M. sartorius_
Semim. _M. semimembranosus_
Semit. _M. semitendinosus_
Tib. ant. _M. tibialis anticus_
Tib. cart. Tibial cartilage

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-passerine 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 bipartite condition. The greater mass of the bipartite muscle probably makes possible a stronger extension of the tarsometatarsus.

Thus, the divided or undivided conditions of the _m. obturator externus_ and the _pars interna_ of the _m. gastrocnemius_ seem to be correlated with the degrees of strength of certain movements of the leg. It is conceivable that these differences in structure are correlated with the manner in which food is obtained, the birds having the bipartite muscles being those which spend the most time on the ground searching and scratching for seeds and other sorts of food. Yet, in _Leucosticte_, a cardueline, and in _Calcarius_, an emberizine, whose foraging habits are rather similar, the structure is unlike. _Leucosticte_ does resemble the emberizines and also _Piranga_ and _Spzia_ in the extension of a band of muscle fibers from the _pars interna_ of the _m. gastrocnemius_ around the front of the knee. A band of muscle fibers of this sort strengthens the knee joint and gives still more strength to the _pars interna_. This condition has been reported in a number of birds by Hudson (1937) and is, in all probability, an adaptation for greater strength of certain leg movements. The development of this band in _Leucosticte_ seems to parallel that in the other birds studied and does not indicate relationship, since in _Leucosticte_ this band arises from the undivided muscle which (as stated above) resembles only the posterior portion of the bipartite muscle described for the other birds. In the latter, the muscular band arises from the anterior part of the muscle.

Minor differences in muscle pattern, like those already mentioned, are consistent also between subfamilies, but correlation of these minor differences with function is difficult. There is the implication, however, that in all the groups except the carduelines and ploceids, the emphasis is on greater strength and mobility of the leg. In the carduelines that were studied the origin of the _m. sartorius_ does not extend so far craniad as in the other species. In the latter, at least half of the origin is from the last one or two free dorsal vertebrae; in the carduelines no more than one third of the origin is anterior to the ilium. It is conceivable that the more craniad the origin, the stronger the forward movement of the thigh would be.