University of Kansas Publications


Museum of Natural History



Volume 12, No. 13, pp. 553-573, 7 figs.



March, 2, 1964

Thoracic and Coracoid Arteries In Two Families of Birds, Columbidae and
Hirundinidae

BY

MARION ANNE JENKINSON

University of Kansas

Lawrence

1964



University of Kansas Publications, Museum of Natural History



Editors: E. Raymond Hall, Chairman, Henry S. Fitch,

Theodore H. Eaton, Jr.



Volume 12, No. 13, pp. 553-573, 7 figs.

Published March 2, 1964



University of Kansas

Lawrence, Kansas



PRINTED BY THE STATE PRINTER

TOPEKA, KANSAS

1964

[Pg 555]

Thoracic and Coracoid Arteries In Two Families of Birds, Columbidae and
Hirundinidae

BY

MARION ANNE JENKINSON

CONTENTS

PAGE



Introduction 555



Methods and Materials 556



Myology and Angiology: Hirundinidae 557

Myology 557

Angiology 558



Myology and Angiology: Columbidae 560

Myology 560

Angiology 560



Summary of Arterial Arrangement 562



Discussion and Conclusions 562

Individual Variation 562

Intrafamilial Differences 563

Interfamilial Differences 565



Summary 567



Literature Cited 573

INTRODUCTION

Most descriptions of the circulatory system of birds, largely the work
of Glenny, have dealt with arteries of the neck and thorax in a wide
variety of species. As a result of his work, Glenny offered several
hypotheses concerning the phylogenetic, hence taxonomic, significance of
differences in some of these vessels. He also described six types of
thoracic arterial arrangements and stated that these categories might
represent various levels of evolution (Glenny, 1955:543-544).

The families Columbidae (pigeons) and Hirundinidae (swallows) have two
nearly extreme arterial types described by Glenny, and are universally
acknowledged as monophyletic. Differences within the families,
therefore, can be considered as valid intrafamilial differences. I have
investigated the thoracic and coracoid arteries and their branches in
members of these two families to determine the degree of individual
variability of the vessels, and the possible causes of interspecific and
intrafamilial differences.

[Pg 556]

METHODS AND MATERIALS

All specimens studied are in The University of Kansas Museum of Natural
History. They were preserved in alcohol and their blood vessels were not
injected. Dissections were made with the aid of a binocular microscope
at magnifications of 10× and 20×.

Following is a list of the species studied, the number of individuals of
each species dissected, and the catalogue numbers of the specimens. The
nomenclature and classification are those of the American
Ornithologists' Union's Check-List of North American Birds, fifth
edition (1957).

The following descriptions are of Progne subis and Scardafella inca.
Differences in the vascular system in other members of the families
represented by P. subis and S. inca are mentioned at the appropriate
places. The muscles briefly described for each of these two species are
those that are supplied by the thoracic or coracoid arteries or by
branches of the same, and muscles that, by their origin, location, or
insertion, seem to affect the course or origin of one of these arteries.

The following sources have been particularly useful for the terminology
of muscles and of skeletal features: Ashley (1941), Beddard (1898),
Coues (1903), Howard (1929), Howell (1937), and Hudson and Lanzillotti
(1955).

The names used for most arteries are those in common usage for
vertebrates. I have not used the terms "internal mammary" and
"intercostal" artery as substitutes for "thoracic" artery, except when
referring to the work of others. The vessel's homology with the internal
mammary artery of mammals has been denied (Glenny, 1955:541), and the
name "mammary" is certainly not useful descriptively in birds. The term
"intercostal" is less objectionable, except that such a name may call to
mind segmental vessels arising from the dorsal aorta. The term
"thoracic" seems best, as it is reasonably descriptive, and has been
used by Glenny in the majority of his descriptions covering a wide
variety of birds. The name "sternoclavicular" has been used by others as
a synonym for the "coracoid" artery. I have arbitrarily chosen to use
the latter.

ACKNOWLEDGMENTS

I gratefully acknowledge many valuable suggestions in my research and
the preparation of this manuscript from Professors Theodore H. Eaton, A.
Byron Leonard, Richard F. Johnston, Robert M. Mengel, and E. Raymond
Hall. Mr. Abbot S. Gaunt and Miss Sandra Lovett assisted in collecting
specimens. Final drafts of the illustrations were prepared by Mr. Thomas
Swearingen.

[Pg 557]

MYOLOGY AND ANGIOLOGY: HIRUNDINIDAE

Figs. 1, 2, 3, and 4 illustrate the following muscles and arteries
described for Progne subis.

Myology

M. pectoralis thoracica, Fig. 1. The origin is from slightly less than
the posterior half of the sternum, from the ventral half of the keel,
almost the entire length of the posterolateral surface of the clavicle
and adjacent portion of the sterno-coraco-clavicular membrane, and
tendinously from the ventral thoracic ribs. This massive muscle covers
the entire ventral surface of the thorax and converges to insert on the
ventral side of the humerus on the pectoral surface.

M. supracoracoideus, Fig. 1. The origin is from the dorsal portion of
the keel and medial portion of the sternum, and is bordered ventrally by
the origin of M. pectoralis thoracica, and laterally by M.
coracobrachialis posterior. The origin is also from the manubrium and
the anterolateral portion of the proximal half of the coracoid and to a
slight extent from the sterno-coraco-clavicular membrane adjacent to the
manubrium. This large pinnate muscle converges, passes through the
foramen triosseum, and inserts by a tendon on the external tuberosity of
the humerus, immediately proximal to the insertion of M. pectoralis
thoracica.

M. coracobrachialis posterior, Figs. 1 and 3. The origin is from the
dorsolateral half of the coracoid, anterolateral portion of the sternum
(where the area of origin is bordered medially by M. supracoracoideus,
posteriorly by M. pectoralis thoracica, and laterally by M.
sternocoracoideus), and also to a slight extent from the area of
attachment of the thoracic ribs to the sternum. The muscle fibers
converge along the lateral edge of the coracoid and insert on the median
crest of the humerus immediately proximal to the pneumatic foramen. In
passing from the origin on the sternum to the insertion on the humerus,
the belly of the muscle bridges the angle formed by the costal process
of the sternum and the coracoid.

M. sternocoracoideus, Figs. 2 and 3. The origin is from the entire
external surface of the costal process of the sternum, and to a small
extent from the extreme proximal ends of the thoracic ribs where they
articulate with the costal process. The muscle inserts on a triangular
area on the dorsomedial surface of the coracoid. Like M.
coracobrachialis posterior, this muscle bridges the angle formed by the
costal process and the coracoid.

M. subcoracoideus (ventral head), Figs. 2 and 3. The origin is from
the dorsomedial edge of the coracoid at its extreme proximal end, and to
a slight extent from the adjacent portion of the manubrium. The origin
is medial to the insertion of M. sternocoracoideus. The ventral head
passes anterodorsally along the medial edge of the coracoid and joins
the dorsal head (not here described). The combined muscle then inserts
by a tendon onto the internal tuberosity of the humerus.

M. costi-sternalis, Figs. 1, 2, and 3. The origin is from the anterior
edge of the sternal portion of the first four thoracic ribs. This
triangular muscle narrows and inserts on the posterior edge of the apex
of the costal process. The portion arising from the first rib may share
slips with M. sternocoracoideus.[Pg 558]

M. costi-sternalis anterior, Figs. 1, 2, and 3. This muscle is
variously developed, and originates from a small area on the ventral end
of the vertebral portion of the last cervical rib. The insertion is on
the apex of the costal process, immediately anterior to the insertion of
M. costi-sternalis.

Mm. intercostales externus, Fig. 1. These muscles extend
posteroventrally between the vertebral portions of successive thoracic
ribs, and between the last cervical and first thoracic ribs. In the more
posterior intercostal spaces these muscles are poorly developed, but
they become progressively better developed anteriorly, and are fully
represented in the most anterior intercostal spaces.

Mm. intercostales internus, Fig. 3. These muscles resemble the
external intercostal muscles, but extend anteroventrally, with the
muscles being most fully developed posteriorly, and progressively less
so anteriorly.

Costopulmonary muscles, Fig. 3. This diagonal series of muscle slips
from the thoracic ribs attaches to the aponeurosis covering the lungs.

Angiology

Figs. 3 and 4 show all arteries discussed for this family. The numbers
following the names or descriptions of arteries in the text refer to
numbered arteries in one or both of these figures.

The right and left innominate or brachiocephalic arteries arise from the
aortic trunk and give rise to the common carotid arteries (14). The
major vessel continuing across the thoracic cavity is the subclavian
artery. Classically the subclavian is considered as continuing into the
anterior appendage as the axillary artery. However, in the species
studied, the axillary artery can best be described as a branch from the
subclavian; the pectoral stem forms a more direct continuation of the
subclavian. In traversing the thoracic cavity, the subclavian gives rise
to the thoracic, coracoid, and axillary arteries, and leaves the
thoracic cavity as the pectoral trunk, dorsal to the area where Mm.
coracobrachialis posterior and sternocoracoideus span the angle
formed by the coracoid and costal process.

The pectoral trunk bifurcates into two main pectoral arteries (9), which
penetrate M. pectoralis thoracica. Neither the axillary artery nor
these pectoral arteries were traced in my study.

The coracoid artery (2) arises from the ventral face of the subclavian
(1), either opposite the base of, or medial to, the axillary artery
(10). The coracoid artery passes ventrad between the medial edge of the
coracoid and the ventral head of M. subcoracoideus, and an artery (7)
is given off to supply that muscle. The main vessel then penetrates M.
supracoracoideus and bifurcates or ramifies into several vessels (12).

Between the origin of the coracoid artery from the subclavian, and the
point where the coracoid artery passes the medial edge of the coracoid,
several branches are given off. These vessels are highly variable in
origin, as described below, and not all were always found. Along with
the coracoid artery, they are termed a "coracoid complex."

The first artery (11) of this complex arises from any one of several
places: from the lateral face of the coracoid artery at its base;
independently from the subclavian immediately lateral to the origin of
the coracoid artery; and from the thoracic artery near its origin. This
vessel travels laterad, parallel to the[Pg 559] subclavian, and penetrates M.
coracobrachialis posterior at the same point that the pectoral artery
passes dorsal to that muscle.

Another vessel (common stem of 4 and 5) of the coracoid complex in most
specimens arises from the anterior face of the coracoid artery and
branches into several vessels, some of which (5) supply M.
subcoracoideus, and some of which (4) feed M. coracobrachialis
posterior. The vessel occasionally shares a common stem with the main
vessel (11) to M. coracobrachialis posterior, and in some specimens
arises independently from the subclavian, immediately anterior to the
origin of the coracoid artery. The branch (4) to M. coracobrachialis
posterior was also seen to arise independently from any of the
above-mentioned positions.

Two remaining vessels (6 and 8) are often found as branches from the
coracoid artery. They were small and often were collapsed in the
individuals I dissected, but were most clearly seen in Iridoprocne
bicolor. The vessels occasionally had a common base, and in some
specimens only one vessel was found. The first artery (6) passes mediad
into M. sternocoracoideus, or continues across that muscle onto the
inner face of the sternum. The second vessel (8) also supplies M.
sternocoracoideus or the inner surface of the sternum, and often a
large branch continues across the dorsal surface of the coracoid to M.
coracobrachialis posterior. Fig. 3 shows a composite of these vessels;
not all branches were seen in any one specimen. In the specimen of I.
bicolor a foramen existed on the lateral edge of the coracoid where the
branch (of 8) to M. coracobrachialis posterior passed. An examination
of skeletons of five to 10 individuals each of the five species for
which dissections were made, and of Petrochelidon pyrrhonota (Cliff
Swallow) and Tachycineta thalassina (Violet-green Swallow), in the
University of Kansas collection, showed that most coracoids of these
seven species of swallows had a small notch (as shown in Fig. 4) or a
complete foramen there.

The thoracic artery (3) arises from the subclavian opposite the base of
the coracoid artery, or from the base of the coracoid artery. Of the
five specimens of P. subis dissected, one individual had the former
arrangement on both sides, and one had the latter on both sides, whereas
in the remaining three the thoracic artery arose from the coracoid
artery on one side and from the subclavian on the other side. The
distance between these two possible sites of origin is slight.

The thoracic artery usually passes ventral to M. costi-sternalis
anterior. Occasionally a small artery (13) could be traced from the
main trunk of the thoracic artery to that muscle. The main thoracic
artery bifurcates near the insertion of M. costi-sternalis, the
branches traveling posteriad on both sides of the muscle. On one side of
one specimen this artery bifurcated immediately after leaving the
subclavian, the dorsal trunk passing dorsal to M. costi-sternalis
anterior, and the ventral trunk ventral to the muscle. On the other
side of the same individual the artery passed dorsal to M.
costi-sternalis anterior, bifurcating at the normal point.

From the ventral trunk of the thoracic artery a variable number of small
vessels arises to supply the costosternal articulations. The main
ventral trunk bifurcates into two branches, one of which passes onto the
inner face of the sternum, and one of which supplies the posterior two
intercostal spaces.

The dorsal thoracic trunk supplies M. costi-sternalis, several dorsal
intercostal areas, and the costopulmonary muscles. Minor variations in
all of the smaller branches of the thoracic artery were common.

[Pg 560]

MYOLOGY AND ANGIOLOGY: COLUMBIDAE

Figs. 5, 6, and 7 illustrate the following muscles and arteries
described for Scardafella inca.

Myology

M. pectoralis thoracica, Fig. 5. The origin is from approximately the
ventral third of the keel, the lateral and anterior portion of the
clavicle and the adjacent sterno-coraco-clavicular membrane, and from
the lateral portion of the sternum and the fascia overlying the thoracic
ribs. This massive muscle covers the entire ventral surface of the
thorax, converges, and inserts on the pectoral surface on the ventral
side of the humerus.

M. supracoracoideus, Fig. 5. The origin is from the dorsal two-thirds
of the keel and medial half of the sternum (where the origin is bordered
ventrally, posteriorly, and laterally by the origin of M. pectoralis
thoracica) and from the sterno-coraco-clavicular membrane adjacent to
the coracoid. This large pinnate muscle converges, passes through the
foramen triosseum, and inserts by means of a strong tendon on the dorsal
surface of the humerus on the deltoid ridge.

M. coracobrachialis posterior, Fig. 5. The origin is from a prominent
lateral wing on the posterolateral portion of the coracoid, and from the
lateral surface of the proximal two-thirds of the coracoid. The
insertion is by means of a tendon on the internal tuberosity of the
humerus. Of the muscles described here, this one differs most strikingly
from the homologous muscle in P. subis. The difference can be seen by
comparing Figs. 1 and 5.

M. sternocoracoideus, Figs. 5, 6, and 7. The origin is from the
external, and to a slight extent from the internal, surface of the
costal process. The insertion is on a posterolateral triangular area on
the dorsal surface of the coracoid.

M. costi-sternalis, Figs. 5 and 6. The origin is from the anterior
edge of the sternal portion of the first three thoracic ribs. The muscle
converges and inserts on the apex of the costal process.

M. subcoracoideus (ventral head), Fig. 6. The origin is from the
manubrium and from approximately the posterior half of the coracoid and
on the medial and dorsal surface of that bone, and the medial side of
the sterno-coraco-clavicular membrane adjacent to the coracoid. The
ventral head passes anterodorsally to join with the dorsal head (not
here described), and the combined muscle inserts by a tendon on the
internal tuberosity of the humerus.

Mm. intercostales externus, Fig. 5. These muscles extend
posteroventrally between successive thoracic ribs and between the last
cervical and first thoracic ribs.

Mm. intercostales internus, Fig. 7. These muscles extend
anteroventrally between the last three thoracic ribs.

Costopulmonary muscles, Fig. 7. This series of muscle slips from the
thoracic ribs attaches to the aponeurosis covering the lungs.

Angiology

Figs. 5, 6, and 7 show all arteries discussed for this family. The
numbers following names or descriptions of arteries in the text refer to
numbered arteries[Pg 561] in one of these figures. Insofar as possible, the
numbers used for these arteries are the same numbers used for the
homologous vessels in swallows.

The right and left innominate arteries arise from the aortic trunk and
give rise to the common carotid (14) and subclavian (1) arteries. The
latter continues across the thoracic cavity, giving rise to the coracoid
(2) and axillary (10) arteries, and becoming the pectoral trunk. That
trunk swings posteriorly and leaves the thoracic cavity near the apex of
the costal process, as shown in Fig. 7. Where the trunk passes under M.
sternocoracoideus, the thoracic artery (3) is given off.

The various branches of the coracoid artery, again referred to as a
"coracoid complex," are as follows: The first branch, from the posterior
face of the coracoid artery, is a relatively large vessel (6) here
termed the sternal artery; it passes mediad across M.
sternocoracoideus, sending off a branch (6a) to that muscle. The right
sternal artery continues posteriorly on the mid-line of the inner
surface of the sternum, and appears to send branches into the various
pneumatic foramina of the sternum, but these vessels are minute and
exceedingly difficult to trace accurately. The corresponding left vessel
is smaller and ramifies on the anteromedial surface of the sternum.
Variations found in these vessels were the following: In one specimen of
S. inca the sternal artery had, on both sides, an independent origin
from the subclavian, lateral to the origin of the coracoid artery. In
Zenaidura macroura both right and left sternal arteries were similar
to the left vessel described above, no median longitudinal vessel being
seen. In Columba livia no vessel corresponding to the sternal artery
was seen. In Zenaida asiatica these arteries penetrated M.
sternocoracoideus; no branch to the sternum was seen.

A small complex of vessels (4 and 4a) arises from the lateral face of
the coracoid artery and feeds M. coracobrachialis posterior, and
occasionally M. sternocoracoideus. One branch (4a) passes under the
coracoid and travels along the lateral side of that bone, supplying
small branches to M. coracobrachialis posterior, and finally ramifying
on the head of the coracoid. In C. livia, Zenaidura macroura, and
Zenaida asiatica this complex usually arises independently from the
subclavian, and in one case it arose from the axillary artery.

Two other branches from the coracoid artery were regularly seen. The
first (8) passes across M. sternocoracoideus and appears to supply the
area of the coracoid articulation with the sternum; the second (7)
supplies M. subcoracoideus as the main vessel passes between that
muscle and the coracoid and penetrates M. suparacoracoideus. A small
notch on the medial side of the coracoid (shown in Figs. 6 and 7) often
marks the passage of the coracoid artery.

All vessels of the coracoid complex are exceedingly variable, in number,
size, and site of origin.

A prominent vessel (15) is given off from the posterior pectoral artery,
outside the thoracic cavity, passes ventrad, and sends two branches into
M. supracoracoideus. No corresponding artery was seen in the swallows
dissected.

The thoracic artery (3), arising from the pectoral stem,
characteristically bifurcates at the anterior end of M.
costi-sternalis. The dorsal, and larger, branch passes posteriorly,
sends several small branches to M. costi-sternalis, and continues to
the most posterior rib. The ventral trunk bifurcates, one branch passing
along the edge of, and supplying, M. costi-sternalis, the other[Pg 562]
branch passing onto the surface of the sternum. In some specimens two
such branches to the sternum were seen.

SUMMARY OF ARTERIAL ARRANGEMENT

In both families the vessels that are relatively constant in appearance
are: a subclavian giving rise to the carotid and axillary arteries, and
becoming the pectoral trunk; the thoracic artery arising variously, and
passing posteriorly to the rib cage; and the coracoid complex of
vessels. The coracoid complex includes the coracoid artery, the vessels
to Mm. sternocoracoideus and coracobrachialis posterior, and the
sternal artery, which is variously present, and more extensive in some
species than in others.

DISCUSSION AND CONCLUSIONS

In the vessels studied individual variation is marked, but the arterial
arrangement within both families is relatively constant. Interfamilial
differences probably represent responses of the arteries to adaptive
structural differences of other systems of the body.

Individual Variation

The term "individual variation" is used here to mean "continuous
non-sex-associated variation" (see Mayr, Linsley, and Usinger, 1953:93)
found between members of the same species or between the two sides of
the same individual. It is hazardous to define individual variation (and
also interspecific differences, as discussed later) in the origin of one
vessel by relating its location to other vessels, because these may
likewise vary in origin. But, by necessity, certain vessels that are
probably less variable (axillary, carotid, and pectoral arteries) have
been considered here as being constant in origin. If these three vessels
are accepted as reference points, individual variants, as well as
interspecific differences, can easily be described in the thoracic and
coracoid arteries and in their various branches.

The thoracic artery in P. subis arose either from the subclavian
artery, or from the coracoid artery. Likewise in other swallows, both of
these origins were found. In doves the thoracic artery arose
consistently from the pectoral stem, lateral to the origin of the
axillary artery.

The coracoid artery in P. subis and other swallows arose from the
subclavian artery, either opposite the base of the axillary artery, or
medial to that vessel. In all doves studied the coracoid artery arose
from the subclavian medial to the axillary artery. I observed much[Pg 563]
individual variation in the branches of the coracoid artery (that is to
say, in the vessels of the coracoid complex). In S. inca the sternal
artery arose either from the coracoid artery, or independently from the
subclavian. As mentioned earlier, in members of both families the
vessels to Mm. coracobrachialis posterior and subcoracoideus are
highly variable, arising in swallows from the coracoid artery or from
the subclavian artery, and in doves from either of these two sites or
from the axillary artery. The distribution of these arteries after their
origin is also diverse.

Individual variation in the arteries of the thorax has been recorded
previously. Bhaduri, Biswas, and Das (1957:2) state that, in the
domestic pigeon, "the origin and course of various smaller arteries...
show noticeable variation," although they do not specifically state to
which vessels they are referring. Fisher (1955:287-288) found
variability in the Whooping Crane, Grus americana, of the axillary,
coracoid, thoracic, and pectoral arteries. In one specimen he found
these vessels arising on the right side from the subclavian, in the
sequence just listed, and on the left side all arose from the same
point. Berger (1956:439-440) strongly emphasized the variability of the
vascular system, calling it the most variable in the body. As he stated,
this high degree of individual variation seems to be due to the
embryological development of the system, wherein many of the adult
channels of circulation are derived from embryonic plexuses.

Intrafamilial Differences

In spite of the rather extensive amount of individual variability in
some vessels, I found the over-all pattern of arteries to be relatively
constant within the family Columbidae and within the family
Hirundinidae. There are, nevertheless, several intrafamilial differences
needing some further discussion and clarification.

Others have reported the occasional presence of more than one coracoid
artery on each side in some columbids, these arteries being described as
arising from various sites and being variously named. Bhaduri and Biswas
(1954) described the arterial situation in seven species of the family
Columbidae (Columba livia, Streptopelia tranquebarica, S.
chinensis, S. senegalensis, Chalcophaps indica, Treron bicincta,
and T. phoenicoptera) and stated (op. cit.: 348) that "The
sternoclavicular [= coracoid] artery is similar in all the species, but
the domestic pigeon seems to be unique in that it has, in addition, a
small vessel, the accessory sternoclavicular." This artery was described
later, in the domestic pigeon, as[Pg 564] follows (Bhaduri, Biswas, and Das,
1957:5): "A minute and insignificant vessel which has been termed the
accessory sternoclavicular artery... is given off close to the origin
of the sternoclavicular. It passes anteroventrally to supply the
adjacent muscles." Glenny (1955:577) described the arterial pattern
characteristic of members of the family Columbidae (more than 30 species
studied by him) and stated that "three pairs of coracoid arteries are
found in Otidiphaps nobilis, normally one or two pairs may be found."
As suggested by Bhaduri and Biswas (1954:348), the "accessory" vessel
probably corresponds to a vessel previously described by Glenny (1940)
in Streptopelia chinensis and referred to as the "coracoid minor."

Bhaduri and Biswas (1954:348) have suggested that "the accessory
sternoclavicular artery occurring sporadically as it does in some
species of diverse groups may not have any phylogenetic value."

In no case did I find more than one coracoid artery on a side. When one
of the highly variable arteries feeding Mm. coracobrachialis posterior
and sternocoracoideus (arteries 4 and 4a, Fig. 7) arises from the
subclavian or axillary artery instead of from the coracoid artery, that
vessel may have been interpreted by others as a second (accessory or
minor) coracoid artery. If so, this artery probably does not "occur
sporadically." Rather, its origin from the subclavian, axillary, or
thoracic artery may be sporadic, subject to individual variation, and it
may have been overlooked when it arose from the coracoid artery.

Of the vessels described here, the only one that differed distinctly in
one species was the sternal artery. In Scardafella inca the right
sternal vessel was long, extending down the mid-line of the inner
surface of the sternum, whereas in other columbids the right and left
arteries ramified on the anterior part of the inner surface of the
sternum, or were altogether lacking. I am unable to account for the
differential development of this artery in S. inca.

In describing the arterial arrangement in the seven species of Indian
columbids named earlier, Bhaduri and Biswas (1954:348) state that all
species except Treron phoenicoptera have two "internal mammary"
arteries on each side "showing variable sites of origin." These arteries
were later described (Bhaduri, Biswas, and Das, 1957:4-5) as "a slender
(outer) internal mammary artery... to the outer wall of the thoracic
cavity" and "a slender (inner) internal mammary artery... to supply
the inner wall of the chest cavity." From this description, the question
arises as to[Pg 565] whether the "outer" one of these arteries should properly
be called an external instead of internal mammary artery. In any
case, I saw no specimen possessing two thoracic arteries on a side.

Interfamilial Differences

As shown above, there is a high degree of individual variation in the
vessels being considered, while at the same time, few interspecific
differences were noted within the families. On the other hand, the
vascular arrangement of swallows consistently differed from that of
pigeons in the species studied. The differences are most easily
described by discussing the resulting change in the site of origin of
the thoracic artery. In swallows the thoracic artery arises between the
carotid and axillary arteries, either from the stem of the coracoid
artery or independently from the subclavian, but in pigeons the thoracic
artery arises from the pectoral stem, a site of attachment that is
relatively more lateral than in swallows.

This difference, in my opinion, demonstrates well the topological
relationships of various systems of the body, here especially of the
skeletal, muscular, and vascular systems. The location of the thoracic
artery seems to be determined by the particular configuration of
skeletal and muscular elements, although even within the bounds set by
these elements, individual variation in the precise origin of the artery
is possible. In all swallows dissected Mm. coracobrachialis posterior
and sternocoracoideus bridge the angle formed by the costal process
and the coracoid. This arrangement makes it necessary for the subclavian
to leave the thoracic cavity dorsal to the costal process, although it
does pass immediately anterior to that process. The thoracic artery
arises from the vessel next to the apex of the costal process, hence
from the subclavian, between the axillary and carotid arteries.

In pigeons, the wing of the coracoid extends farther laterally than does
the costal process, and the apex of the latter is displaced farther
posteriorly than it is in swallows. M. coracobrachialis posterior does
not arise from the sternum, and only part of the costal process serves
as a point of origin for M. sternocoracoideus. Consequently, this
region differs from that of swallows; the area between the costal
process and coracoid is not entirely bridged by muscle, and the space
between the two skeletal elements is of a different shape and size. It
seems that these differences have resulted, in pigeons, in the
subclavian assuming a more anterior position with reference to the
costal process. The subclavian in these birds leads into the pectoral
artery, which runs posteriad, passing[Pg 566] under M. sternocoracoideus and
leaving the thoracic cavity approximately opposite the apex of the
costal process. The thoracic artery arises immediately opposite the apex
of the costal process from the main artery in the area, as it does in
swallows, except that in this case the adjacent artery from which it
arises is the pectoral stem.

The thoracic area seems to be most "efficiently" arranged when the
thoracic artery arises opposite the apex of the costal process, from
whatever main artery is closest to that site. This arrangement existed
in all species studied. Considering the differences in skeletal and
muscular structures, between pigeons and swallows, it would be much more
remarkable if an alternative were the case, that is to say if the
thoracic artery had the same site of attachment on the subclavian in
both groups.

A comparison of these suggestions with statements made previously about
these arteries seems necessary. When Glenny (1955) summarized his
accumulative findings, concerning the main arteries in the region of the
heart, based on individuals representing more than 750 avian species of
27 orders and 120 families, he described five types of thoracic arteries
that were distinguished by differences in the site of their origin, and
one type in which there were two thoracic arteries on each side. His
statements regarding these differences were as follows (Glenny,
1955:543-544):

The six types of thoracic arteries described in Glenny's classification
were distinguished as follows (Glenny, 1955:544):

Possibly the thoracic artery has undergone migration but apparent
differences in its origin might well be due to differences in other
vessels of the thoracic area. Additionally, there seems to be no reason
to assume that the lateral position of the thoracic artery is the
primitive one, or that the medial is the derived position, as is implied
by the phrase "medial migration." Although the lateral site of
attachment (type 1) is predominant in the lower orders of birds, and the
medial attachment is found primarily in Passeriformes, a fact which may
indicate that type 1 is the more primitive, it must nevertheless be kept
in mind that a sequence of a single morphological character does not
necessarily represent the phylogenetic sequence of the character itself
(see Mayr, 1955:41).

Also, a given arterial arrangement might be independently derived more
than once. If such has been the case, similarities in arterial
arrangements in different taxa would sometimes be "chance similarities,"
that is to say, "resemblance in characteristics developed in separate
taxa by independent causes and without causal relationship involving the
similarity as such" (Simpson, 1961:79).

The particular arrangement of the arteries of the thoracic area also
seems to be of limited value as a clue to taxonomic relationships. If
the origin of any artery is determined by skeletal and muscular
features, as I suggest, the artery perhaps ought not be considered as a
separate character, but as part of a "character complex" that varies as
a unit (see Mayr, Linsley, and Usinger, 1953:123). The skeleton offers a
potential fossil record for consideration. Changes in the skeleton and
muscles, great enough to affect the blood vessels, would probably be
detected more easily than would the resulting vascular changes. Also, I
did not find as much individual variation in the skeleton and muscles in
the area studied as I did in the vascular system. In other words, within
the bounds established by the skeletal and muscular features, the artery
still exhibited individual variation in exact origin.

SUMMARY

The origin, distribution, and individual variation of the thoracic and
coracoid arteries, and their branches, have been studied in four species
of the family Columbidae (pigeons) and in five species of the family
Hirundinidae (swallows). These arteries are described for Scardafella
inca (Inca Dove) and Progne subis (Purple Martin). Muscles that are
supplied by these vessels, and muscles the particular[Pg 568] configuration of
which seems to effect the arrangement of the arteries have also been
described. Correlation of the arteries observed with those named and
described by other workers has been attempted.

In most of the vessels studied there is a high degree of individual
variation, but few interspecific differences were noticed within either
family. Differences in the arteries of the thorax between the two
families are described by discussing the resulting different origins of
the thoracic artery. In swallows the thoracic artery arises from either
the subclavian artery or the coracoid artery, whereas in pigeons it
arises from the pectoral trunk. This difference in site of attachment
seems to be a result of differences between the two families in muscular
and skeletal elements of the thorax.

The particular site of attachment of the thoracic artery is of limited
value as a taxonomic character. Several considerations influenced this
conclusion. (1) If the location of the artery is determined by skeletal
and muscular elements, these associated structures must be considered
taxonomically as a "character complex" (a set of characters varying as a
unit). (2) Even within the bounds established by the skeleton and
muscles, the artery displays a high degree of individual variation in
exact origin. (3) A given arterial arrangement could have been derived
independently many times. (4) Because differences are defined relative
to other likewise variable vessels, supposed similarities or differences
in the one artery may be artifacts of the system of description.

My findings and interpretations do not support previous suggestions that
the thoracic artery has undergone a mediad migration, and that the
various sites of attachment of that vessel may come to represent various
levels of evolution. The primitive site of attachment of the vessel is
unknown, and it seems to me that it has not been sufficiently
demonstrated that the vessel has undergone any "migration."[Pg 569]

Fig. 1. Progne subis. Lateral view of left half of
thorax. M. pectoralis thoracica (area of insertion indicated by dotted
line) has been removed. Muscles not described in text are not shown. (×
1.5.)

Fig. 2. Progne subis. Lateral view of left half of
thorax. Same view as shown in Fig. 1, but with Mm. supracoracoideus,
coracobrachialis posterior, and intercostales externus removed. (×
1.5.)

[Pg 570]

Fig. 3. Progne subis. Medial view of left half of
thorax. Not all muscles shown. See Fig. 4 for identification of
arteries. (× 1.5.)

Fig. 4. Progne subis. Lateral view of left half of
thorax. (× 1.5.)

(Applies also to Fig. 3.)

[Pg 571]

Fig. 5. Scardafella inca. Lateral view of left half of
thorax. M. pectoralis thoracica (area of insertion indicated by dotted
line) has been removed. Muscles not described in text are not shown. See
legend for Fig. 7 for identification of arteries. (× 1.)

Fig. 6. Scardafella inca. Lateral view of left half of
thorax. See legend for Fig. 7 for identification of arteries. (× 1.)

[Pg 572]

Fig. 7. Scardafella inca. Medial view of left half of
thorax. (× 1.)

Key

(Applies also to Figs. 5 and 6.) Numerals not used are those used for
Progne subis for which no homologous artery occurs in Scardafella
inca.

[Pg 573]

LITERATURE CITED

American Ornithologists' Union

1957. Check-List of North American birds. Baltimore, Maryland, Amer.
Ornith. Union, xiv + 691 pp.

Ashley, J. F.

1941. A study of the structure of the humerus in the Corvidae. Condor,
43:184-195.

Beddard, F. E.

1898. The structure and classification of birds. London, Longmans,
Green, & Co., xx + 548 pp.

Berger, A. J.

1956. Anatomical variation and avian anatomy. Condor, 58:433-441.

Bhaduri, J. L., and Biswas, B.

1954. The main cervical and thoracic arteries of birds. Series 2.
Columbiformes, Columbidae, part 1. Anat. Anz., 100:337-350.

Bhaduri, J. L., Biswas, B., and Das, S. K.

1957. The arterial system of the domestic pigeon (Columba livia
Gmelin). Anat. Anz., 104:1-14.

Coues, E.

1903. Key to North American birds. Vol. I, Fifth edit. Boston, The Page
Company, xlii + 535 + [55] pp.

Fisher, H. I.

1955. Major arteries near the heart in the Whooping Crane. Condor,
57:286-289.

Glenny, F. H.

1940. The main arteries in the region of the heart of three species of
doves. Bull. Fan Mem. Inst. Biol., Zool. ser., vol. 10, pt. 4, 271-278.
(Not seen.)

1955. Modifications of pattern in the aortic arch system of birds and
their phylogenetic significance. Proc. U. S. Nat. Mus., 104:525-621.

Howard, H.

1929. The avifauna of Emeryville Shellmound. Univ. Calif. Publs. Zool.,
32:301-394.

Howell, A. B.

1937. Morphogenesis of the shoulder architecture: Aves. Auk, 54:364-375.

Hudson, G. E., and Lanzillotti, P. J.

1955. Gross anatomy of the wing muscles in the family Corvidae. Amer.
Midl. Nat., 53:1-44.

Mayr, E.

1955. Comments on some recent studies of song bird phylogeny. Wilson
Bull., 67:33-44.

Mayr, E., Linsley, E. G., and Usinger, R. L.

1953. Methods and principles of systematic zoology. New York,
McGraw-Hill Book Co., x + 336 pp.

Simpson, G. G.

1961. Principles of animal taxonomy. New York, Columbia Univ. Press, xiv
+ 247 pp.

Transmitted June 24, 1963.