Part 4

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 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 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 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 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 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 thoracic, intercostal, or internal mammary artery of birds... is found to arise at slightly different relative positions--from a point at the base of the inferior pectoral artery to a point near the base of the coracoid or sternoclavicular artery, and in some instances both of these vessels have a common root from the subclavian artery. Such differences are found to be of common occurrence within several orders of birds. In the Galliformes and the Passeriformes there appears to be a graded series in the sites of attachment of the thoracic artery from a lateral to a medial position. As a result of these observations, numerical values can be assigned to the site of attachment of the intercostal or thoracic artery, and these values may come to be used as an index in specific levels of evolution....

"The medial migration of the thoracic artery appears to have some phylogenetic significance as yet not understood."

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