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.