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 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."
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.)
