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."
[Illustration: 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.)]
[Illustration: 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.)]
[Illustration: FIG. 3. _Progne subis._ Medial view of left half of thorax. Not all muscles shown. See Fig. 4 for identification of arteries. (× 1.5.)]
[Illustration: FIG. 4. _Progne subis._ Lateral view of left half of thorax. (× 1.5.)
1. Subclavian artery. 2. Coracoid artery. 3. Thoracic artery. 4. (Unnamed.) Supplies _M. coracobrachialis posterior_. 5. (Unnamed.) Supplies _M. subcoracoideus_. 6. (Unnamed.) Supplies _M. sternocoracoideus_ and sternum. 7. (Unnamed.) Supplies _M. subcoracoideus_. 8. (Unnamed.) Supplies _M. sternocoracoideus_, _M. coracobrachialis posterior_, and sternum. 9. Pectoral artery. 10. Axilliary artery. 11. (Unnamed.) Supplies _M. coracobrachialis posterior_. 12. (Unnamed.) Supplies _M. supracoracoideus_. 13. (Unnamed.) Supplies _M. costi-sternalis anterior_. 14. Carotid artery.]