n 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 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."

[Illustration: FIG. 1. _Progne