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):

"Type 1: attachment to the pectoral stem lateral to the
axillary.

"Type 2: attachment to the subclavian between the axillary
and coracoid.

"Type 3: attachment to the subclavian at the base of the
coracoid.