ces are most easily
described by discussing the resulting change in the site of origin of
the thoracic artery. In swallows the thoracic artery arises between the
carotid and axillary arteries, either from the stem of the coracoid
artery or independently from the subclavian, but in pigeons the thoracic
artery arises from the pectoral stem, a site of attachment that is
relatively more lateral than in swallows.

This difference, in my opinion, demonstrates well the topological
relationships of various systems of the body, here especially of the
skeletal, muscular, and vascular systems. The location of the thoracic
artery seems to be determined by the particular configuration of
skeletal and muscular elements, although even within the bounds set by
these elements, individual variation in the precise origin of the artery
is possible. In all swallows dissected _Mm. coracobrachialis posterior_
and _sternocoracoideus_ bridge the angle formed by the costal process
and the coracoid. This arrangement makes it necessary for the subclavian
to leave the thoracic cavity dorsal to the costal process, although it
does pass immediately anterior to that process. The thoracic artery
arises from the vessel next to the apex of the costal process, hence
from the subclavian, between the axillary and carotid arteries.

In pigeons, the wing of the coracoid extends farther laterally than does
the costal process, and the apex of the latter is displaced farther
posteriorly than it is in swallows. _M. coracobrachialis posterior_ does
not arise from the sternum, and only part of the costal process serves
as a point of origin for _M. sternocoracoideus_. Consequently, this
region differs from that of swallows; the area between the costal
process and coracoid is not entirely bridged by muscle, and the space
between the two skeletal elements is of a different shape and size. It
seems that these differences have resulted, in pigeons, in the
subclavian assuming a more anterior position with reference to the
costal process. The subclavian in these birds leads into the pectoral
artery, which runs posteriad, passing under _M. sternocoracoideus_ and
leaving the thoracic cavity approximately opposite the apex of the
costal process. The thoracic artery arises immediately opposite the apex
of the costal process from the main artery in the area, as it does in
swallows, except that in this case the adjacent artery from which it
arises is the pectoral stem.

The