Thoracic and Coracoid Arteries In Two Families of Birds, Columbidae and Hirundinidae

by Marion Anne Jenkinson

Interfamilial Differences

As shown above, there is a high degree of individual variation in the vessels being considered, while at the same time, few interspecific differences were noted within the families. On the other hand, the vascular arrangement of swallows consistently differed from that of pigeons in the species studied. The differences 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 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.