Individual Variation
The term "individual variation" is used here to mean "continuous non-sex-associated variation" (see Mayr, Linsley, and Usinger, 1953:93) found between members of the same species or between the two sides of the same individual. It is hazardous to define individual variation (and also interspecific differences, as discussed later) in the origin of one vessel by relating its location to other vessels, because these may likewise vary in origin. But, by necessity, certain vessels that are probably less variable (axillary, carotid, and pectoral arteries) have been considered here as being constant in origin. If these three vessels are accepted as reference points, individual variants, as well as interspecific differences, can easily be described in the thoracic and coracoid arteries and in their various branches.
The thoracic artery in P. subis arose either from the subclavian artery, or from the coracoid artery. Likewise in other swallows, both of these origins were found. In doves the thoracic artery arose consistently from the pectoral stem, lateral to the origin of the axillary artery.
The coracoid artery in P. subis and other swallows arose from the subclavian artery, either opposite the base of the axillary artery, or medial to that vessel. In all doves studied the coracoid artery arose from the subclavian medial to the axillary artery. I observed much individual variation in the branches of the coracoid artery (that is to say, in the vessels of the coracoid complex). In S. inca the sternal artery arose either from the coracoid artery, or independently from the subclavian. As mentioned earlier, in members of both families the vessels to Mm. coracobrachialis posterior and subcoracoideus are highly variable, arising in swallows from the coracoid artery or from the subclavian artery, and in doves from either of these two sites or from the axillary artery. The distribution of these arteries after their origin is also diverse.
Individual variation in the arteries of the thorax has been recorded previously. Bhaduri, Biswas, and Das (1957:2) state that, in the domestic pigeon, "the origin and course of various smaller arteries... show noticeable variation," although they do not specifically state to which vessels they are referring. Fisher (1955:287-288) found variability in the Whooping Crane, Grus americana, of the axillary, coracoid, thoracic, and pectoral arteries. In one specimen he found these vessels arising on the right side from the subclavian, in the sequence just listed, and on the left side all arose from the same point. Berger (1956:439-440) strongly emphasized the variability of the vascular system, calling it the most variable in the body. As he stated, this high degree of individual variation seems to be due to the embryological development of the system, wherein many of the adult channels of circulation are derived from embryonic plexuses.