Angiology
Figs. 3 and 4 show all arteries discussed for this family. The numbers following the names or descriptions of arteries in the text refer to numbered arteries in one or both of these figures.
The right and left innominate or brachiocephalic arteries arise from the aortic trunk and give rise to the common carotid arteries (14). The major vessel continuing across the thoracic cavity is the subclavian artery. Classically the subclavian is considered as continuing into the anterior appendage as the axillary artery. However, in the species studied, the axillary artery can best be described as a branch from the subclavian; the pectoral stem forms a more direct continuation of the subclavian. In traversing the thoracic cavity, the subclavian gives rise to the thoracic, coracoid, and axillary arteries, and leaves the thoracic cavity as the pectoral trunk, dorsal to the area where Mm. coracobrachialis posterior and sternocoracoideus span the angle formed by the coracoid and costal process.
The pectoral trunk bifurcates into two main pectoral arteries (9), which penetrate M. pectoralis thoracica. Neither the axillary artery nor these pectoral arteries were traced in my study.
The coracoid artery (2) arises from the ventral face of the subclavian (1), either opposite the base of, or medial to, the axillary artery (10). The coracoid artery passes ventrad between the medial edge of the coracoid and the ventral head of M. subcoracoideus, and an artery (7) is given off to supply that muscle. The main vessel then penetrates M. supracoracoideus and bifurcates or ramifies into several vessels (12).
Between the origin of the coracoid artery from the subclavian, and the point where the coracoid artery passes the medial edge of the coracoid, several branches are given off. These vessels are highly variable in origin, as described below, and not all were always found. Along with the coracoid artery, they are termed a "coracoid complex."
The first artery (11) of this complex arises from any one of several places: from the lateral face of the coracoid artery at its base; independently from the subclavian immediately lateral to the origin of the coracoid artery; and from the thoracic artery near its origin. This vessel travels laterad, parallel to the subclavian, and penetrates M. coracobrachialis posterior at the same point that the pectoral artery passes dorsal to that muscle.
Another vessel (common stem of 4 and 5) of the coracoid complex in most specimens arises from the anterior face of the coracoid artery and branches into several vessels, some of which (5) supply M. subcoracoideus, and some of which (4) feed M. coracobrachialis posterior. The vessel occasionally shares a common stem with the main vessel (11) to M. coracobrachialis posterior, and in some specimens arises independently from the subclavian, immediately anterior to the origin of the coracoid artery. The branch (4) to M. coracobrachialis posterior was also seen to arise independently from any of the above-mentioned positions.
Two remaining vessels (6 and 8) are often found as branches from the coracoid artery. They were small and often were collapsed in the individuals I dissected, but were most clearly seen in Iridoprocne bicolor. The vessels occasionally had a common base, and in some specimens only one vessel was found. The first artery (6) passes mediad into M. sternocoracoideus, or continues across that muscle onto the inner face of the sternum. The second vessel (8) also supplies M. sternocoracoideus or the inner surface of the sternum, and often a large branch continues across the dorsal surface of the coracoid to M. coracobrachialis posterior. Fig. 3 shows a composite of these vessels; not all branches were seen in any one specimen. In the specimen of I. bicolor a foramen existed on the lateral edge of the coracoid where the branch (of 8) to M. coracobrachialis posterior passed. An examination of skeletons of five to 10 individuals each of the five species for which dissections were made, and of Petrochelidon pyrrhonota (Cliff Swallow) and Tachycineta thalassina (Violet-green Swallow), in the University of Kansas collection, showed that most coracoids of these seven species of swallows had a small notch (as shown in Fig. 4) or a complete foramen there.
The thoracic artery (3) arises from the subclavian opposite the base of the coracoid artery, or from the base of the coracoid artery. Of the five specimens of P. subis dissected, one individual had the former arrangement on both sides, and one had the latter on both sides, whereas in the remaining three the thoracic artery arose from the coracoid artery on one side and from the subclavian on the other side. The distance between these two possible sites of origin is slight.
The thoracic artery usually passes ventral to M. costi-sternalis anterior. Occasionally a small artery (13) could be traced from the main trunk of the thoracic artery to that muscle. The main thoracic artery bifurcates near the insertion of M. costi-sternalis, the branches traveling posteriad on both sides of the muscle. On one side of one specimen this artery bifurcated immediately after leaving the subclavian, the dorsal trunk passing dorsal to M. costi-sternalis anterior, and the ventral trunk ventral to the muscle. On the other side of the same individual the artery passed dorsal to M. costi-sternalis anterior, bifurcating at the normal point.
From the ventral trunk of the thoracic artery a variable number of small vessels arises to supply the costosternal articulations. The main ventral trunk bifurcates into two branches, one of which passes onto the inner face of the sternum, and one of which supplies the posterior two intercostal spaces.
The dorsal thoracic trunk supplies M. costi-sternalis, several dorsal intercostal areas, and the costopulmonary muscles. Minor variations in all of the smaller branches of the thoracic artery were common.