[Examination on the courses of the arteries in the axillary region I. The course of the subscapular artery system, especially the relationships between the arteries and the posterior cord of the brachial plexus].

The subscapular artery (Sbs) is the largest and most variable branch of the axillary artery (Ax). Although this Sbs has been defined as a common trunk of the thoracodorsal artery (TD) and the circumflex scapular artery (CS), TD and CS do not always form a common trunk, and the posterior circumflex humeral artery (CHP) is often associated with CS or the CS+TD trunk. Moreover, the subscapular branch (RS) usually arises from CS or CS+TD. Therefore, the subject of the study on Sbs must at least include RS and CHP. However, many anatomical studies on Sbs variation have not included them, and the mechanism causing the variation of Sbs has not been treated except in a few papers. Yamada distinguished two types of subscapular artery in 1967. The first type arose from Ax proximal to the point where Ax passed between the lateral and the medial cord or penetrated the ventral stratum of the brachial plexus. The second type arose from Ax distal to the point. The former type crossed over the medial cord of the plexus to reach the upper lateral thoracic wall, gave off the lateral thoracic artery (TL) and then ran backward on the serratus anterior muscle to reach the deep region of the axilla. Yamada named the former type Sbs "A.subscapularis superficialis" and considered it to be derived from TL. Yamada made an epoch-making report regarding the arteries of the axilla in 1967. He did not, however, refer to the relationship between the arteries to the deep region of the axilla and the posterior cord of the plexus. We called Sbs and the related arteries to the deep region of the axilla "the subscapular artery system" (Sbs system) and examined the courses of the Sbs system in 202 sides of cadavers during the dissecting practices from 1988 to 1992. Our examination revealed that there were three types of Sbs systems, i.e., S-type, I-type, and P-type Sbs systems. The S-type Sbs system was almost the same as Yamada's "A. subscapularis superficialis", branching off from the lateral thoracic artery arising from Ax proximal to the point where Ax penetrated the ventral stratum of the brachial plexus, and ended up in TD or bifurcating into TD and CS. CHP arose from the proximal part of the TD+CS-trunk and ran via the proximal course, or from CS and via the distal course. The S-type CHP ran posterior to the radial nerve (R). The frequency of appearance of the main arteries of the S-type Sbs system decreased sequentially from the medial artery to the lateral one as follows: TD > CS > CHP. Both the I-type and P-type Sbs system arose from Ax distal to the point where Ax penetrated the ventral stratum of the brachial plexus. But the former ran posterior across the medial side of R and the latter ran posterior across the lateral side. The branching point of the I-type Sbs system from Ax was located more proximally than that of P-type. The I-type Sbs system more often gave off CS than TD and CHP. The P-type Sbs system arose from the most distal part of Ax just in front of the lateral axillary hiatus. The sequence of the frequency of appearance was CHP > CS > TD. Both CS and TD of P-type passed behind R. Different types of Sbs systems coexisted. In such cases, the more proximal type gave off more medial arteries and the more distal type gave off more lateral arteries. Exceptions to the rule, for example, coexistence of P-type CS with S-type CHP or I-type TD with S-type CS etc., were never observed. Although different features were distinguished in the stem parts of the three types of Sbs systems, the courses of the peripheral branches of Sbs systems displayed a common pattern on the surface of the subscapular muscle when they were analyzed collectively disregarding the direction of the flow of blood. Therefore, the peripheral part of each Sbs system on the surface of the subscapular muscle was considered to be derived from a common arterial network. In conclusion, the mechanism of the formation of variations in the Sbs system can be understood by the combination