(Vide AXILLARY ARTERY.) The quadrilateral compartment is bounded above, by the capsular ligament of the shoulder joint, by the prominence of the head of the humerus, and by the tendinous attachments of the teres minor and of the sub-scapularis muscles ; below, by the teres major and latissi mus dorsi ; externally, by the neck of the humerus ; whilst, internally, it is separated from the triangular compartment • last de scribed by the long head of the triceps muscle; it transmits, from within outwards, the circum flex nerve and posterior circumflex artery.
This artery contributes to form the great scapular anastomosis ; some of its branches ascending in the substance of the deltoid muscle, inosculate freely with the superior acromial branches of the infra-scapular artery, whilst others pass backwards and unite in the infra-spinal fossa, with branches of the sub scapular and tbe posterior scapular arteries. The cirenny'lex nerve is distributed almost exclusively to the deltoid muscle ; but two collateral branches are detached from it, which are distributed in the scapular region; the first, a branch to the teres minor muscle ; the second, a cutaneous filament, which escapes froni beneath the posterior edge of the deltoid muscle, and is distributed to the integument.
The posterior scapular artery, although placed beyond the limits of the scapular region, may, nevertheless, be here described, as it is distributed chiefly to the parts con tained within it. Under the name of "trans versalis colli," this artery arises in the neck from the thyroid axis, near to, and sometimes by a common trunk with, the transversalis humeri ; it sometimes comes from the sub clavian, external to the scaleni muscles,—an irreoularit which is by no means uncommon. When derived from its more usual source, this branch runs transversely across the scalenus anticus muscle and the phrenic nerve, covered by the clavicular portion of the stern°. cleido-mastoid muscle : it then traverses the apex of the supra-clavicular triangle, lying above the level of the curve of the subclavian artery, and placed before or between the for mative roots of the brachial plexus ; passing still further outwards it gets under the trapezius muscle, and here gives off its ascending cer vical branch • at the posterior superior angle of the scapula, the artery bends backwards, under cover of the levator anguli scapulw ; here it changes its direction, and inclining downwards, runs along the vertebral edge of the scapula. Its course may, therefore, be
divided into two stages ; the first extends from the origin of the artery to the superior angle of the scapula, and so far its direction is nearly horizontal, and it is properly desig nated the " arteria transversalis colli." In its second stage, the artery runs vertically, parallel to, and about an inch distant from, the vertebral margin of the bone. This portion of the artery, which alone should be termed " posterior scapular," is covered by the great er and the lesser rhomboid muscles, and by the trapezias. To these, and to the other muscles attached to the scapula, it furnishes numerous branches, and at the inferior angle of that bone, it anastomoses very freely with the posterior branch of the sub-scapular artery.
The structures which occupy the infra spinal fossa may here be briefly recapitulated : first the integument and the sub-cutaneous layer of areolar tissue ; secondly, the fleshy edges of the deltoid and the latissimus dorsi muscles, and the triangular tendinous expansion of the trapezius, covered by their respective portions of fascia: in the interval between these muscles, and partly covered by them, lie, third/y, the infra-spinatus, the teres major, and the teres minor, muscles ; these are con tained in distinct sheaths, formed by their investing fascia, and the aponeurotic septa detached from its deep surface ; fourth/y, the anastomoses of branches from all the scapular arteries; fifth/y, the bone (fossa infra-spinata).
Around the margins of the scapula there exists, as has thus been shown, a chain of large blood-vessels, which, by numerous branches, anastomose freely at the angles, and on the different aspects of the bone, forming a vascular circle of great interest to the surgeon ; for by means of it the upper extremity is mainly supplied with blood, when the current through the subclavian is inter rupted, at the distal side of the branches which spring. from its first stage. At the acromial end of the scapula two series of anastomoses may be observed; thefirst, su perficial to the acromion process, is formed by the union of the superior acromial branches of the supra-scapular artery with the ascending (inferior acromial) branches of the circum flex, and with the acromial thoracic divisions of the axillary artery.