The deltoid, trapezius, and latissimus dorsi muscles, where they overlap the scapular re gion, circumscribe a triangular space, in which may be seen part of the posterior edge of the scapula, with the attachment to it of the rhomboid muscle, and also a portion of the infra-spinatus and of the teres minor muscles.
By the removal of so much of these super ficial muscles as encroach on the scapular re gion, and of the strong fascia already de scribed, the deeper seated muscles of the infra-spinal fossa, viz. the infra-spinatus and the teres major and minor, are completely exposed.
The infra-spinatus nzuscle arises from the upper four-fifths of the dorsum of the scapula below its spine. The strong fascia of invest ment already described also furnishes an extensive surface of origin to its fibres ; the muscle is triangular, the fibres all converging anteriorly to their common tendon, which, passing beneath the spine of the scapula and the acromion process, approximates closely to the tendon of the supra-spinatus muscle, and is inserted immediately beneath it, into the great tuberosity of the humerus.
The teres minor muscle seems to be little else than a fasciculus of the last described muscle, to which it is parallel, and along the lower edge of which it is placed : anteriorly, it is inserted by a separate tendon into the lowest portion of the great tuberosity of the humerus. The teres minor and infra-spina tus muscles might be regarded as different portions of one and the same muscle, not only from the similarity of their anatomical rela tions, but also from the identity of their physiological functions; both draw down vvards and backwards the humerus, and pro duce the rotation outward of the arm at the shoulder joint. The teres minor is placed between the infra-spinatus superiorly and the teres major inferiorly, in close contact with the former, from which an aponeurotic septum and branches of blood-vessels alone separate it ; whilst a very considerable space, containing the long head of' the triceps, and some im portant nerves and vessels, intervenes between it and the teres major.
The teres major and the teres minor mus cles arise close together from the lowest portion of the dorsum of the scapula. The teres major (the more superficial of the two at its origin) is attached to the rough surface on the outer aspect of the inferior angle of the scapula, whilst the teres minor arises more anteriorly, from a narrow but well-marked groove, situated just above the axillary margin of the bone. At their origin the teres minor
is concealed by the teres major, but as they pass towards the humerus they gradually diverge, and are inserted on different aspects of the bone, and at different levels, so that the long head of the triceps firstly, and the neck of the humerus secondly, intervene be tween them. The teres minor is inserted on the outside of the humerus into its great tuberosity', below the insertion of the infra spinatus muscle, Inhilst the teres major, in company with the latissimus dorsi, passes on the inner aspect of the humerus, and is in serted along with it into the bottom of the bicipital groove: at their insertion, the tendon of the teres major is posterior, and a little inferior to the tendon of the latissimus dorsi.
The teretes muscles, in diverging to the humerus, form with the upper part of that bone a triangular space, of which the base is at the humerus, and the apex at the inferior angle of the scapula. The scapular origin of the triceps extensor muscle in its vertical course down the arm, crosses this space, and divides it into comeartments. a posterior triangular, and an anterior quadrilateral one, through both of which the axillary cavity communicates with the posterior region of the scapula and shoulder.
The triangulir compartnzent, overlapped posteriorly' by the deltoid muscle, is bounded, above, by the teres minor and axillary edge of the scapula; below, by the teres major. Its base, situated externally, is formed by the long head of the triceps; whilst its apex, directed inter nally, corresponds to the point of contact of the teretes muscles, where they arise together from the scapula. In this compai tinent is seen the posterior branch of the sub-scapular artery' (cireziNflexus .seapitlex, Scemmering), forming here a curve, the convexity of which is directed downwards and backwards. The artery' leaves this space by bending upwards and backwards, beneath the teres minor and infra-spinatus muscles. It thus arrives in the infra-spinal fossa, lies next the bone, and ramifying minutely anastomoses, superiorly with the descending branch of the supra scapular artery, and posteriorly with the termination of the posterior scapular artery.