Beneath the trapezius muscle, and sepa rating it from the fascia which covers the supra-spinatus muscle, a layer of fatty areolar tissue is always placed, which varies, however, in its amount, in different persons. In chronic disease of the shoulder joint, such as ulcera tion of the articular cartilage, and, in fact, in all cases where inflammatory action has ex isted in the articulation for any considerable length of time, this intermuscular fatty stra tum becomes absorbed ; and to this circum stance, as also probably in some degree, to atrophy of the muscular fibres, is due the pe culiar flattening, or even the depression, so constantly observed above the spine of the scapula in such cases; appearances analogous to the flattening of the gluteal region, which is one of the most remarkable external fea tures of " morbus coxae." Deeper still is placed an aponeurosis of great strength, which forms, with thc smooth concave surface of the supra-spinal fossa, an osteo-fibrous canal, containing the fleshy por tion of the supra-spinatus muscle. This fascia is stretched between the superior costa and the spine of the scapula ; by its under surface, posteriorly, it affords attachment to the fibres of the supra-spinatus ; whilst ante riorly it accompanies the tendon of that muscle, under the acromial end of the cla vicle and the triangular ligament, losing itself on the capsular ligament of the shoulder joint.
By the removal of this fascia, we bring into view the supra-spinatus muscle, filling accu rately the fossa from which it derives its name, and from nearly the entire of which it derives its origin ; anteriorly, however, the muscular fibres have no ossific attachment. They here glide over the smooth, pulley-like surface presented by the bone, and then, bending downwards and outwards, they form the tendon of the muscle, which is inserted further on into the upper facette of the great tuberosity of the humerus, becoming also in corporated with the capsular ligament. In this part of their course the supra-spinatus muscle and tendon are concealed by the acro mio-clavicular articulation, and more exter nally by the coraco-acromial, or triangular li gament, which is stretched in the form of an arch above them ; a bursa of large size inter venes between the under-surface of the liga ment and the superficial, or upper aspect of the tendon. By the removal of the trapezius muscle we are also enabled to see the attach ments of several muscles to the edges of the supra-spinal fossa ; thus the insertion of the levator anguli scapulm into the posterior su perior angle of the scapula becomes apparent ; also the attachment of the upper fibres of the serratus rnagnus anticus to its superior costa; and that of the omo-hyoid muscle to " the ligament of the notch," and the base of the coracoid process. In this situation also the
supra-scapular nerve and artery enter the supra-spinal fossa, usually separated from one another by ".the ligament of the notch." The nerve, in the majority of instances, being beneath, and the artery above, the ligament, the nerve is transmitted through a foramen, formed by the notch in the upper edge of the scapula and the ligament of the notch, whilst the artery enters the fossa through a small triangular interval, the respective sides of which are constituted by the ligament of the notch, coracoid process, and the posterior belly of the onto-hyoid muscle.
The supra-scapular nerve is a branch from the upper division of the brachial plexus : in the neck it follows the course of the omo hyoid muscle to the scapula, passes beneath the origin of that muscle and through the foramen above described ; after which it enters the supra-spinal fossa, and is distri buted, first/y, to the supra-spinatus muscle ; secondly, to the infra-spinatus and teres minor muscles, by a branch which passes beneath the acromion process ; and, thirdly, by, a few twigs to the exterior of the capsule of the shoulder joint.
The supra-scapular artery (sometimes de signated arteria transversalis hunzeri) has elsewhere been described as arising from the thyroid axis of the subclavian trunk ; it passes at first downwards, and then nearly trans versely outwards, anterior to the phrenic nerve, and between the sterno-cleido mastoid and the anterior scalenus muscles; it next runs along the base of the supra-clavicular triangle in close contact with the front of the subclavian vein, behind the clavicle and sub clavian muscle, and below the level of the subclavian artery, here in the third stage ; more externally it crosses this great trunk near the commencement of the axillary ; it then passes in front of the brachial plexus of nerves, running along with the supra-scapular branch, parallel to the omo-hyoid, and covered by the trapezius muscle, to the superior costa of the scapula, where it enters the supra spinal fossa above " the ligament of the notch." Whilst under cover of the trapezius muscle, the supra-scapular artery gives off a large muscular branch, the ramifications of which have been alluded to as assisting to form the acromial anastornosis. The final distri bution of the artery is by two branches.
1. The supra-spinal branch, which is distri buted to the supra-spinatus muscle, and which anastomoses near the posterior superior angle of the scapula, with branches from the pos terior scapular artery.