SHOULDER JOINT (NoartiAL ANA.. TOMY OF). t The scapular and the axillary regions are each limited externally by the region of the shoulder-joint ; the latter also unites the two former regions to each other.
The region of the shoulder joint (le morgnon de repaule) exhibits a rounded projection, due to the angle formed by the union of the arm with the shoulder ; and to the surgical anatomist it possesses extreme interest, be cause its skeleton is formed by the shoulder joint.
Some difficulty arises in assigning to this surgical region precise limits. Anterior/ y, it is separated froin the pectoral region, by the narrow space between the deltoid and the great pectoral muscles (the groove, Velpeau) ; above, it is limited by the convex projection of the acromion pro cess, and by the outer end of the clavicle ; posteriorly, it is confounded with the scapular region ; whilst inferiorly, it extends as far as the insertion of the folds of the axilla.
The elements of which this region is com posed, are the following : under the super ficial coverings lie the deltoid muscle (the greater portion of which belon„as exclusively to this space), and in it and beneath it, the branches of the circumflex arteries, and of the great circumflex nerve ; still deeper are situ ated the exterior of the capsule of the shoulder joint, the neck and tuberosities of the humerus, the acromion and coracoid processes, with the attachment to them of numerous muscles and ligaments.
In this article it is proposed to notice,first, the structires in the scapulo-humeral region which are superficial to the joint ; and, se condly, to describe the anatomical characters ai the shoulder joint itself.
In removing the integuments and subcuta neous layer of areolar tissue which covers the deltoid muscle, the anatomist brings into view numerous small branches of nerves from the cervical plexus twigs), some of the fibres of origin of the platysnia myoides, and some small venous branches which, after anastomosing freely with one another, termi nate in the cephalic or axillary trunks.
By the removal of its investing fascia, the deltoid muscle is next exposed : in its origin it corresponds accurately to the insertion of the trapezius ; hence thes6 two muscles are direct antagonists of each other. The fibres of the
deltoid muscle arise from the anterior edge of the outer third of the clavicle and of the acromion process, and froin the lower margin of the spine of the scapula : from this extensive line of origin, the fibres in descending con verge to the humerus, and are inserted on the outer aspect of that bone into a rough surface called the deltoid impression. The insertion of the muscle is outside the limits of the scapulo-humeral region, and belongs to that of the arm whilst its posterior portion is contained in the scapular region ; so that the anterior, upper, and central portions of the deltoid alone belong to the region under con sideration. Immediately beneath the clavicle, the anterior edge of the deltoid is separated from the pectoralis major by a triangular in terval, of which the base, placed superiorly, is formed by the bone, whilst the edges are constituted by the adjacent muscles ; in feriorly, the interstice becoming smaller de generates into a groove, which continues to separate the muscles from each other, until at length the clavicular fibres of the great pectoral unite with the deltoid, and are in serted conjointly with it into the humerus. In this muscular interstice the cephalic vein is lodged, which, ascending to the triangular space below the clavicle, there dips into the axilla, and joins the axillary vein. The descending branch of the thoracica acro mialis (arteria thoracica hunwraria) descends in the same groove, twisting in a spiral tnanner around the cephalic vein. More deeply still the ligamentuni hicorne, enclos ing between its layers the subclavius muscle, may be seen. The axillary vein and artery, brachial plexus of nerves, and inferior to these and crossing before them, the lesser pectoral muscle, may also be made apparent in this space ; but to bring these latter parts into view, the anatomist must first freely separate the muscles from each other.