2. The infra-spinal brazzch, which enters the infra-spinal fbssa by passing beneath the acromion process, and the " spino-glenoid ligament " of Sir Astley Cooper; here it is distributed to the deep surfaces of the muscles of this region, and anastomoses freely with the termination of the posterior scapular, and with the posterior branch of the subscapular arteries.
The structures thus shown to be contained in the supra-spinal division of the scapular region are the following : — 1. Integument, dense areolar tissue, and superficial nerves ; 2. A thin aponeurosis covering, 3. The trapezius muscle ; 4. A layer of fatty areolar tissue ; 5. The strong supra-spinal aponeu rosis ; 6. The supra-spinatus muscle ; 7. The supra-scapular vessels and nerve; 8. The smooth concave surface of the bone (fossa supra-spinata).
Below the spine of the scapula portions of the trapezius, deltoid, and latissimus dorsi muscles overlap the scapular region, and partly conceal from view the intrinsic muscles of the infra-spinal fossa. These muscles are cove' ed by an aponeurotic expansion, which is thin over the trapezius and latissimus dorsi muscles ; more dense and strong where it covers that part of the deltoid which belongs to the region of the shoulder ; and much stronger still, where it invests the infra-spi natus and teretes muscles ; superiorly, it is attached to the lower edge of the spine of the scapula ; posteriorly, it is connected with the tendinous expansion of the trapezius muscle, and the base of the scapula. From its deep surface, septa are detached, which pass in between the subjacent iuuscles, and contract firm adhesions to the bone ; whilst, at the posterior edge of the deltoid niuscle, it divides into two lanimm, between which that muscle is enciosed ; the superficial layer covers the outer surface of the deltoid, and so becomes identified with the fascia of the arm ; whilst the deeper layer, passing beneath the deltoid muscle, becomes continuous with the capsule of the scapulo-humeral articulation.
The trapezius and latissinzus dorsi nzuscles overlap—the one, the posterior superior, the other, the inferior angle of the scapula. The trapezius is tendinous where it glides upwards and forwards over the smooth triangular sur face situated behind the spine of the scapula.
A bursa here intervenes between the bone and the flat tendinous expansion of the muscle. The latissimus dorsi, by its fleshy fibres, over laps the inferior angle of the scapula. The di rection of the muscle at this part of its course is nearly horizontal. As these fibres pass off the scapula, they are joined by those of its costal origin, and thence they all run upwards and forwards, presenting a twisted appearance to their insertion, which takes place by a narrow flat tendon into the bottom of the bi cipital groove of the humerus. Both these muscles, from their peculiar relation to the scapula, serve to compress it against the thorax, and so to prevent its being unduly separated from the trunk in the varied and extensive movements which it enjoys.
A peculiar displacement of the scapula, the result of accident, has been described by Vel peau, who supposes it to depend on paralysis of the serratus-magnus from injury of the great posterior thoracic nerve (external respi ratory, Ch. Bell), which is distributed to that muscle. The appearances observed in the case detailed by Velpeau were, remarkable projection backwards of the scapula especially of its posterior border, and inability on the part of the patient to bring it in contact with the side of' the thorax : cases corresponding in their general features to this description have been seen by almost every surgeon. In those which have occurred in the writer's ex perience, the projection of the posterior edge and (f the lower angle of the scapula was very remarkable, and the movements of the upper extremity were greatly impeded. Mr. Adams has suggested, as a more plausible ex planation of the deformity in these cases, that the lower angle of the scapula escapes from under the latissimus dorsi muscle ; an accident which may occur from too great elevation and abduction of the upper extremity ; and the more easily, as, in the majority of instances, either the muscle is not attached to the bone at all, or else it adheres to it by a few small fibres only.