Shoulder Anatomy |
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Free Art Book for Learning Human Anatomy The ShoulderANATOMY Form is given to the shoulder by the deltoid (triangle) muscle. An almost perfect triangle is this muscle, its apex downward and wedging into the outer groove of the arm, its base upward and bent around to attach to the shoulder girdle. Just below the base is a ripple which marks the head of the arm bone. The shoulder girdle is made up of the collar bone and a ridge of the shoulder blade, meeting. They both point outward, the ridge a bit the lower, but both turn straight forward before meeting. The collar bone is an S-shaped bone, its outer curve and tail made by this forward turning. Over the point ot union is a flat space. From the hollow of this S-curve a groove sinks first downward and then at an angle outward, marking the border between the shoulder and the great breast muscle. Behind the inner two-thirds of the collar bone is a triangular depression between it and the trapezius muscle behind; its base to the neck, its apex pointing outward. MOVEMENTS In the shoulder are found two joints. At the point ot the shoulder is the joint between shoulder blade and collar bone, a flat hinge pointing straight forward, allowing the shoulder blade to slide freely over the flat surface of the back. Not only may the shoulder blade slide freely over the back, but may even lift from it at the point and inner edge, slightly amplifying its range. Below it under the deltoid is the joint of the shoulder blade with the humerus or arm bone, the shoulder proper, facing sideways and a little forward. It is a universal joint, with a right angle and a halt of movement in two planes; but its sweep is always increased by the movement of both shoulder blade and collar bone. At the juncture of the collar bone with the sternum or breast plate is a universal joint, with movement in two planes and also twisting, but with very narrow range. Its movements are chiefly lifting forward and up and twisting forward. Its shape expresses an important spring function, it being the only bony union of arm and shoulder with the trunk. MASSES Constructing these masses first as blocks, we will have the mass of the shoulder, or deltoid muscle, with its long diameter sloping down and out, beveled off at the end; its broad side facing up and out; its narrow edge straight forward. This mass lies diagonally across and overlaps the mass of the arm, whose long diameter is vertical, its broad side outward, its narrow edge forward. The mass of the forearm begins behind the end of the arm and passes across it at an angle forward and out. It is made of two squares. The upper half of the forearm is a block whose broad side is forward, its narrow edge sideways; while the lower half, smaller than the upper, has its narrow edge forward, its broad side facing out (with the hand held thumb up). These blocks are joined by wedges and wedging movements, and to the straight lines are wedded the curved lines of the contour of the muscles. The deltoid is itself a wedge, whose apex sinks into the outer groove of the arm half way down. The mass of the biceps ends in a wedge which turns outward as it enters the cubital fossa. The mass of the forearm overlaps the end of the arm on the outside by a wedge (supinator longus) that arises a third of the way up the arm, reaches a broad apex at the broadest part of the forearm and tapers to the wrist, pointing always to the thumb; and on the inside by a wedge that rises back of the arm and points to the little finger (flexor-pronator muscles). In the lower half of the forearm, the thin edge of the mass, toward the thumb, is made by a continuation of this wedge from the outside; while the thin edge toward the little finger is made by the end of the wedge from the inside. When the elbow is straight and the hand turned in, the inner line of the forearm is straight with that of the arm. When the hand is turned out, this line is set out at an angle that corresponds with the width of the wrist. The little finger side (ulna) being the hub of its movement. The flexor tendons on the front of the forearm point always to the inner condyle; the extensor tendons on the back point always to the outer condyle. The breadth of the hand corresponds with that of the lower mass; not joining it directly, but with a step-down toward the front. In the back view of the arm, the mass of the shoulder sits across its top as in the front view. The back edge of this mass is seen to be a truncated wedge arising under the deltoid and focusing on the elbow. The upper end resolves itself into the three heads of the triceps; the lower or truncated end is the triceps tendon, to which is to be added the tiny The ArmpitThe hollow ot the arm, filled with its friction hairs, is made into a deep pit by the great breast muscle (pectoralls major) in front, and the greater latissimus dorsi behind. Its floor slopes forward, downward and outward, following the slope of the chest wall. Its rear wall is deeper, since. the latissimus attaches farther down the back; thicker because made of two muscles (latissimus and teres major), and rounder because its fibres turn on themselves before attaching to the arm bone. The front wall is longer because the pectoral muscle attaches farther down the arm. Into this pit the biceps and triceps muscles plunge, with the coraco-brachialis between them. The bottom of the pit may, when the arm is fully raised, be bulged by the head of the arm bone and the lymph glands that lie there. Mechanism of the Armpit, front view:1 Biceps. 2 Triceps. 3 Latissimus dorsi. 4 Teres major. 5 Deltoid. Latissimus Dorsi: From spine, sixth dorsal to sacrum and iliac crest; passes inside of humerus to fasten to front side near head. Teres Major: From lower corner of scapula to front of humerus.
THE ARMWedging and interlocking of the Masses OF THE Arm and Shoulder
THE SHOULDERMechanism of the Shoulder, back view:I Deltoid, 1 Triceps. 3 Teres minor. 4 Teres major. Deltoid: From clavicle, acromion and ridge of scapula to outside of humerus. Triceps: Outer head, back of humerus above musculo-spiral groove. Inner head, back of humerus below musculo-spiral groove. Middle or long head, shoulder blade below socket to olecranon process of ulna. Teres Minor: From scapula to inner tubercle of humerus. Teres Major: From lower corner of scapula to front of humerus. 
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