139. The soft parts of the SHOULDER are composed chiefly of muscles; its bones are two, the scapula or the blade bone, and the clavicle or the collar bone (fig. LXV. 2, 4).

1. Sternum; 2. clavicle; 3. ribs; 4. anterior surface of
scapula; 5. coracoid process of scapula; 6. acromion process
of scapula; 7. margin of glenoid cavity of scapula;
8. body of the humerus or bone of the arm; 9. head of the
humerus.

140. The SCAPULA is placed upon the upper and back part of the thorax, and occupies the space from the second to the seventh ribs (fig. LXV. 4)

1. Posterior surface of scapula; 2. margin of scapula;
3. acromion process; 4. margin of glenoid cavity; 5. clavicle;
6. body of humerus; 7. head of humerus.

Unlike that of any other bone of the body, it is embedded in muscles, without being attached to any bone of the trunk, excepting at a single point. From the bones of the thorax it is separated by a double layer of muscles, on which it is placed as upon a cushion, and over the smooth surface of which it glides. Originally, like the bones of the skull, it consisted of two tables of compact bone, with an intermediate layer of spongy bony substance (diploë); but, by the pressure of the muscles that act upon it, it gradually grows thinner and thinner, until, as age advances, it becomes in some parts quite transparent and as thin as a sheet of paper.

141. The figure of the scapula is that of an irregular triangle (fig. LXVI.). Its anterior surface is concave (fig. LXV. 4), corresponding to the convexity of the ribs (fig. XLV. 7); its posterior surface is very irregular (fig. LXVI. 1), being in some parts concave and in others convex, giving origin especially to two large processes (figs. LXV. 5, and LXVI. 3); one of which is termed the acromion (fig. LXVI. 3), and the other the coracoid process of the scapula (fig. LXV. 5). The margins of the bone, whatever the thinness of some portions of it, are always comparatively thick and strong (fig. LXVI. 2), affording points of origin or of insertion to powerful muscles. At what is called the anterior angle of the bone there is a shallow oval depression covered with cartilage and deepened by a cartilaginous margin, called the glenoid cavity of the scapula (figs. LXV. 7, and LXVI. 4), which receives the head of the humerus or bone of the arm (figs. LXV. 9, and LXVI. 7, 6).

142. The clavicle, the second bone of the shoulder, is a long and slender bone, of the form of an italic