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 [Illustration], projecting a little forwards towards its middle, so as to give a slight convexity of outline to the top of the chest and the bottom of the neck (fig. LXV. 2). It is attached by one extremity to the sternum (fig. LXV. 2) and by the other to the scapula (fig. LXV. 2), by moveable joints. The nature of an immoveable joint has been explained (63). In the connexion of the bones of the trunk, while the main object is to secure firmness of attachment, some degree of motion is at the same time obtained (81 et seq.): but the mode in which the several bones of the extremities are connected with each other and with the trunk, admits of so great a degree of motion, that these articulations are pre-eminently entitled to the name of moveable joints. The component parts of all moveable joints are bone, cartilage, synovial membrane, and ligament. The great character of a moveable joint is the approximation of two or more bones; yet these bony surfaces are never in actual contact, but are invariably separated from each other by cartilage. The cartilage either covers the entire extent of the articulating surface of the bones, as in the shoulder-joint, where both the head of the humerus and the cavity of the scapula that receives it are enveloped in this substance (fig. LXV. 7. 9), or a portion of it is placed between the articulating surfaces of the bones, as in the joint between the clavicle and sternum (fig. LXVII. _a_); which, when so placed, is termed an interarticular cartilage (fig. LXVII. _a_). By its smooth surface cartilage lessens friction; while by its elasticity it facilitates motion and prevents concussion. Slightly organized cartilage is provided with comparatively few blood-vessels and nerves. Had it been vascular and sensible like the skin and the muscle, the force applied in the movements of the