SCAPULO-HUMERAL ARTICULATION.- The scapulo-humeral articulation is formed by the contact of the head of the humerus with the glenoid cavity of the scapula. This, the prin cipal articulation of the upper extremity, is placed at the superior and external portion of the trunk, behind the line of the axis of the hip-joint ; an arrangement which is. produc tive of this advantage, that the niost impor tant motions of the upper extremity (those in the direction forwards)lhave a more exten sive range than if the articulation had been located nearer to the anterior aspect of the thorax.
The arrangement of the articular surfaces and of the ligamentous structures belonging to the shoulder joint, accords with the general plan on which the bones and articulations of the upper extremity are constructed ; " the disposition and structure of the bones of the upper extremity afford a marked contrast to those of the lower ; the latter are organs of support, and therefore are solid, firm, strong, and withal elastic. The former are destined to perform extended motions, as well as minute and nicely adjusted ones, and there fore, while they possess all the requisite strength, they are light, present little expanse of surface, and are articulated by numerous very moveable articulations." (Todd and Bow
man's Physiological Anatomy, vol. p. 147.) The varied uses fulfilled by the upper ex tremity, added to its remarkable mobility, especially predispose the shoulder joint to accidents; but as we proceed we shall take occasion to point out the abundant provisions which exist to counteract this tendency.
As regards its motions, and the anatomical dispositions of its connecting media, the shoulder joint belongs to the class of " Enar throdial Articulations ;" but, if its bony con stituents alone be considered, it seems more nearly allied to the " Arthrodia." This is owing to the imperfect development of the glenoid cavity which is opposed to the head of the humerus.
The shoulder joint is constructed after the same plan in all vertebrate animals whose anterior extremities are developed.
In this article the several components of the scapulo-humeral articulation shall be de scribed in the following order:—