The posterior surface is remarkable for its division into two portions by a large process which projects from it nearly horizontally back wards and slightly upwards. This process, called the spine of the scapula, is fixed to the bone at the line of union of its superior and mid dle thirds; it commences at the triangular surface already noticed at the termination of the superior fourth of the vertebral border of the scapula, thence it proceeds outwards, in clining a little upwards, and just where the neck of the scapula is united with the rest of the bone, this spine ceases to be connected with the scapula, and is continued outwards in a slightly arched form, as a broad and flattened process, denominated the acromion process, (ageoe, summus, wihoc, humerus.) The spine presents posteriorly a thick and rough edge, which by its superior border gives attachment to the trapezius muscle, and by its inferior to the deltoid, the intervening space being covered by the aponeurotic expansion which connects the muscles last-named. The superior surface of the spine looks nearly directly upwards; it is concave, and contributes to form the fossa supra-spinata. The inferior surface, on the other hand, forming part of the fossa supra spinata, is convex anteriorly and slightly con cave posteriorly, and looks downwards and backwards on each surface we observe a large nutritious foramen. The posterior edge of the spine is quite subcutaneous, and the physician often finds it desirable to practise percussion upon it.
Above the spine of the scapula is the fossa supra-spinata, which lodges the muscle of the same name, formed in front by the scapula, behind by the spine, both surfaces being slightly concave. Below the spine is the fossa supra-spinata much larger than the preceding, slightly convex, except towards its anterior part. This fossa is formed by the scapula below and the inferior surface of the spine above ; it is limited in front by a ridge which proceeds downwards and backwards, from the glenoid cavity to the inferior angle, and bounds behind a surface which gives attachment to the teres major and minor muscles. Into this ridge itself is inserted a fibrous fascia, which separates the attachment of the last-named muscles from the fossa infra-spinata and the insertion of the muscle of the same name. The two fossx, thus separated by the spine, com municate through a channel formed on the posterior part of the neck of the scapula and bounded behind by the spine ; through this channel pass the arterial and nervous ramifica tions from the superior to the inferior fossa.
The acromion process is evidently continu ous with the posterior thick edge of the spine of the scapula, and viewed from above ap pears to be merely an expansion of it. The narrowest part of the process is where it seems to spring from the spine, forming a sort of pedicle. Its posterior surface is convex, rough, covered with fibrous tissue in the recent state ; its aspect is upwards and backwards. here the process is quite subcutaneous as the pos terior part of the spine of the scapula. The anterior surface is concave, smooth, looks downwards and forwards to the posterior and superior part of the shoulder-joint. The
posterior or inferior edge of the process con tinuous with the corresponding edge of the spine of the scapula forms a curve, convex downwards and outwards, and terminates in the pointed extremity or apex of the pro cess; all this edge affords attachment to the deltoid muscle. The superior edge is con cave ; near the apex we observe upon it a plane oval articular surface to which the acromial extremity of the clavicle is articulated ; into this edge the trapezius muscle is inserted. The apex of the acromion, which is imme diately in front of the articular surface for the clavicle, gives insertion to the apex of the liga ment, whose base is attached to the outer edge of the corticoid process.
The scapula is connected to the trunk through its articulation with the clavicle, but chiefly through the intervention of muscles, so that muscles are inserted into all its edges, and its surfaces are " cushioned with muscles." It is, then, as might be anticipated, a very moveable bone, and its motions consist in more or less extensive revolutions round an axis through its centre. This bone, then, being the medium of connexion between the pectoral extremity and the trunk, it is evident that the great move ments of the former must depend upon the movements produced in the scapula by the muscles which pass to it from the trunk ; more over, when some of these muscles fix the scapula, it becomes the point whence the others act in producing the motions of the ribs. The scapula, then, is an essential element in the upper extremity, and it exists wherever we find that limb in a perfectly developed state, but it experiences various modifications in position and shape according to the uses to which the upper extremity is applied. In quadrupeds the position of the scapula is more forwards and on the side of the chest, for in them the anterior extremity is employed as an instrument of support. It is interesting to observe the variation in the aspect of the glenoid cavity, according to the oblique or upright position of the scapula, indicating whether the pectoral extremities are used chiefly as instruments of support or as instruments of prehension, &c. When freedom and rapidity of motion are required conjoined with strength, we find the scapula placed obliquely over the ribs, and a corresponding obliquity between the humerus and scapula. " In the horse, as in most quadrupeds, the speed results from the strength of the loins and hinder extremities, for it is the muscles there which propel the animal. But were the anterior extremities joined to the trunk firmly and by bone, they could not withstand the shock from the descent of the whole weight thrown forwards; even though they were as powerful as the posterior extremities they would suffer fracture or dis location. We cannot but admire, therefore, the provision in all quadrupeds whose speed is great, and whose spring is extensive, that, from the structure of their bones, they have an elastic resistance by which the shock of descend ing is diminished.