[Illustration: FIG. 15.--VERTICAL AND TRANSVERSE SECTION, AT THE PLANE OF THE SHOULDERS, OF THE THORAX OF A DOG (DIAGRAMMATIC FIGURE).
1, Outline of the thorax at the level of the third dorsal vertebra; 2, 2, scapula; 3, spinal border of the scapula; 4, contour of the skin.]
Its external surface is divided into two parts by the spine of the scapula; which, in some animals, terminates inferiorly in a flat and clearly distinct process, the homologue of the acromion process of the human scapula. The two regions separated by the spine are known as the supraspinous fossa and the infraspinous fossa. The supraspinous fossa is anterior to the spine, and the infraspinous is posterior to it. The surfaces of the scapula are, in quadrupeds, flatter than in the human being, and in particular the subscapular fossa, which is also less concave. Some authors attribute this to the lesser curvature of the ribs in quadrupeds. A few words will suffice to prove that there must be another reason. The scapula is not in immediate contact with the ribs; the subscapular fossa is not moulded on them. Besides, the form of the scapula is, as in other parts of the skeleton, dependent on the disposition of muscles, and the development of these latter is correlated to the extent and energy of the movements which the individual is able or required to execute. But the movements which those muscles produce (more especially the rotation of the humerus) are, in quadrupeds, less extensive than in the human being; and, consequently, the muscles which produce them are, proportionally, less strongly developed. The inferior angle (superior and external in man), situated at the junction of the cervical and axillary borders, presents the glenoid cavity, which, looking downwards, receives the articular surface of the superior extremity of the bone of the arm--that is to say, the head of the humerus. Above this cavity, on the lower part of the cervical border, is situated a tubercle which reminds us of the coracoid process of the human scapula. The region occupied by the glenoid cavity is separated from the body of the bone by a constriction--the neck of the scapula.
In birds the scapula is elongated in a direction parallel to the vertebral column, and very narrow in the opposite (Fig. 18): it is also flat, and has no spine. Its coracoid process is represented by a peculiar bone--the coracoidean or coracoid bone--which we shall describe later on when we come to the study of the clavicle and of the anterior region of the shoulder (see p. 26).