Fig. 36.—Left Scapula of the Cat: External Surface.
1, Posterior or axillary border; 2, superior or spinal border; 3, anterior or cervical border; 4, spine of the scapula; 5, coracoid process; AA′, length of the spinal border.
In the dog, the posterior angle, formed by the junction of the axillary and the superior (spinal) borders, is obtuse; the spine rises perpendicularly from the surface of the bone. The width of the scapula, measured at the level of the spinal border (from A to A′, [Fig. 35]), equals about half the length of the spine. We must, however, make an exception for the turnspit dog, in which the superior border equals three-fourths of that length. The scapula is, in this case, of a more compact type; it is broader, but shorter. In the cat, the anterior outline of the scapula, formed by the union of the cervical border and the corresponding half of the spinal, is more convex; the posterior angle is not obtuse, as in the dog. The spine is bent slightly downwards and backwards; before terminating in the acromion process it presents a triangular projection, the apex of which is directed downwards. The tubercle which represents the coracoid process is curved inwards more strongly than that of the dog, thus resembling more closely the appearance of this process in the human being.
All proportions considered, the scapula of the cat is broader than that of the dog; its width, measured along the length of its spinal border (from A to A′, [Fig. 36]), equals three-fourths of the length of the spine.
The clavicle is rudimentary; it is, however, better developed in the cat than in the dog. The clavicle of the cat is represented by a small, elongated bone, curved in outline, the convexity being turned forward; it is united to the acromion and the sternum by ligamentous fibres; that of the dog is merely a scale-like osseous plate situated on the posterior surface of a muscle of this region (see [Figs. 16] and [17]).
The humerus is long and twisted in the shape of an S. The inferior articular surface has the form of a simple pulley, for the condyle is very slightly marked. The internal part of this articular surface descends lower than the external; this condition resembles that found in the human being, where the inner lip of the trochlea is lower than the condyle.
In the dog, the olecranon fossa communicates with the coronoid by an opening.
In the cat, there is a supra-epitrochlear canal (see [Fig. 19]), but no olecranon perforation.