The cheiroptera (bats) possess an extremely well-developed clavicle, on account of the varied movements which their thoracic limbs execute.
This formation of the shoulder which favours flight in the bat is even more remarkable in birds. In these latter ([Fig. 18]) the clavicles, fused together by their lower extremities, form one bone, having the shape of the letter V or U, which is known as the fourchette; this bone, acting as a true spring, keeps the shoulders apart, and prevents their approximation during the energetic movements which flight necessitates.
In birds whose power of flight is strong, the two limbs of this bone are widely separated and thick, and the fourchette is U-shaped. Those whose flight is awkward and but slightly energetic have the limbs of the fourchette slender; they unite at a more acute angle, and the bone is V shaped.
Furthermore, a bone named the coracoid joins the scapula to the sternum; this bone, often fused with the scapula, where it contributes to the formation of the glenoid cavity, represents in birds the coracoid process of the human scapula. If we fancy this process directed inwards, and sufficiently lengthened to join the sternum, we shall have an idea of the disposition of the bone we are now discussing, and the reasons for which the name has been chosen by which it is designated. The coracoid bone, like the fourchette which it reinforces, offers to the wings a degree of support proportionate to the efforts developed by those limbs; for this reason it is thick and solid in birds of powerful flight.
The superior extremity of each branch of the fourchette, at the level of its junction with the coracoid and the scapula, bounds, with these latter, a foramen which gives passage to the tendon of the elevator muscle of the wing, or small pectoral. The importance of the fourchette being, as we have seen, in proportion to the movements of flying, it is easy to understand that the bone is not found in the ostrich.
The Arm
A single bone, the humerus, forms the skeleton of this portion of the thoracic limb.
The Humerus.—The bone of the arm is, in quadrupeds, inclined from above downwards and from before backwards.
It is, with relation to other regions, short in proportion as the metacarpus is elongated, and as the number of digits is lessened. In the horse, for example, whose metacarpus is long, and in which but one digit is apparent, the humerus is very short. The slight development in length of the humerus explains its close application to the side of the animal as far as the elbow.
In animals in which the humerus is longer, the bone is slightly free, as well as the elbow, at its inferior extremity. [Later on] we will return to the consideration of this peculiarity and of the proportions of the humerus, after we have studied the other parts of the fore-limbs.