Comparative Anatomy-Brachium

Here we have a demo drawing of the dog brachium used as a background for actual human, dog, and horse  skeletal front limbs.This illustrates the shoulder mechanism - the scapula, humerus, ulna, and radius , and more accurately, the evolution of the radius.In the full  pronating radius in the human, where the upper limb has lost it's prime function of  translocation we compare this with the dog  where the radius exhibits  a semi fixed position, more suitable to the survival imperative of running.Finally we observe the horses's radius-ulnar anatomy, where the translocation function has finally perfected itself in the actual fusion of the ulna with the now massive and immobile radius.

Canine Brachium

Dogs have disconnected shoulder bones (lacking the collar bone of the human skeleton) that allow a greater stride length for running and leaping.Consequently this specialization shrinks the biceps, as the scapula lacks the bony coracoid process onto which it can anchor. The supraspinatus muscle enlarges, filling out over half of the scapula surface. The deltoid muscles shrink as they lack the bony ridge of the clavicle from which to emerge, and the triceps take on a more important role in translocation. Most interesting is how the radius has pronated and then locked itself in, although it hasn't fused like a horses radius and ulna .