At the most fundamental level, reciprocation means energy exchange between linked parts of a biomechanical system.

At first glance, the stifle joint appears to be a huge mistake — the worst-designed part of the musculoskeletal system.

Weight-bearing upon flexed hocks puts enormous strain on all parts of the hock joint. Along with the equally complex stifle joint, the hock is crucial to the horse’s ability to flex and extend the hind limb and create the forward thrust that is the “impulsion” so often sought by horse owners who compete in the Olympic disciplines.

Throughout this series, I have illustrated the concept of systems anatomy. Veterinary education and that of traditional farriers are based on regional anatomy, which studies regions and the anatomical components of each. The perspective of systems anatomy is that bodies are organized hierarchically, i.e. building up from small functional units (for example, cells or single tissues) to subsystems (for example, organs), which work together to make up a major system such as the nervous, circulatory or musculoskeletal system.

Over much of the past year, through this series I have presented the building blocks of information needed to understand the structure of the forelimb reciprocating apparatus in horses. Starting from collagen and elastin at the molecular level and working through all the various connective tissues structured by those two molecules, you have had the opportunity to learn equine anatomy and terminology as enrolled university zoology, pre-medicine or pre-veterinary majors would.

Writing this installment in our Equine Reciprocating Limb series, I could not help but hear the lyrics to Bob Dylan’s “Subterranean Homesick Blues” running through my head — especially the punch line: “The pump don’t work ’cause the vandals took the handles.” That very aptly sums up one of my main points: if anatomical parts aren’t connected, reciprocation does not happen.

Most, if not all, farriers are already familiar with the chain of bones that forms the equine forelimb: scapula, humerus, radius-ulna, carpal bones, cannon bones and splints, pastern bones and coffin bone. Added to this list are the three pseudo-bones called sesamoids, which lie behind the ankle and coffin joints.

The periosteum, thin but tough connective tissue that enwraps all true bones, was the focus of the third installment of this series, “Equine Reciprocating Systems: Connecting Tendon to Bone.” Knowledge of the periosteum creates a good starting point for learning the structure of joints.