The equine forelimb (aka a hind foot) has four weight bearing digits with a proximal (P1/long pastern), middle (P2/short pastern) and distal (P3/coffin bone/pedal bone) phalanx. The forelimb also has a horny (aka insensitive) laminae of the hoof capsule called collateral sulci which are surrounded by a thin epidermal membrane known as the periople. The periople produces a waxy coating that protects the hoof wall against drying.
The distal phalanx of each digit has two prominent extensor processes that provide the muscle bellies (aka digital flexor tendons) with a place to insert. Each digit also has a flat ‘ground’ surface on its ‘toe’ which is called the solar surface and is where the deep digital flexor tendon inserts.
There are two interdigital ligaments that join each digit to the adjacent digit and prevent digits from splaying too far away from each other. There is also a distal sesamoidean ligament that forms a groove over each distal phalanx and is filled with dermal papillae from the coronary dermis/band (aka corium) of the hoof capsule.
Each weight bearing digit has three articulations – one with the femur (aka tibia) and two with the metacarpals. The articulations are formed by condyles on the femur that meet with a patella (aka kneecap) at each tarsometatarsal joint (aka a metacarpal joint). The lateral femoral trochlear ridge and medial trochlear ridge of the femur have a notch between them called the Devitt notch which the long digital extensor tendon passes through to extend the digit.
A sagittal cut through the hoof shows the horny laminae of the frog and collateral sulci and the inner rim of the coronary groove (Figs 3-5/CD, 8, 10). The frog has a central sulcus (aka a heel sulcus) which is connected to a broad ridge that extends over the proximal and middle phalanx and is known as the frog wall. There is also a sulcus between the bars of the hoof capsule and the frog apex (Figs 3-2/LD, 3, 4, 6, 8).
HORSES have a passive system of forelimb stay apparatus which conserves muscle energy by stopping flexion or hyperextension of specific joints in order to prevent collapse of the limb while the animal is standing. The forelimb stay apparatus is a complex set of structures spanning from the 3rd metacarpal (Mc3 aka cannon bone) to the proximal phalanx and involves a combination of connective tissue, a deep digital flexor tendon and the omotransversarius muscle. This allows the horse to stand without putting any load on the limb during periods of rest and without sacrificing its ability to generate large amounts of force when needed. To achieve this, the muscle bellies of the digital flexor tendons are wrapped in a strong and flexible connective tissue called the deep digital flexor tendon of insertion which is embedded within the navicular bursa or ‘digital cushion’. There are other important features of the equine forelimb but these will be covered in the next article.