The skeletal system of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses typically have 205 bones. The pelvic limb typically contains 19 bones, while the thoracic limb contains 20 bones.
Functions of bones Bones serve four major functions in the skeletal system; they act as levers, they help the body hold shape and structure, they store minerals, and they are the site of red and white blood cell formation. Bones can be classified into five categories
Long bones: aid in locomotion, store minerals, and act as levers. They are found mainly in the limbs. Short bones: Absorb concussion. Found in joints such as the knee, hock, and fetlock. Flat bones: Enclose body cavities containing organs. The ribs are examples of flat bones. Irregular bones: Protect the central nervous system. The vertebral column consists of irregular bones. Sesamoid bones: Bones embedded within a tendon. The horse's proximal digital sesamoids are simply called the "sesamoid bones" by horsemen, his distal digital sesamoid is referred to as the navicular bone. Ligaments and tendons hold the skeletal system together. Ligaments hold bones to bones and tendons hold bones to muscles. Synovial membranes are found in joint capsules, where they contain synovial fluid, which lubricates joints. Bones are covered by a tough membrane called periosteum, which covers the entire bone excluding areas of articulation.
Ligaments Ligaments attach bone to bone or bone to tendon, and are vital in stabilizing joints as well as supporting structures. They are made up of fibrous material that is generally quite strong. Due to their relatively poor blood supply, ligament injuries generally take a long time to heal. Ligaments of the upper body include:
Nuchal and supraspinous ligaments: the nuchal ligament is composed of strong elastic tissue originating from the occipital protuberance of the skull (the poll) and extending to the withers. The main (funicular) portion has attachments on the second cervical vertebra, and the flattened fanlike (lamellar) section has bands extending to the third through at least 5th cervical vertebrae, each getting smaller, with attachments less commonly found on the sixth and seventh cervical vertebrae. After attaching caudally at the withers, the nuchal ligament continues as the supraspinous ligament, attaching to each of the subsequent thoracic and lumbar vertebrae. Its main purpose is to support the head and allow it to be moved upward or downward. Intercapital ligaments: lie between the first through eleventh ribs. Help to prevent thoracic disk herniation. Sacrosciatic ligament: Originates from the sacrum and coccygeal vertebrae, inserts into the pelvis. Ligaments of the legs include:
Suspensory ligament: runs from the back of the cannon bone (between the two splint bones), then splits into two branches and attaches to the sesamoid bones at the bottom of the fetlock. Branches continue downward and attach to the extensor tendons. The main purpose of the suspensory is to support the fetlock joint, preventing it from overextending. Injury to this ligament is an important cause of lameness in performance horses. The suspensory is a modified muscle, the equine equivalent of the interosseous muscle, which contains both tendon fibers and residual muscle fibers. Interosseous ligaments: connect the cannon bone to each splint bone. Injury to this ligament produces the condition known as "splints". Proximal and distal check ligaments: The proximal check ligament originates from the radius and attaches to the superficial digital flexor tendon. The distal check originates from the palmar carpal ligament and attaches to the deep digital flexor tendon, approximately 2/3-way down the metacarpus. Plantar ligament: in the hind leg, originates on the calcaneus, running down the lateral side of the tarsus, attaches to the 4th tarsal and 4th metatarsal bones. Injury leads to a condition known as "curb." Inter-sesamoidean ligament: supporting ligament connecting the two sesamoid bones. Distal sesamoidean ligaments: run from the sesamoid bones to the two pastern bones. Important in the stay apparatus as a functional continuation of the suspensory ligament. Impar ligament: runs between the navicular bone and the 3rd phalanx. Annular ligament: goes around the back of the fetlock, surrounding the flexor tendons and their tendon sheath, attaching to the sesamoid bones. It helps to support the fetlock, and provides an enclosed "pulley" for the flexor tendons to run through. Collateral ligaments: With the exception of the shoulder and hip, all joints in the fore and hind limbs have collateral ligaments which allow flexion in the sagittal plane, but prevent significant lateral-medial collateromotion, thereby stabilizing the joints. The collateral ligaments of the stifle allow for some rotational movement, and those of the distal interphalangeal joint allow for moderate collateromotion to allow for hoof contact on uneven surfaces.
Axial skeleton
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