Horse Skeleton: Axial and Appendicular

• The horse skeleton consists of about 206 bones containing living cells and minerals.
• Bones have a blood, lymph, and nerve supply allowing them to repair themselves when diseased or injured.
• During growth, bones can adjust and respond to changes in stress levels caused by movement.
• The skeleton is divided into two parts, the axial and appendicular skeletons.

Axial Skeleton

• Consists of the skull, vertebrae, sternum, and ribs
• Acts as the main axis and support of the body.

Appendicular Skeleton

• Includes the front and hind limbs, scapula, and pelvis
• Attaches to the axial skeleton and facilitates movement.

Purpose of the Skeleton

• Acts as the support of general framework for the body.
• Internal skeletal frame that allow horses to be upright, and not creep along the ground like a slug.
• Framework, giving attachment for tissues, muscles, tendons, and ligaments.
• Gives body rigidity.
• Jointed to assist in movement.
• Protects vital internal organs.
  • The skull protects the brain.
  • The spine, made of vertebrae, protects the spinal cord running down the central vertebral canal.
  • The scapula and rib cage protect the heart and lungs.
  • The pelvis and sacrum protect the urinary and reproductive organs.
• Locomotion
  • Essential for locomotion.
  • Muscles pull on bones to produce movement at a joint.
• Respiration
  • The ribcage is movable and assists with breathing.
• Storage
  • Stores calcium, phosphorus, and magnesium, which can be laid down in or removed from the bone as needed.
  • Fat can be stored in the internal cavity of the bone or marrow.
• Blood cell production:
  • Red and white blood cells, and platelets for blood clotting, are made in the bone marrow.
  • Only certain long bones can produce and store these cells.

What is Bone?

• Bone is living connective tissue that constantly modifies its structure in response to internal and external stresses.
• Bone is the second hardest substance in the body.
• It is tough, strong, and slightly flexible.
• Minerals stored in the bone make it more rigid but brittle.
• Younger horses' bones have fewer minerals than older horses.

Bone Cells

• Osteoclast: absorbs bone
• Osteoblast: builds new bone
• Osteocyte: static bone cell

Types of Bone

• Dense or compact bone:
  • Hard bone found in the long bones and provides bone strength and rigidity
• Cancellous or spongy bone:
  • Found at the ends of long bones and in short bones.
  • It is a fine network of bony threads and lines the dense bone and is filled with marrow, to make bone lighter for its size.
• Both are found in differing proportions in one bone

Anatomy of a Bone

• Surface of bone: covered by the periosteum.
  • Periosteum: a tough protective membrane that provides for attachment of ligaments and tendons, and is blood-rich and bone-producing.
• The endosteum : a membrane that lines the bone marrow cavity.
• Bone consist of:
  • Epiphysis is a head either side of spongy bone.
  • The shaft or diaphysis are in the middle of compact bone.
• Bones have articular cartilage at joint ends.
• Bones have cartilaginous growth plates (epiphyseal plates).
• Marrow cavity: lined by the endosteum.
• Nutrient artery : enters at the nutrient foramen (hole).

Bone Marrow

• Four types of bone marrow exist, depending on the animal's age:
  • Type 1: in young horses, marrow is red and makes red and white blood cells.
  • Type 2: as horse gets older, production of cells slows and is yellow
  • Type 3: when production stops and marrow becomes white.
  • Type 4: in the old horse marrow is gelatinous.

Bone Growth

• Two ways bone formation/growth can occur:
  • Intramembranous: grows in thickness or diameter.
  • Endochondral/intercartilaginous: grows in length.
• Bones heal and remodel after a fracture/external stressors from these methods
• Intramembranous:
  • Seen in flat bones and shaft of long bones.
  • Osteoblasts beneath periosteum form new bone matrix, becoming calcified and resulting in thicker bone.
  • An osteoblast surrounded with calcified bone becomes an osteocyte.
  • Marrow cavity enlarges as compact bone grows to keep bones from becoming too heavy.
  • Osteoclasts enlarge the marrow cavity by removing bone lining the cavity.
• Endochondral/intercartilagenous
  • Bone grows in length from epiphyseal or grow plates.
  • Cartilage is created by chondrocytes (cartilage cells).
  • Growth plate becomes wider.
  • Cartilage is systematically replaced by bone as osteoblasts move in, and cartilage becomes calcified.
  • Immature bones stop growing lengthwise when growth plates are completely ossified.

Growth Plate

• Closure times of long bones is the process of fusion to stop them growing in length.
• It is not advised to overwork horses before they are mature in order to avoid damage to growing epiphyseal plates.
• Closure stars at the bottom bones and move upwards.

Classification of Bones

• Bones can be classified according to appearance.
• Long Bones
  • Largest in body and cylindrical
  • Contain marrow (produce RBC)
  • Joint surface at either end
  • Function as levers in support and movement
  • Appendicular skeleton
    • Examples
      • Cannon
      • Femur
      • Radius
      • Ulna
      • Humerus
      • Tibia
      • Long pastern
• Short or cuboid bones
  • Strong and compact
  • Absorb concussion
  • Have no marrow but have spongy bone
    • Examples
      • Carpal knee bones
      • Tarsal hock bones
      • Short pastern
      • Pedal bone
• Flat bones
  • Thin, broad, flattish surface
    • Provides protection
    • Muscle attachment
    • Examples
      • Skull (true flat bone)
      • Scapular
      • Ribs
      • Sternum
      • Pelvis
• Sesamoid bones
  • Lie within tendons or ligaments
  • Add strength to these tendons/ligaments
  • Reduce friction
    • Examples
      • Patella
      • Proximal sesamoid (fetlock)
      • Distal sesamoid (navicular)
• Irregular bones
  • Different shapes
    • Protect spinal cord
    • Provide support and form a base for muscle attachment
      • Examples
        • Vertebrae (cervical, thoracic, lumbar, saccral, coccygeal)

The Skull

• It is the heaviest bone in the body, gives protection to the brain.
• Sinuses are air filled cavities that help decrease the weight of the skull.
• The frontal sinus is the largest, then the two maxillary sinuses, where the molar tooth roots can lie.
• The sinuses on the one side all communicate with each other and the nasal chamber, but not with the sinuses on the other side of the skull.

The Vertebral Column and Ribs

• It includes:
  • 7 cervical (neck) vertebrae
  • 18 thoracic (chest) vertebrae
  • 6 lumbar (loin) vertebrae
  • Sacrum – 5 fused vertebrae
  • Coccygeal (tail) vertebrae which varies from 15 – 25.
• Eighteen ribs either side of the thoracic vertebra, 8 true ribs attached directly by cartilage to sternum.
• Ten false ribs joined in a “rope” of cartilage which eventually joins to the posterior sternum.
• The joint between the lumber vertebrae and sacrum (lumbar sacral joint) is a very important joint involved in collection of the hind limbs and propultion.

Forelimb

• It is attached to the body by the thoracic sling (muscle and ligaments).
• There is no bony or joint connection like in hind limb giving benefits such as:
  • Ease of grazing because one leg is flexed and one extended to take support so trunk and base of neck can sink nearer to the ground.
  • Increased agility with balanced turning at speed and a longer stride length

Joints

• A joint is an area where two or more bones meet. Joints may or may not allow movement (articulation).
• Three types of joints:
  • Fixed
    • Fibrous joint
    • Fused together
    • Little to no movement
    • Held together by fibrous connective tissue
      • Example
        • Skull
        • Sacrum
  • Slightly movable
    • Cartilaginous joint
    • Only very slightly movable
      • Example
        • Vertebral joints
        • The pelvis (symphysis)
  • Movable
    • Synovial joints
    • Freely movable
    • Modified shock absorbers
      • Types of synovial joints
        • Hinge type
          • Allowing movement in one plane only
            • Examples are elbow and fetlock
        • Ball and socket type
          • Allows rotation adduction and abduction
            • Examples are hip and shoulder
        • Pivot type
          • Allows horse to turn head
            • Joint between skull and first neck vertebrae (atlanto-occipital joint)
        • Gliding type
          • Where bones glide over one another
            • Examples are the stifle and patella Movement in Synovial joints is due to the contraction and relaxation of muscles associated with that joint. Joints are stabilised by ligaments, tendons and muscles. Both ends of the bone at a Synovial joint are covered in cartilage, a dense connective tissue containing collagen and elastic fibres. This articular cartilage reduces friction and helps absorb shock. Cartilage contains no blood vessels or nerves. Cartilage metabolism is predominantly anaerobic. Nutrients, including glucose, oxygen, and amino acids must reach the chondrocytes by diffusion from the synovial fluid. Movement of the joint aids in pumping the synovial fluid around and therefore also aids in diffusion. Synovial fluids gets it's nutrients by diffusion from the highly vascular joint capsule.