As you will know from my Anatomy blog horses have about 205 bones in their bodies. To understand how to look after them and what can go wrong it is useful to understand a bit about the function of the skeleton and the composition of bone.
The skeleton is a framework of bone and cartilage found within the body tissue it:
- gives the body it's shape and posture
- allows movement through the action of joints and by providing attachment for the muscles
- protects internal organs
- assists with breathing (the ribs)
- produces red blood cells (in the bone marrow)
- acts as a reservoir for calcium and phosphorus
(see my Anatomy blog)
- The skull is large to accommodate the jaw as the horse is a grazer. However, it is also to balance the weight of the abdomen. When moved with the neck it allows the horse to alter his centre of gravity.
- The spine comprises the cervical, thoracic, lumbar, sacral and coccygeal vertebrae. The cervical vertebrae allow lateral movement and curvature. Arching of the neck is mostly movement of the muscles! The thoracic vertebrae attach to the ribs and any movement is limited. The lumbar vertebrae do have the ability for slight lateral movement but become more fixed with age. The sacral vertebrae are fused. Side to side movement is obvious at the walk but at faster paces the muscles resist this and make the spinal column rigid thus eliminating the wasteful sideways movements. In addition, muscles above and below the spine prevent it 'rounding' as in cats and dogs.
- The scapula is large and flat and allows the attachment of the forelimb to the trunk by muscles and ligaments. Horses do not have collar bones (clavicle) which means that the forelimbs are less restricted.
- The humerus, radius and ulna are all heavily muscled.
- The carpus has 7 (or 8) carpal bones which are arranged in 2 rows. These little bones mean there are several surfaces, this allows for more concussion absorption.
- The hindlimb is directly attached to the spine this means that any concussive forces will go to the spine and not the limb causing the potential for back problems rather than hind limb problems.
- The pelvis (ilium, ischium and pubis) provides extensive areas for muscle attachment.
- The femur is very strong but can be shattered by uncoordinated muscle movement. Hip problems are usually as a result of rotation or muscle spasm and not the femur being out of place!
Bone appears to be hard and inelastic but it is actually dynamic and is able to respond to changes in the environment - it is affected by nutrition and stress. Bone is made up of living material which can grow with the horse. Some adaption occurs before birth. In the embryo the skeletal material is cartilage, this is mostly replaced by bone as the horse becomes an adult. Cartilage will remain where firmness is needed but also some flexibility such as the ends of bones and the rings of the trachea and larynx.
Compact bone is much harder and denser and forms the main bulk of the shafts of long bones. In the long bones it has a large central cavity filled with yellow bone marrow. This bone marrow produces white blood cells (see my Immune system blog). In other bones the compact bone has tiny spaces. Spongy (cancellous) bone is made up of a network of hard bars and larger spaces filled with red bone marrow. Short bones are made up of spongy bone as are the ends of long bones. The combination of the two types of bone and the spaces mean that the bones can be both strong and hard but also light! Bone is resistant to 'squashing' forces but weak under 'pulling' forces. Therefore it is great for support.
Bone also contains nerves, blood and lymph vessels. New bone can be laid down and also in contrast the size of bones can be reduced.
Functional Anatomy of Bone
Mature bones are made up of cells called osteocytes, these are surrounded by the intercellular matrix. The cells are located in spaces (called lacunae) which are connected by small canals (canaliculi) which transmit tissue fluid containing the nutritional needs of the bone cells. The entire bone is covered by a protective membrane (called the periosteum)which also assists with the transport of nutrients and serves as an attachment for tendons and ligaments.
New bone cells are formed by special cells called osteoblasts, these are capable of rapid division. Most initial modification of bone occurs before the age of 3 months - called modelling. Remodelling involves cell turnover. The first steps of this involve weakening the area further by excavating a tunnel using osteoclasts (which dissolve old bone cells). This then allows the transport of nutrients and materials required for rebuilding to the area. The osteoblasts will then form new bone cells. Obviously this means that bones are WEAKER before they become stronger so applying further stress at this time may lead to a stress fracture!
Growth of Long Bones
The diameter of bones is increased by osteoblast action within the periosteum, the length is increased as cartilage is converted to bone. Cartilage is then replaced within the epiphyseal (growth) plate. The growth plates of bones stop 'growing' at different times. Problems can occur if growth is too rapid or one side grows more rapidly than the other.
The development, modelling and remodelling of bone should be taken into consideration in youngsters and horses recovering from injury. Even a knock can cause changes to the bone and nutrition can affect the health and strength of them. Basil has 'new' bone which has formed around the area affected by his tooth abscess - something you would perhaps not think would happen in a horse of 14!
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Until next time!