| The Skeleton of The Horse |
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By Stephen Champion, BSc (Hons) BVM&S MRCVS, The primary functions of the horse’s skeleton are to support the body, to provide the system of levers used in locomotion and to protect soft tissue parts. These functions are performed mainly by the bones of the horse, whose shape and microscopic design are determined by biomechanical factors, such as the way the horse moves. Bones also have a secondary role to play; supplying a reserve of calcium, phosphorus and other chemicals, in conjunction with the diet of the horse. The skeleton can be divided into the cranial skeleton (the head) and the post-cranial skeleton (the rest of the horse!). The post-cranial skeleton can further be divided into the axial skeleton of the trunk and the appendicular skeleton of the limbs. The neck, back and tail of the horse exhibit a serial repetition of similar elements, most notably the vertebrae, extending from the skull to the tip of the tail. The vertebral column was once likened, by a famous zoologist, to the structure of a suspension bridge. For any of us who have owned a horse with a chronic “back problem”, the well-known upkeep of the Forth Road Bridge will seem relatively insignificant! The bones that make up the skeleton are simply classified according to their shape. Thus, long bones are typical of the limbs of the horse and come under enormous strain when the horse is exercised. The short bones are rather small and square in shape and make up the complicated structures of the hock and knees. These bones can be damaged, for example in young racehorses on the gallops for the first time or during a fall. The flat bones are found in the shoulder and pelvis and also in the skull. Being flat, they are prone to damage through hard knocks but afford protection to underlying soft tissue structures, such as the brain (in certain breeds!). There exist some specialised varieties of bones, which are found in important areas of the horse’s body. Some bones are found within tendons where these tendons change direction. These bones are known as sesamoid bones and are found for example in the stifle (as the patella or kneecap) and in the fetlock (as the proximal sesamoids). Another sesamoid bone is found in the foot, as the familiar but enigmatic navicular bone, famous for the devastating navicular disease and in serious penetrations of the frog. Bones meet each other at joints, some of which are designed to unite bones firmly and others to allow free movement. The latter type, found in the horse’s legs, are called synovial joints, where the articulating bones are separated by a fluid filled space, the joint cavity. The ends of the articulating bones are covered by a layer of cartilage, several millimeters thick, and these are bathed in a thick, lubricating liquid called synovial fluid. The joint capsule is lined by the synovial membrane. During the horse’s life, these structures are placed under enormous friction. Often the cartilage is worn away, the fluid is depleted and the membrane is inflamed, leading to the painful condition of osteoarthritis. Many treatments of this crippling disease are associated with trying to reinstate the status quo within the joint, often with anti-inflammatory drugs, joint suppliments and modern “nutraceutical” preparations. The bones of the horse’s skeleton are attached together by ligaments, short and very tough structures, often found incorporated into joints. Bones in addition have to be attached to muscles, in order to move. These attachments are called tendons, and in the horse can be very long and vulnerable structures. The best known tendons are located running down the backs of the horse’ s lower legs, attaching the very powerful muscles of the upper leg to the bones of the foot. Much more on these exciting and expensive structures will be covered in a later, gripping article. For those of you with their noses in a tub of garlic powder in a bid to get through this article, a few clinical and relevant words on which to finish. When does a fracture of the horse’s leg bones mean the end of a horse? Several years ago, I suspect it meant almost always. Nowadays, many fractures can be repaired, but there are some which due to the considerable forces applied to the horse’s skeleton, are still beyond the scope of veterinary orthopaedic surgeons. These fractures comprise complete fractures of the femur; complete fractures of the humerus in mature horses over 300kg; complete fractures of the tibia; and many compound fractures in many parts with severe tissue damage. This is intended only as a guide and there will always be exceptions. Don’t let it keep you awake! |
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