Welcome back. In previous lessons, we looked at equine anatomy including the hoof and locomotion. In this lesson we'll look at the muscular skeletal system, which is the system within the horse that allows it to move and perform its various athletic functions. By the end of the lesson, you'll be able to identify the major bones and muscle groups that shape the horses' anatomy. Let's start with the skeletal system. The skeletal system gives the horse's body form and strength. It is the bony structure that supports the soft tissues of the horse. We need to know the horse's basic skeletal anatomy to understand how the normal horse's body moves and functions. This understanding will help us identify problems early. First, let's look at the components of the axial skeleton. The axial skeleton is the part of the skeleton that includes the head and the trunk. Starting at the head, we have the skull, jaws, and teeth. We can see the nasal and orbital or eye cavities. The incisor and molar teeth are separated by what is called the interdental space. This is where the bit of a bridle sits. The skull articulates or connects to the vertebra which extend all the way to the tail. With different section names along the way. The first section is in the neck area where we have the cervical vertebrae. The cervical vertebrae are the most flexible and they control the motions of the head and neck. Next we have the thoracic vertebrae where the ribs attach. The lower portions of the ribs are connected to the breastbone or sternum, by thick, flexible cartilage that expands with the horse's breathing. There is a lumbar vertebral group in the loin area. A sacral group which is fused to form the sacrum. And a coccygeal group which is the bony columns of the tail. The axial skeleton also includes the pelvis, which is made of a pair of flat bones and, like our pelvis, it seats the hip. Remember, that the point of the hip on the horse is an area that is prone to injury. But it's a misnomer. The point of the hip that you're seeing is actually part of the pelvis. The hip joint is located farther back and buried under significant musculature. The upper surface of the vertebrae each have a spinous process of varied length. The tallest one form the withers. And the actual topline of the horse that we see is the topline of these dorsal spinous processes and not the vertebrae themself. Now let's discuss the appendicular skeleton, which includes the bones of the front and rear legs. This part of the skeleton is used principally for locomotion, and these are the bones most involved in lameness issues. The foreleg includes the scapula, shoulder blade, humerus, radius and ulna, the carpal bones, the medial and lateral metacarpals or split bones, the third metacarpal or cannon bone, and the first, second, and third phalanx. The third phalanx is the last bone on the leg, and is also called the coffin bone. It is the tip of your finger, and prone to injury. Horses, unlike any other species, walk on one digit. There are two smaller bones, the proximal or upper sesamoid and distal or lower sesamoid bones. The lower sesamoid bone is also known as the navicular bone. And a tendon travels over this small but important bone, which works like a pulley or lever arm. This particular area can be a source of long term, low grade pain. You may notice, as we discuss in a previous lesson, that the coffin bone is the same shape, relatively, as the hoof that incloses it. The tarsal bones form the tarsus which is more commonly known as the hock which is the common source of hing limb lameness. The horse's hock is the equivalent of our ankle. And it contains four different joints. The largest tarsal bone protrudes upwards to the rear. And it is where their equivalent of our Achille's tendon attaches. This protuberance is called the point of the hock which is prone to becoming calloused and fluid filled from repeated trauma. Now let's look at the soft tissues which account for the muscular part of the musculoskeletal system. Approximate 50% of a horse's weight is attributed to their muscle mass. The skeletal muscles are voluntary muscles which cause movement under direct command of the brain. Muscle groups act as either flexors or muscles that control bend. Or extensors, muscles that stretch or extend the limb. Sometimes these actions occur at the same time to create balanced movement. Muscles have a rich blood supply, but voluntary muscles can contract for only short periods of time before fatigue sets in. The muscle groups of the shoulder direct the position and movement of the head and neck. The muscles on either side of the backbone extend forward and backward and contribute to the effort needed for lateral flexion, rearing and kicking. This area of muscles also supports the weight of the saddle and rider. The large muscle groups of the croup and thigh provide the power for propulsion. Some muscle fibers are directly attached to bone. But usually, the ends of muscles taper off and connect to the bone via cord like tendons. The tendons transmit the pull of the muscle during contraction to result in movement. Ligaments connect bone to bone. The lower legs of the horse have minimal muscle mass, but a strong system of tendons and ligaments, which have less blood supply than the muscles. This means the lower limbs are prone to injuries, which are sometimes difficult to heal even if they're superficial. In the legs, many muscles have long tendons which pass joints and extend over bony prominences. There are tendon sheaths and bursa at these various friction points to allow the muscle to perform more efficiently. That concludes our discussion of the equine musculoskeletal system. Next week we'll turn our attention to the topics of hydration and nutrition. Two of the most important aspects of equine welfare. We'll look at nutritional requirements, as well as types of feed and how the horse processes food. See you there.
