Written by Wendy Murdoch
The horse and human skeletons are quite similar, even though we stand in completely different orientations, on either four legs or two. There are some major differences between our skeletons: a horse’s bones are much larger than ours, the proportions are different and we each have a few bones that are unique. But, overall, our skeleton and the horse’s skeleton are a lot alike.
In this series of articles on Comparable Parts, I am going to examine the similarities and differences between our two skeletons. Sometimes I will discuss individual bones, while in other cases I will look at a collection of bones. The idea is to help you understand, recognize, locate particular boney landmarks on both you and your horse and learn how the bones determine movement in each species. Hopefully you will find this information useful in understanding how your horse moves. More importantly, it will help you understand how you can work together in whatever equestrian discipline you enjoy.
Let’s start with the overall number of bones in each species. We have more individual bones when we are born, about 300, than we do as adults. Some of these bones fuse together as we get older. Our skull, for instance, is made up a number of different bones as is the horse’s head. These bones fuse to form the skull.
Horses and humans, on average, vary by only one in total number of bones. Horses average 205 bones and humans 206. While we both have a pelvis, only humans have collar bones. Horses have muscles that act like collar bones, but there is no skeletal attachment of the front leg to the rib cage as in humans.
Bones are extremely important for living on earth due to gravity. Gravity is the force that pushes you towards the earth. It is what causes you to go down when you get off your horse and makes jumping exciting. If it weren’t for gravity, jumping a big fence would be boring. There is no thrill without the sense of leaving the pull of the earth behind for a brief moment. Gravity is also what makes the older rider afraid. When you realize you could break bones falling from your horse, your nervous system sends you warning messages to be cautious.
Bones help us resist gravity, protect our internal organs and allow us to move over the earth. Joints are the sites where two or more bones meet. Joints let you and your horse move gracefully. Have you ever seen someone walking around on stilts? They look very tall but quite stiff and awkward in a walking movement. That’s because there are no joints in the stilts.
Joints provide for fluid movement and dexterity. Imagine typing on your laptop with only one joint in each finger. Just for a moment try to send an email without bending your fingers. I think you will appreciate your joints much more after that. Having bones with joints is great, but if you don’t have anything cushioning the places where the bones meet, it can be pretty painful. That’s the job of cartilage, which also acts as a soft skeleton.
Next time you have roast chicken for dinner, help carve it up. Notice the tough whitish stuff at the end of the chicken leg bones. This material is cartilage. Cartilage cushions the bones so that they don’t grind together. The end of your nose is made of cartilage, which is why it is flexible. Your horse has cartiledge at the end of his nasal bones. This is why it is important to make sure your noseband is properly adjusted. If it sits on the cartilageous part of your horse’s nose it, can be quite uncomfortable and potentially damaging, not to mention interfering with his breathing.
Cartilage cushions the joints, but it does not hold them together. That is the job of ligaments. Ligaments are tough fibrous tissue connecting bone to bone. Remember when you tore the chicken leg apart to see the cartilage? You had to tear the ligaments to get to the ends of the bones. Ligaments have very little blood supply, which is why a ligament injury, say in the stifle, is so serious. It takes much longer for ligaments to heal than muscle or bone injuries due to this lack of blood supply.
The ligaments have a very serious job to do and require minimal energy to perform their function. They have to keep the bones in place and restrict motion in certain directions. In some cases ligaments have to keep bones so close together there is almost no movement in the joint. Take your horse’s pelvis for example. The pubis is where the two halves of his pelvis meet in the front. The ligaments hold this area together so that the force of the hind leg pushing against the ground propels the horse forward. If the horse were to fall on ice and tear these ligaments, he would have tremendous difficulty standing or walking.
Bones support, cartilage cushions and ligaments hold bones together as well as restrict movement so that joints don’t overextend. But how does all this boney mass move? That’s the job of the skeletal muscles. They are the brawn of the body. They are attached to bones by tendons, which is connective tissue running from the muscle belly to the bone.
Muscles can only contract or decontract (relax). How they contract determines if the overall length of the muscle increases or decreases. If you pick up an object, your biceps contract and your triceps have to decontract to allow you to bend the elbow. If both contract equally at the same time, you can’t move the elbow joint.
It is this interplay of contraction and decontraction that allows horses and humans to move. However, the skeletal muscles are really quite dumb. Without the nervous system they wouldn’t do anything at all. In fact, they would wither away or atrophy without signals from the nerves to contract.
Nerve impulses cause muscles to contract, moving the bones so that we can run and our horses can jump. Muscles are attached to bones by tendons, fibrous tissue going from muscle to bone. Ligaments keep the bones in place without effort and prevent over-extending joints while the cartlige cushions the process. Coordinating muscular contraction is the job of the brain and nervous system. If you had to think about all the muscles that go into standing up from a chair, you probably would never get up. Your nervous system does this for you all the time without you ever having to think about it. This is true for your horse as well. Pretty amazing when you think about it, isn’t it?
In future installments we will look at specific bones you and your horse’s skeleton share, and learn how they are similar and different.