Hoof Structure

The amount of information available about horse hooves and their trimming, shoeing, barefooting, wrapping, booting, soaking, and “orthotic-ing” is mind-boggling.  So the first thing I want you to know in this post is that I’m not going to summarize all that or wade in to the “natural hoof care” debate or encourage you to purchase a product of some sort.  My goal in this post is the same as it is for the blog as a whole: to offer you a different point of view on a subject — one that doesn’t seem to be readily available otherwise — that is based in biomechanics and good, solid biology.  It will be your job to take what you learn here and figure out how you want to integrate it with what you already do.

This post will discuss specific aspects of the biology of hoof structure, and then the next post will consider functional aspects of those structures in terms of basic biomechanics.  In both posts, the place we’re heading is a consideration of the nature and size of the element called the “coffin bone” by horsepeople and the “terminal phalange” by anatomists, and how that element functions in the stress environment of a large animal’s hoof.

We’ll start by looking at what bones are in a horse’s legs and feet.  These bones have names as well as “definitions” based on where they’re located in the body and what other bones they connect to on the ends.  For instance, animals have a lower jaw bone called the mandible.  In mammals, the mandible is entirely mde of a bone called the dentary, which bears teeth on its upper surface (if the animal has teeth in its lower jaw at all, which some do not).  There is a right dentary bone and a left dentary bone, one on each side of the animal’s lower jaw, and they join each other in the middle at the front (to form what we would consider the animal’s “chin”).  The two halves are “sutured” there (that is the anatomical term) by a joint that is fixed or immovable and that looks like a wavy line of back-and-forthing such as might be done by a needle and thread.  As the animal ages, this line disappears and there seems to be only one bone instead of two.  On the back end of each mammal’s dentary bone there is a process that sticks upward, that makes a fairly high crest and that has a lower piece sticking back with a roller joint on it.  When the lower jaw is put in proper life position against the skull, this roller joint articulates or fits against a socket in the skull, and the jaw rolls to open or close the mouth at this joint.  The high crest projects up through a part of the skull that sticks out from the rest of the bone as a big arch.  In life, it can be seen that strong jaw muscles attach to both the crest of the dentary bone and that arch on the skull.  These muscles help to pull the jaws together so the teeth can chew food.  Here is what the dentary looks like in a horse, when it is not attached to the skull.

The picture below (left) shows you how the dentary bones look in position, articulated with the skull.  You can see the crest goes through an arch on the skull and that the roller joint articulates with or fits up against a part of the skull that is shaped to receive it.  The dentary in this picture is the white (uncolored) bone.  The lower teeth are hidden by the upper teeth, which “lap” around them to the outside as this is drawn.

The skull to the right of the horse, above, is that of a human.  It shows the human dentary bone and the way it fits to its skull, for comparison.  I have colored the dentary a light bluish-purple.  The point is to notice that the basic shape, position, and relationship to the bones around it are the same for the human dentary bone as in the horse.  That’s why we consider both bones to be dentary bones, is that they meet the same “criteria” of definition for that bone.  The dentary of the horse is therefore said to be homologous to the dentary of humans. It should be added that homologous structures may be seen to develop from the same embryonic tissues during development of different animals.  So the similarity of structure that we call homologous reflects a real commonality of genetic origin and development.

Comparative anatomy gives us powerful tools for understanding animal structures.  That’s probably more true for the hoof than for just about any other structure, because we all know how vitally important a horse’s hoof is.  “No hoof, no horse” is an adage that’s been around about as long as people have been riding and working horses.  And it turns out that the hoof is an astonishingly “little” bone.

Here is a horse skeleton with the bones of its front leg colored in a specific way.  The circle I’ve drawn is around the bone horsepeople call the cannon bone, to serve as a place-marker with which you’re familiar.  I’m not going to worry here about what other names horsepeople have coined for different parts of a horse’s limb anatomy, because that will actually confuse matters.  I’m going to use terms that tell us about the homologies of these structures instead. Next to the horse skeleton is the skeleton of a human arm.  I have colored the bones there so they correlate to their homologues in the horse skeleton.  As we run through these bones, you can use your own arm and shoulder to help you learn the structures experientially.

At the top of the of the arm, the first good-sized bone that moves the arm in a mammal, is the scapula or shoulder blade bone.  It’s colored pale turquoise here.  In a horse, the scapula lies beneath (and gives the sloping ridge-shape to) the withers.  In a human, the scapula is “down behind” our arm in our back because of the way our chest is flattened out side to side.  There is a bone called the clavicle or collar bone in humans that is shown in white on the diagram above, that is not present in the horse.  Then the next main bone in the sequence is the humerus bone.  We looked at the humerus bone already in our discussion of the trotting gait in horses, and it’s colored in red on the diagram.  It’s also in red on the human skeleton.  In you, the humerus is inside your upper arm.  (You’ll have to disregard the horse’s back leg here, on which I previously colored the femur red as well for a previous blog.  The horse’s back leg is homologous to a human’s leg, too, but we are going to focus on the foreleg and arm as our example for this blog.)

Beneath (or “outward from”) the humerus there are two bones, side by side:  the ulna and the radius.  Here, I have colored the ulna a light green and the radius yellow.  In humans (and many other mammals), the two bones are totally separated their entire length.  They twist back and forth across one another when we rotate our forearms to turn our palms up or palms down.  (If you sit with your right arm bent at the elbow, with your right hand just above your lap, you can move your arm in a way that allows you to feel these two bones.  Put the fingers of your left hand across your right arm, about halfway between the inside of your elbow and your wrist.  Press down firmly.  Then, without moving your upper arm, simply twist your wrist to rotate the palm of your right hand so it faces the floor, then faces the ceiling again. (Your right thumb will face left when your hand is palm-down, and right when your palm is face-up.)  The fingers of your left hand should feel the radius cross over the top of the ulna when you do this.  The ulna is the longer of the two bones, and the back end of it “sticks out” to form our elbow.

Horses have an ulna and radius, too, but theirs are fused together not too far below the elbow area.  (The next time you saddle your horse, take a good look at the part of the horse’s arm just in front of the cinch or girth.  You will see a very obvious elbow there.)  It’s hard to see what they look like in the skeleton above, so here’s a closer picture of just those two bones in a horse.  I colored the ulna light green and the radius yellow.  You can see the dark line between them near the top of the radius, and then you can see that the ulna fuses to, and essentially becomes one with, the radius farther down the shaft. This is because the separate ulna and radius allow the hand to be twisted around the long axis of the leg — as is so in us — and that would be a really bad way for a horse’s leg to move.  Can you imagine what would happen if a horse was running at 30 miles an hour and suddenly twisted its front hoof on its leg?  So there is an adaptation of this structure that keeps the twisting motion from taking place.  The radius and ulna are “locked” down.

Beneath the radius and ulna are the wrist bones, or carpals.  In the big skeleton, those are not even drawn in, they are of such little consequence to many horsepeople learning anatomy.  But of course, those bones are extremely important and you sometimes hear about one or more of them being injured in a performance horse.  Since we’re heading for the hoof, though, we’re going to skip the carpals for now and head on downward.  So the carpals are simply white on the human skeleton as well.

Beneath the carpals are the finger bones.  The first of these is a long metacarpal bone that is actually positioned inside the fleshy part of the hand (where the palm is).  Then, at the end of each metacarpal, there is a series of three little bones, each called a phalange.  There are three phalanges on each finger except the thumb, which has only two phalange bones.  If you take a look at your hand, you will see all this.  On the human skeleton, I have colored the metacarpal a dark blue, the first phalange dark red, the second dark green, and the last orange or dark gold.  However, I have only colored the bones of the middle finger — because that is the finger homologous to the hoof bones of a horse.

Look at the colors of the bones in the skeleton above, showing the horse hoof.  You will see that the cannon bone is actually a metacarpal.  And the bones that make up the pastern and the hoof are phalanges. Here is a diagram that shows you the end of this sequence a little more clearly.  It’s a photograph of a horse hoof that was injected with plasticine so it could be sectioned cleanly and show you the structures.  On the left is an image without any modification, and on the right is the same image with the bones colored so they correspond to the key I’ve just run through. (The diagram, from Wikimedia commons, is labelled in German.)

What should blow you completely out of any sense of complacency about your horse’s abilities, here, is realizing how very tiny the bone is that’s inside your horse’s hoof.  I mean, LOOK at it!  Here’s a nice shot of this coffin bone from a horse care website, that happens to have the edge of a person’s thumb and thumbnail in it, for scale.  Take a look at just how small this bone really is.  And if you consider the fact that the “hoof bone” or “coffin bone” is, in fact, a terminal phalange — the last tiny bone of a single finger — the fact that it bears so much weight, running at high speeds is nothing short of astonishing.

We are impressed, and rightfully so, by ballet dancers who leap and pivot en pointe, with their feet positioned so that their weight is borne only on the tips of the toes.  Depending on the gender of the dancer, they may weight perhaps 80 to 150 pounds a person.  What you can see here is that horses are also en pointe, minus the ballet slippers, but carrying closer to 1000 or 1200 pounds when they run, jump, and spin on those toe tips.  It takes nothing away from human ballerinas, but I think it certainly adds to our appreciation of horses.

Or at least, I hope it will after we consider the consequences this has for the stresses (force per unit area) in the bones of horses’ feet.  Coming up, next time . . .

 

This post was a featured blog entry on BarnMice, Dec. 1, 2012.

 

Learning How Horses Walk

Sometimes the best way to understand how your horse moves is to mindfully experience how your own body moves.  This is called experiential learning.  When I used to teach comparative anatomy and functional anatomy in the university, I always told my students that the number one tool we all have to understand how animals move is our own bodies.  See if you don’t discover something really cool about horse movement by trying this exercise in experiential learning.

What you need to do is read the next four paragraphs (down to the break in the text), and then set your computer aside and get down on the floor on your hands and knees.  Yes, really.  Don’t just think about it, and don’t ask someone else (like your teenager) to do it so you can watch them.  You need to experience this for it to have the substantial learning impact it can have.

Get on your hands and knees, supporting yourself.  In other words, you don’t want to crawl like you’re scooting under barbed wire.  War games are next week.  (OK, not really.  But don’t scoot.)  Then just crawl across the floor as far as you can without hitting the furniture.  Don’t think about it;  just do it.

Once you’ve done that, and have your rhythm established, turn around and go back the other way, still crawling, but this time just pay attention to — but do not try to control — the order in which your hands and knees hit the floor.  I suggest you choose your left knee to start with as you make mental notes, and that you then notice what hits the floor next after your left knee, then next after that, and so on.  Do it several times so that you have a good idea of the pattern that exists when you crawl.  You can even stop and jot that pattern down.

Now go crawl.  :-)  I will skip some lines here so you can’t easily see what I write next, and can instead scroll down to it after you do the crawling exercise.

 

 

 

 

 

 

Unless something really interesting has happened (and bodies are interesting things, that surprise us sometimes), your order of motion was left knee, then left hand, then right knee, then right hand.  Of course, if you started by paying attention to the right hand coming down first, then the sequence would be right hand, left knee, left hand, right knee, right hand, and so on.  But the sequence is still identical.  It is that you put down the back “leg” (knee) on one side, and this is followed by putting down the front “leg” (hand) of that same side, and then you put down the back leg of the other side, which is then followed by the front leg of that other side.

Note:  It is very important to realize that the sequence is always the same, but that it will at first look different if you “start” with a different foot.  Each time a front foot comes down and hits the ground, the next foot that comes forward will be the hind foot of the opposite side.  But this is then followed by the forefoot on the same side.

Here’s what it looks like in a walking horse, in slow motion.  These images are stills edited from a video by British videographer “Mike Snail”, who posts his work series at Youtube under “Slow Motion Connection.”  I left in the time stamp information so you can see the sequence of things.

 

Starting with the left hind foot touching the ground…

 

 

 

 

Then the left front foot touches the ground…

 

 

 

 

Then the right hind comes forward and strikes the ground…

 

 

 

 

And finally the right front foot strikes the ground.

 

 

 

The entire clip is at youtube, here.  I strongly urge you to watch it, paying close attention to the sequence in which the feet strike the ground.

Now of course the point of all this is to go back and compare the horse’s sequence of limb movements at the walk to your own sequence of limb movements at a crawl.  They are identical:  left rear to left fore to right rear to right fore.  And they are identical to crawling in a human baby as well.  It is thought that this similarity is due to neurological homology — that the firing mechanisms that activate the limb muscles in that particular pattern are literally the same in both horses and humans.  In fact, Susan Patrick and two other researchers published a paper in the Journal of Neurophysiology in 2008 suggesting the structural/ functional pattern of the nerves that activate crawling and walking is so conservative that it’s the same in almost all quadrupeds — which is to say, in every animal with four legs.  (Note:  In biology, “conservative” means it has not changed evolutionarily.  It has remained the same over vast spans of time during which other structures have evolved and changed.  For more discussion of homology in gaits, see the previous blog post.)

What does this mean for you, as a horseperson who wants a better experience with your horse?  It means you have an inborn mechanism that allows you to understand where your horse’s feet are.  You have probably been told by at least one trainer, on a DVD if nothing else, how important it is to know where your horse’s feet are as it moves.  (That’s because it’s only really possible to move a foot with rein or seat if that foot is off the ground at the time, not supporting the horse’s weight.  You can’t move a foot someone is standing on.)  And if so, you’ve probably also been told one or more complicated ways to learn how to tell where your horse’s feet are.  But, as long as you’re willing to use your experiential knowledge, you’ve got all the tools you need right inside your own body.

Here’s a suggestion:  crawl around some more.  Get a good “feel” for your own body.  (Yes, I am suggesting that good “feel” for your horse begins with good “feel” of yourself.)  Then go out to your horse and ask someone to hold its lead line for you, and just halter it.  Then do one of two things: (1) get on your horse with its normal saddle, but place your hands on the withers right in front of the saddle, as far down the slope as you can in that position, or (better yet!) (2) get up on your horse bareback and lay your hands on the withers on each side, on the sloping area.  Remember that if a friend is holding your horse’s lead line, you don’t need to worry about holding the reins. Then close your eyes and pause, for a moment, to remember — as viscerally as you can — how it felt to crawl.  Fix that in your mind.  You can even let your legs and arms twitch a little bit as you remember their motion.

Then have your friend slowly lead your horse forward at a walk.

What you want to do here is focus on the similarity between the four feet your horse is moving and the pattern of your own limb motion when you crawled.  If you can let those two “walking feelings” overlap in your mind and in your body-awareness, you will suddenly have very strong “feel” for your horse’s feet.  And although at first you will be focused on when each foot hits the ground, the awareness will grow to the point that if you just turn your mind a little bit, you will know when each foot comes up off the ground to swing forward.

If it helps, particularly if you are on your horse bareback, you can lean forward so that you are oriented with your back more parallel to your horse’s back when you do this, which will overlap your fore and hind limbs more completely with your horse’s fore and hind limbs.

Either way, with a saddle or without, this exercise allows you to use your own body’s proprioceptive senses to feel your horse.  Your horse, in other words, becomes an extension of your own body.

And how cool is that?

Many thanks to my business partner Jo Belasco for the work she’s done with bareback riders who have their eyes closed as they learn to feel a horse’s feet at a walk.

 

This post was a featured blog entry on BarnMice, Nov. 19, 2012.

 

Gaits in Horses and Humans

In a very interesting conversation with a trainer whose observations are consistently brilliant, I realized that it’s hard for riders to see the similarities between human and horse gaits.  This is because the vertical position of a human’s torso, due to our being bipedal, changes the orientation of the arms or forelimbs, thereby obscuring their motion.  So I thought it might be useful and interesting to compare the motions of humans and horses, anatomically.

First, here is a horse in a trot.  If you ride, you are familiar with this gait in which the legs on one side of the body either come together beneath the midline at the same time or extend away from the midline at the same time.  In this case, the left fore limb is pulled behind its neutral position, and the left hind limb is moved forward or ahead of its neutral position. The neutral position of the uppermost limb bones is important to understand in this case.  So let’s look at a horse skeleton very quickly, to see what’s happening to make the limbs look this way.

In this horse skeleton, the animal is simply standing, not moving.  So the limbs are said to be in a neutral position because the horse isn’t going anywhere right now.  I have colored two bones red.  The one in the fore limb is called the humerus.  It articulates with the scapula at the top — and you know the scapula because it underlies the withers.  At the bottom, the humerus forms part of the horse’s elbow, just in front of where you put the cinch or girth.  Most of the humerus bone is not easy to see when you look at a horse, because it is so closely bound into the chest that we just don’t notice it.  But it’s there, and you can see its approximate position marked by a red line in the diagram of the trotting horse, below, that I have modified to show you where these crucial bones are.

I have also drawn a little turquoise arrow to show you the direction the humerus has moved (from the neutral position) to get to where you see it, in this photo of the trotting horse.  You may wonder, “If that’s where it is when it’s pulled backwards, how far forward does it go?”  You can tell by looking closely at the front of the horse in the picture.  You can see where its left elbow is;  now look at its right forelimb and imagine about where the right elbow is.  That elbow is a lot farther forward, and it’s because the humerus of the right leg is oriented about as far forward of its neutral position as it can get.  Horse humerus bones are not able to reach out as far as human humerus bones can.  But they do move, and you can see how much by comparing the positions of the two elbows on this horse.

Now let’s look at the back leg, comparing it to the skeleton above just as we did with the front leg.  Remember there is a bone colored red on the back leg as well, and that’s the femur.  I have marked the position of the femur with a red line in the trotting horse diagram, too, and also put in a small turquoise arrow to show you the direction it has moved (from the neutral or standing position) to get to where it is now.  If you look at the skeleton one more time, with the legs straight down as the horse is simply standing, you can see that the fore limb has moved behind that position, and the hind limb has moved ahead of that position.

So take a moment to familiarize yourself with the positions of the horse’s upper limb bones in a trot, and then let’s look at a human being jogging.

Here is a photograph of a jogging human being.  To many people, this does not look like the trot in the horse that we just saw.  But that’s only because your eye is tricked by the human’s vertical torso.  Imagine the person standing in a neutral position, not going anywhere.  The legs would be straight under the body, and the arms would be hanging straight down.  Now look at the left arm and leg, facing the camera.  Where is the left arm, with respect to the neutral position?  It’s behind it.  It has been drawn backwards.  And where is the left leg, with respect to the neutral position?  It is in front of it. It has been drawn forwards.  Does this remind you of anything we just saw?

Here is the same image, drawn and labeled as I did the trotting horse.  Compare the two labeled images, particularly the motion arrows of the humerus and femur bones in the human and the horse.  This still might look to you like it’s not the same thing, in which case it might help to turn the woman in the picture so that her torso is level, like the horse’s torso is, and then put her arms and legs back in place.  I cut the picture up and put it back together to help you see what this might look like, and it’s below — with my apologies for the rather Frankenstein’s monster effect of the cutting-and-pasting!

Her right arm, away from the camera, looks a bit disconnected here because of course it would be more visible if she wasn’t standing upright.  (When she is upright, her right humerus is partially hidden by her chest.)  But pay particular attention here to her left arm and her left leg.  The humerus of her left arm is drawn back, and the femur of her left leg is forward.  This is exactly the positions we see in the left limbs of the trotting horse in the first picture I posted on this blog.  And that is no coincidence.

Humans “trot” when they jog or run.  This is not metaphor, but actual homology.  Homology is an anatomical term meaning that something is similar because of an underlying commonality.  In this case, there are structures that are the same in both horses and humans — the humerus bone and the femur bone, for instance.  (Both articulate to the same bones at both ends, in both animals, and both are derived from the same types of embryonic tissues.)  The muscles are largely the same as well, and these are the muscles that move the humerus backwards and forwards, or move the femur backwards and forwards.  But, even more importantly when it comes to discussion of gaits, there are neurological commonalities as well.

Muscles contract to move bones when they are stimulated by nerve impulses sent by the brain.  Different patterns of limb movement that are highly stylized and predictable — like the trot — are created by very specific patterns of nerve firings on both sides of the front and hind limbs.  These firings must be coordinated very precisely for the animal to move properly without losing its balance.  For instance, simply to move the humerus backwards requires that a number of muscles fire in a very specific sequence and also that a number of arm muscles do not fire at all!  When you think about this having to happen on all four limbs simultaneously, you can begin to get an idea of the complexity and precision of neural firing patterns that generate movement in a certain gait.  Neural firing patterns — the actual placement of the nerves as well as the systems that regulate the patterns of firing in the brain — are very conservative, meaning they are much more similar between different animals than we might think they would be.  So even though a horse and a human look very different to our eyes, they both have a humerus bone and a femur bone — and the pattern of neural firing that produces a trot is not just similar, it’s nearly identical!

In my next post, I will talk about the walking gait in horses and humans.

(Note:  If you wonder why the woman’s lower forearm and hand are drawn up so close to her body rather than being extended on back and down as in the horse, it’s because she is not putting weight on her hand as she runs, as the horse must do.  Joggers are commonly taught to flex their elbows and “tuck” their forearms and hands close to their bodies to reduce drag and make running more efficient.  One reason we pay such close attention to the humerus and femur to understand gait is that they are under the closest neurological control and so show us the pattern most clearly.  Runners who try to run without at least letting their humeri swing as shown here generally find they cannot do much more than hobble — because the neural firing pattern is so tight that the femur needs the humerus to move in a trot in order for it to move in a trot;  the two are literally connected by the neural firing patternA person can learn to move only the femur bones to jog, but the firing patterns of the nerves are demonstrably very, very different — and not the “normal” pattern typical of a trot, even in the muscles around the femur.  This is why so much physical therapy is necessary to help people re-learn how to move if they suffer injuries that interfere with the normal firing patterns in any way.)

 

This post was a featured blog entry on BarnMice, Nov. 11, 2012.