Wall Growth

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Written By Walt Friedrich

At first glance, it seems obvious – but there exists a subtle mystery of sorts at play. We know the wall is firmly attached to the coffin bone – yet it grows downward at the same time. Experts, including David Hood, Robert Bowker, Pete Ramey and others, agree that there is a paradox, and provide independent, similar explanations, while also agreeing that although we have a pretty good idea of how it all works, nobody has proved it yet. The explanations are quite plausible, but they are not etched in stone. KC LaPierre explains it well; what follows is based upon Mr. LaPierre’s work, but remember, this is just my “take” on the matter.

As you know, the components involved with the hoof wall are the coffin bone, laminar layer,  inner wall, outer wall and coronary band. We have evidence that the outer wall grows down from the coronary, as demonstrated by the downward “movement” of the scar left from a popped abscess at the coronary band. And while we can’t see it, we also know that in a healthy hoof there is a very tight attachment between all those components. The sticky part is understanding how the wall can move downward while at the same time remain locked to the coffin bone by the laminar connection.

Good question. The consensus explanation seems to be that the inner wall,
firmly attached to the coffin bone by the laminar layer, has two components: loosely packed tubules, originating from a corium at the coronary, and a thick, dense “glue” referred to as intertubular horn — an immensely strong substance that fills the inner wall, completely encapsulating its tubules. The outer wall is likewise constructed of tubules growing groundward and held together with intertubular horn, but its tubules are very densely packed.

This leads us to two important concepts: first, that intertubular horn, while very dense and tight, is actually a fluid (more on this later). One might consider the inner wall as being composed of intertubular horn with tubules embedded therein to keep that horn in place. That construction makes the inner wall extremely strong and shock-resistant. Thus a primary function of the inner wall is to provide strength for weight support as well as shock absorption for protection for everything inside and above the hoof itself. Second, the outer wall, composed mostly of tubules with enough intertubular horn to hold it together, presents an almost impenetrable shield against external damage. The ancient Greeks couldn’t have asked for a more efficient shield material, even though they did pretty well with what they had.

Back to the intertubular horn being a fluid, and enter a physics concept
called “fluid dynamics”; it says that a fluid in motion is essentially
motionless at its base, and the farther away from the base you go, the
faster it moves. It’s the way rivers work — the water’s velocity is greatest at the surface, diminishes as you go deeper. In the hoof, that property of the
intertubular horn means that while the inner wall’s base remains almost motionless, attached to the coffin bone by the laminae, its outer surface (abutting the outer wall which is moving downward) is moving right along with the outer wall, at exactly the same velocity, albeit very slowly. Intertubular horn cells initiate from the laminar layer, and grow outward, perpendicular to the wall surface and filling the inner wall’s tubular space, but as they reach the junction between inner and outer walls, they have begun to move downward, in parallel with the outer wall’s movement, thus keeping everything smoothly locked together.

Think of it: outer wall resembles a broom — stiff and strong, made of
tubules, constantly growing longer, forming an almost impenetrable shield – while inner wall performs the task of keeping everything locked to the coffin bone yet allowing the outer wall’s downward growth at the same time. The outer wall corium, located in the coronary band, has just one job, constantly generating new cells. The inner wall’s cells actually have two sources — some developing at the coronary that generate the tubules, and others developing at the laminar surface, generating the intertubular horn.

Incidentally, I’ll add an interesting side note: As long as everything is flowing smoothly and normally, the hoof will have a smooth, even outer wall — no ridges or striations. But any disturbance in the evenness of growth between the two layers will show up as a “glitch” at the outside surface – trauma to the inner layer, such as with a laminitic attack for example, or perhaps a sharp enough strike on the outer wall surface will interrupt its rate of contribution of intertubular horn to the outer wall as it grows downward, resulting in a “fold” in the outer surface, hence those rings we sometimes see running side-to-side across the toe of the hoof, and it explains why they can and do grow out. In addition, the inner wall, thanks to heavy keratinizing of the intertubular horn, is quite waterproof. But when the inner wall “thins out” due to some trauma, it loses a certain amount of its tightness against leakage, allowing some blood to find its way out, showing up eventually at the bottom of the hoof at trim time as those disheartening red areas we sometimes see. They may also indicate a trauma in the past, but do not necessarily indicate that the trauma is actually past. In addition, it’s my personal opinion that toe-first landings that send shock waves through the entire hoof are also responsible for damage to the inner wall’s intertubular horn that allows some adjacent blood vessels to rupture, the results being the blood spots we see weeks later at ground level during a trim.

Thus, the simplest, undetailed answer to the question, “what makes the hoof wall grow”, may be that:

The wall is a two-layered structure: the outer wall grows downward, and consists of densely packed tubules with enough intertubular horn from the inner wall to hold the tubules together, while the inner wall grows outward, and consists of intertubular horn with just enough of its own tubules to hold the horn together. The seam between the two layers is an active place, where the descending outer wall “pulls” the outward-growing intertubular horn downward as they flow together toward the ground. Thus, as long as their coriums are functional, both inner and outer walls’ growth is guaranteed, and their functions of support and protection can exist because of the fluid characteristics of the intertubular horn. It is truly a remarkably efficient design.

Everybody Admires a Well-Turned Leg…

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Written By Walt Friedrich
 …especially when it’s on a horse! Same goes for a pretty foot, also especially when it’s on a horse. I’m going to talk a little about horses’ legs and feet here, but I’ll refer to the feet as “hooves” for the sake of both accuracy and clarity. Know how you think of your foot as everything from the heel to the toe? If we do the same when referring to a horse, we’d be talking about fully the bottom half of an entire visible leg! (More on that a little later.) His hoof is the hard shell at the very bottom of each leg, plus its contents.
But the hoof, simple as it looks, is more than just a hard shell on the end of a horse’s leg, it’s actually a very complex system with a number of movable parts that all need to work together. And I do mean work – a feral (wild) hoof is in actual work typically twenty hours per day, seven days per week, all year long, for many years. With that kind of usage, the hoof had better be well-designed, well-built, tough, strong, and self-rejuvenating. If a horse moves twenty miles every day, and lives for twenty years, he puts 150,000 miles on each hoof over his lifetime, and even at the end most of those hooves are still functional! Only his teeth and jaws give his hooves a run for the money, so to speak, and they don’t support all that weight while they’re doing it.
Here’s a little food for thought: Enough fossilized remains have been studied that we know something of how today’s horse has evolved over the millennia. For example, vestigial bones in a horse’s leg suggest that that big old hoof is actually the equine equivalent of the last joint in your middle finger!
Let’s follow the finger analogy: his hoof is part of an “assembly” consisting of three bones, each articulating with its neighbors, and whose joints are held together with extremely strong and tough ligaments. Each of these bones moves with respect to its adjoining bone(s), controlled by tendons at the ends of muscles. Those tendons are encased in sheaths, to keep them positioned properly. You and I call this three bone assembly a “finger” – but on a horse, they’re known as the “pastern”. The last bone in the set, the bone that is encased in the hoof capsule itself, is known as the “coffin bone” – though there’s nothing at all eerie about it.
But let’s get back to that well-turned leg and examine it, top to bottom. Because they look so different, you’d never think that a horse’s foreleg and your own arm are very much alike – but they actually are, as you’ll now see. Remember the old song, “Dry Bones”? Let’s play an equine version of it — you can follow it on the adjoining sketch, and you’ll see how it relates to your own arm, step-by-step:
His shoulder blade’s connected to his upper arm bone —
 Technically, that’s his scapula connecting to his humerus – both are buried inside his body, so you can’t see them;
His arm bone’s connected to his elbow bone —
Which, finally, you can actually see (but probably have never noticed). It’s very easy to feel, though — run your hand up the back of a foreleg, and just as you get to his torso, you’ll feel a hard, round knob – that’s part of his elbow;
His elbow’s connected to his forearm bones —
Radius and ulna, that is – his forearm is the top section of his leg that’s visible to you and me;
His forearm’s connected to his wrist bones —
These are a collection of small, vestigial bones called the carpals, the “wrist” joint forming what most of us call his “knee”, because it’s in the middle of his leg and it bends forward like our own knees;
His wrist bone’s connected to his hand bone —
— by way of the one carpal bone that survived the evolutionary process to become his “lower leg”, which is actually the equine version of your hand. It’s called the Cannon bone;
His hand bone’s connected to his knuckle bone —
That joint is called his “fetlock”, the equivalent of your middle finger knuckle;
His knuckle bone’s connected to his finger bone —
Which, as we have learned, is his three-boned pastern.
Now, that’s the leg of the horse!
To help you orient yourself, that well-turned leg is vertical down to the fetlock, at which point his pastern takes an angle of 30 degrees, give or take, forward.
Let’s not omit his hind leg: the hoof is pretty much the same as those in front, right up to the fetlock. The long bone that extends from there half-way up his leg meets the joint we refer to as the “hock” – which is analogous to your heel and ankle joint! Continue upward to find his knee, which you won’t see unless you look very closely – but you can find it by running your hand up the front of his back leg, and just as you reach the torso you’ll feel the bony knob that is part of his actual knee. His thigh bone is inside his body, attaching to his hip.
Two incidental points of interest: one, his hind leg, from his hip down, appears longer than his foreleg, which leads us to question how he can run so smoothly. If they truly are longer, you’d expect many more steps by his forelegs than his hinds in order to keep up. But remember, his “upper arm” bone is buried inside his body so you can’t see it; furthermore, a horse has no collarbone (clavicle) to lock his scapula in place, as do you and I, so effectively he has an extra “leg bone” in his fores, which makes front and hind legs essentially of equal length. And the second point, the “cowboy tale” that you can predict the adult height of a newborn foal by measuring the distance between his “knee” and fetlock, and substituting “hands” for “inches”, turns out to have some basis in fact. It’s not an exact science, but as empirical evidence, observe a newborn standing beside his dam: his lower leg (cannon bone) will be close to the same length as his dam’s, and will grow but little more – except in girth, as it develops muscle. That makes it a fairly reliable predictor of adult height.
Whew. Congratulations, if you’ve stayed with it thus far.
Now let’s get to the hoof. We’re going to take something shaped roughly like a slip-on shoe (his hoof) and stuff it full of several interesting items, the items that make up the hoof’s “innards”.
We are all familiar with the shape of the hoof – rather like half a cone, with no top. Well, the coffin bone itself is shaped in very much the same way, attached to the leg’s bony column at an angle so its base can sit flat in the hoof capsule. It tucks neatly into the front section, and fits like a glove. The back half of the capsule contains a large wad of very tough, fibrous tissue, known as the “digital cushion” – flexible as well as tough, as it supports the horse from directly under the bony column of the leg, and it absorbs the shock when the hoof lands. The digital cushion is held firmly and tightly in place by a “belt” of even tougher material known as “lateral cartilage” – it “cups” the digital cushion from underneath, behind and both sides.
All we need to hold it all together is to sort-of glue the coffin bone to the inside of the capsule around the toe. That gets done by Velcro-like layers composed of billions of cells forming what are called “laminae”. One layer of laminae is part of the coffin bone, the other layer is part of the hoof wall; these two layers interlock like Velcro, and these form one of nature’s strongest bonds.
Finally, what’s underneath the hoof? At the very back, the wall forms two ultra-strong columns, called the “heels” – one on each side – capable of slight sideways and vertical movement to stabilize the horse when he’s moving. Between the heels and stretching toward the toe sits the “frog” — triangular-shaped, tough fibrous tissue that provides both support at the back of the hoof and stimulation for the digital cushion, immediately above it. And what’s left, covering most of the bottom of the hoof, is the “sole”. It holds everything together, and in conjunction with the wall and heels, provides the total support for the horse’s entire weight – for an average horse, that amounts to a load in the area of 200 to 300 lbs per hoof, just standing; imagine how much greater when the horse is walking, running, jumping…
Tying it all together is the blood supply. And it is RICH. It has to be – it’s the only protection the hooves have from the cold, and that protection is superb. The hoof’s components, together, are very demanding of constant, steady blood flow. They get help from what’s known as “hoof mechanism”; because of the hoof’s architecture, its blood supply is cut off for an instant with every step the horse takes, allowing a momentary pressure build in the arteries feeding the hoof and a small pressure decrease in the hoof itself. But as the hoof completes each step and raises off the ground, that blockage is released, and the built up pressure forces a spurt of blood flow through the hoof. The hoof itself also expands slightly as it takes weight with each step; that forces blood through the hoof, and when the hoof is raised, the expansion relaxes, allowing the blood pressure in the hoof to restabilize. These two actions are synchronized, with the result that each hoof is referred to as a sort-of auxiliary heart – that means five hearts working to pump blood with every step the horse takes. The laminae in the hooves especially require a strong and steady supply of rich blood, and Nature’s design provides it for them.
While the hoof and the horse date back into antiquity, you might note that the hoof is also the first four-wheel independent suspension system on the planet. The shock absorbers are the digital cushions together with the frogs, and the springs are the heels, working independently of each other. That’s why when his feet are healthy, he can stride fast across a path of rocks – each heel retracting and returning as necessary on the uneven terrain.
So you see, those four little hooves – little in comparison to the bulk and weight of the rest of the horse – actually do wonderful work, far greater than their own size and weight. But then, that sort of thing is true of the horse in general.