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.