All parts of the equine hoof grow groundward and, because the form of the hoof is a slanted, truncated cone, it also grows slightly forward. The mysteries of the conveyor belt arrangement - how the hoof wall and sole stay attached as the horn grows, and importantly, why the system fails, are only just being unravelled.
You can mark the lines of growth by drawing a line in a contrasting pen down a tubule from coronet to distal margin. Do that all round the hoof at regular intervals and you get a clear picture of the way the hoof is growing and whether there are areas that are growing at significantly different angles to the ground.
The slanted, truncated cone of the hoof capsule should reduce evenly in its length (linear distance from hair bearing skin to distal margin) and its height (vertical distance from the ground to hair bearing skin) from the toe region to the heel buttress.
The hoof wall is open at the heels. At the heel buttress, the wall turns inwards and becomes the bars - so the bars are, in effect, a continuation of the wall and share many of the same characteristics eg there is corium at the top of the bar which produces the bar horn and a laminar corium which produces interdigitating laminae that bind bar and sole horn together.
The bars' outer zone, like the wall, is pigmented (most obvious in a dark hoof) and more tubule dense than the bright white and moister inner zone of horn which interfaces with the sole. At the top of the bars (i.e. at the top of the collateral groove) the corium that produces the bar horn merges with the frog corium.
The bars both allow and control lateral expansion of the heels; they share load, and they help in deceleration if the wall and bar sit slightly proud of the sole. As well as expanding laterally under load, the heels can also deform vertically when the hoof loads on uneven ground.
This flexibility is achieved because the rearward third or so of the hoof grows from dense cartilage - not bone. The wall in the heel region is a little thinner than the toe region (or should be but often isn't because the toe is too weak) but the wall and bar work together to add substance and stability to the heel region.
Just as the wall proper reduces in length/height and width from the toe region to the heel buttress, the bars - also should reduce in length/height and width from the heel to the bar apex where bar horn merges seamlessly with sole horn.
This region, forward of the apex of the bar and about half way along the frog, where bar horn merges seamlessly with sole horn is a zone unlike any other in the hoof. The two types of bar corium (coronary and laminar) merge with each other and merge with the sole corium.
In the hoof several different types of horn and other tissue meet: hair bearing skin and capsular horn at the coronet; hair bearing skin and the bulbs and frog; sole and wall horn; sole and frog; frog and bar…. and each junction has properties which ensure it's not subjected to too much strain.
Eg. the wall and sole junction is buffered both by the white line - which is formed by terminal papillae at the distal margins of the coffin bone - AND by the change in the structure of the wall to a less tubule dense, more moist horn that makes the inner zone of the wall more like sole horn. The frog/bar junction is buffered by the interdigitation of laminae and by the elastic and dampening properties of the frog (the region of the bar corium is aligned with the twin folds of the frog in its rearward third). The sole/frog junction is buffered by the elastic frog and by solar concavity - and so on.
We don't actually know much about how these coalescing zones function - what is obvious is that they can become problematic if they are exposed to severe or long term unphysiological stresses.
As a slanted cone, the hoof should diverge towards the ground, i.e. have a smaller circumference at the coronet than at the distal margin. In order to be able to deform laterally under load, the heels have to diverge to some degree.
If the heels are vertical, ground reaction forces will result in them being deformed upwards.
If they are beyond the vertical i.e. they slant towards the midline of the hoof, they will tend to narrow under load which compresses the bulbs and the frog.
If they are too shallow (under-run), impact forces will result in too much lateral or forward spread and the heel horn may completely collapse and lie over the sole.
Each region of the hoof has a certain optimal angle of growth. This may be a range and it can change according to environment etc, but it is a fairly narrow one. If hoof form is too steep or too shallow, or there is too great a difference between sections of the same hoof, problems will arise.
For example, it's often not understood that, in a splayed or 'forward running' hoof, where wall and bar sit at too shallow an angle, the sole also will have too shallow an angle of growth. If the hoof is too steep and convex, the angle of growth of the sole will be too steep.
The angle of growth of the bar is always matched by the angle of growth of the heel. The steeper the heel is, the steeper the bar; the more sloping the heel is, the more shallow the growth angle of the bar is. If a heel is bent inwards beyond the vertical, the bars will also be bent beyond the vertical and / or it will form a curve. If the heel has completely compressed and is lying flat, so is the bar.
As with the wall, the angle of growth of the bar determines how the different zones of the bar meet the ground and how loading and abrasive forces affect it.
A feral horse can cope with all manner of pathology in its hooves if it lives in a benign environment but if its environment is harsh and it cannot adapt its hooves to cope with the demands on it, the horse will not survive.
In domesticity, people often ask horses with severe hoof pathology and all manner of related conformational issues to cope with being ridden or driven and they use shoes and boots to allow them to push the horse past the point it would be able to go to if it had bare hooves. Sometimes the demands people make are too much and the domestic horse also does not survive - because it is put it down.
Some specific issues:
'The bars help form the sole'
In a hoof with an 'white' wall and sole, it's not always easy to distinguish the inner zones of the wall from the sole but in a dark hoof, the unpigmented region of the bar is very obvious. If the bar does contribute to the production of the sole in the way that Bowker suggests, given the bar has an inner bright white zone, all hooves with a dark sole should have areas of bright white in them running forward from the bar apex towards the toe. They don't.
I don't think it's as simple as 'bars help form the sole' - and there is the question of which bit of the bar, given the complexity of the junction at a cellular level. We just don't know enough about the bar-sole junction to be able to say for certain what is going on in any given hoof.
I have here at the moment, one horse with all black hooves, two with one white hoof and one with one black hoof. The one with all black hooves has no white horn forward of the bar apex - although when I first started trimming him he did have pools of white horn that spread out from the bar apex. The horse with one black hoof has areas of white sole in his black hoof but their location never alters and it forms discrete patches towards the toe. In the two with one white hoof, the black hooves have no white at all. In the white hooves of course the outer horn is not pigmented or not heavily so. (See post on pigmentation) I have seen bicolored hooves with white soles that have dark bars and dark horn running from the bar apex all the way round the tip of the frog. Is this bar? Some would say yes, some no.
The effects of ground reaction forces
The hoof and the structures it protects have to be able to deal with massive deceleration and impact forces - and the persistent pressure of rest stance.
When a horse stands on a level, hard surface, the toeward region of the anatomically normal bar in a normal hoof is passive when the horse is in rest stance. The buttress region of the bar shares load with the wall, which shares load with the rearward third (folded) region of the frog and the sole adjacent to the wall. The concave nature of the hoof means the sole under the sharp distal margin of the pedal bone is well protected.
But the world is not usually completely level and smooth and on a conforming surface the load is more evenly shared, and under maximum impact on any surface the entire volar surface shares load.
However, the outer regions always impact first and carry most load; maximum impact is fleeting on any given hoof, and concussion should be damped/dissipated by many different mechanisms.
Problems for the horse arise when any of those mechanisms is impaired and any given part of the hoof receives too much load at a given point or over time.
For example, in a severely contracted hoof the bars may not touch the ground on a level surface, or a shoe will lift them out of ground contact even further but - the internal structures of the digit - specifically the region of the navicular bone - can be impacted by the bars even if they never actually touch the ground, even on a flat surface where they are sitting above the ground because gravity ensures that body weight pushes downwards onto them. If the digital cushion is weak and suspensory apparatus is compromised, the situation is worsened.
Photographs of a dissection of a foundered hoof with ingrown bars are shown in this blog post. I have seen far worse impacted bars in foundered hooves. The worst I have seen were in a pony, whose bars were arched up into the hoof past the level of the coronet and curved inwards under the frog. The pony had spent years denied grazing and movement because she was prone to laminitis. No-one responsible for her care understood the connection between her metabolic and endocrine issues and her persistent attempts to avoid loading her increasingly painful long and contracted heels and bars.
The bottom line is that the bars are a critically important part of the hoof - in horses which cannot grow, load and wear their hooves optimally or have them trimmed optimally, the bars can grow in ways and into places which impact on sensitive structures and affect the correct functioning of the hooves and associated muscle-skeletal structures. The long term effects can be profound.
Photographs of a dissection of a foundered hoof with ingrown bars are shown in this blog post. I have seen far worse impacted bars in foundered hooves. The worst I have seen were in a pony, whose bars were arched up into the hoof past the level of the coronet and curved inwards under the frog. The pony had spent years denied grazing and movement because she was prone to laminitis. No-one responsible for her care understood the connection between her metabolic and endocrine issues and her persistent attempts to avoid loading her increasingly painful long and contracted heels and bars.
The bottom line is that the bars are a critically important part of the hoof - in horses which cannot grow, load and wear their hooves optimally or have them trimmed optimally, the bars can grow in ways and into places which impact on sensitive structures and affect the correct functioning of the hooves and associated muscle-skeletal structures. The long term effects can be profound.
