Anatomy of the Hoof
In order to explain how a hoof functions
as a whole we must first break it apart and understand each of it's
pieces. And the best way to do that is from the inside out.
So let's start with the bare bones...
The Bones
The
Coffin Bone
The shape of the front half of the hoof is dictated primarily by the
shape and placement of the third phalanx, commonly called the coffin
bone. The coffin bone is a hoof-shaped bone. People often
think of the coffin bone as pointy because cut-away diagrams and
x-rays portray the coffin bone this way. But what one is seeing
in the cut-away is only the middle of the bone. It is actually a
conical crescent with curved wings, which sweep toward the back of the
foot, and a concaved sole.
The coffin bone is the only bone in the
body that does not have a periosteum, the lacquer-like "skin" of the bone.
This is because the coffin bone is covered with the highly vascular
corium with its laminae that produce hoof wall and bind that hoof wall to the bone.
It is the shape of the coffin bone that gives shape to the front of
the foot, and conversely, it is the integrity of the exterior foot
that keeps the coffin bone correctly positioned.
Place a coffin bone on the table and it
becomes immediately apparent how the bone was designed to sit in the
hoof. The crescent shaped bottom rim sits perfectly on the flat
service. In a healthy hoof, the coffin bone will sit within 5
degrees of ground parallel so that when the bone is weight bearing,
that pressure is dispersed around the entire sole of the bone.
The Coffin Joint
The coffin joint consists of three
bones: the coffin bone itself, the navicular bone and the short
pastern bone. The navicular bone is a tubular bone that sits sideways
between the wings of the coffin bone. The navicular bone acts as
a guide for the deep digital flexor tendon, which runs down the back
of the leg, over the navicular bone and attaches to the underside of
the coffin bone. The other tendon in the foot is the extensor
tendon, which runs down the front of the leg and attaches to the
coffin bone. The last bone of the joint is the short
pastern bone, which allows the leg to flex at the connection between
the pastern and hoof. The front edge of this joint should sit at
or just below the coronary band.
Softer Structures
The Digital Cushion
The digital cushion plays a
vital role in the shock absorption of the hoof. A well-developed
cushion snugs up into the space between the wings of the coffin bone
(as well as under part of the bone) and then expands to fill the back
half of the foot. The cushion should be dense and fibrous.
Not only does it absorb the impact of a heel first landing but it also
cushions the joint under which it sits and provides a protective mass
around the major nerves of the foot.
The Lateral Cartilages
These
wing-shaped cartilages originate on the palmar wings of the coffin
bone. They spread back along the sides of the digital cushion,
encasing it and giving shape to the bulbs of the heels. It also
spreads forward above the coffin bone, under the coronary band and can
be seen as a very slight bulge above the coronet.
The lateral cartilages also absorb impact
and allow the back of the foot to flex and distort to dissipate that
impact. They also serve to protect and support the digital
cushion.
The Corium
The corium covers not only the coffin bone on the sides but also the
lateral cartilages. The sole corium covers the sole of the
coffin bone and merges with the frog corium, which in turn, connects
to the coronary band. The corium produces all the horny outer
structures of the hoof and binds those structures to the inner
structures. The corium is very vascular and requires excellent
circulation to produce quality horn with good laminar attachment.
The Laminae
The laminae are the "Velcro" that bind the inner structures
of the hoof to the outer wall. There are two types of laminae:
sensitive, which are part of the corium, and insensitive, which make
up the inner lining of the hoof wall. The laminae are arranged like
vertical fins or blinds. Each fin has secondary laminae, which
are smaller fingers that interlock to form a tight bond between the
inner and outer foot. The tubules of the inner wall form at the
laminae and migrate out and downward. In the sole of the foot,
the laminae are presented as the "white line," which joins the sole to
the wall.
The Coronary Band
The coronary band and the frog corium are interconnected
and have separate, yet complimentary, functions. The coronary band is
a tough, fibrous band that encircles the top of the hoof. The
frog anchors the coronary band. So as the foot lands and weight
is loaded, the coronary band, which has little give, tightens and
allows blood pressure to build in the foot. As the hoof breaks
over, pressure is released and blood is effectively pumped back up the
limb.
The Horn
The Inner Wall
The inner wall is thick with a
looser configuration of tubules than the outer wall; therefore it is
much more flexible than the outer wall. It is designed to
dissipate impact by flexing. When examining the wall from the
bottom of the foot, the inner wall is often referred to as the "water
line" or "unpigmented wall." It is white in color, as opposed to
the white line, which is really yellowish.
The Outer Wall
The outer wall is the hard
keratinized shell of the hoof. It's what we see when we picture
a hoof in our minds. The outer wall has densely packed tubules
and it's hard, smooth surface repels excess moisture, bacteria and
infection in addition to acting as an armor plate, protecting the hoof
from the hard knocks it endures.
The
Sole
In the natural foot, the
tubules of the sole are densely packed into a tough, callused
material. The sole is generally concave and acts as support as
well as a springboard for the coffin bone. The sole plays an
important role in keeping the coffin bone well suspended in the hoof.
On all but the flattest surfaces, the sole aids the walls in
supporting hoof.
The
Heel Buttresses
The heel buttresses are located
where the hoof wall turns back at the palmar end of the foot.
They are designed to receive the impact of a heel-first landing.
The Bars
The bars act as a spring, allowing the hoof wall to expand and the
sole to descend as the hoof is weighted. But they also add
strength to the hoof capsule and protect the back of the foot from
concussion.
The Frog
The frog is the hinge that, when weighted on impact spreads the spring
created by the bar/wall configuration. The back of the frog also
creates added traction for the foot as well as anchors the coronary
band.
Put it all together and it should come out
something like this.
Continue to
MECHANICS...
