We’ve learned about the sole region, the wall capsule, and the bars, but we have one more piece of our imaginary hoof to go before we can learn how all of our hoof capsule pieces connect! The next structure we are going to learn about which leads to the final hoof capsule piece is the digital cushion. The digital cushion is located between the lateral cartilages and is attached to the lateral cartilages and the coffin bone.
The digital cushion is composed of fibrocartilage, cartilage that contains fibrous bundles of collagen, which has a firm, springy structure—making the digital cushion an ideal shock absorber for the hoof while also providing protection for the deep digital flexor tendon and coffin joint. It is a triangular, wedge with the apex (point of the triangle) being attached to the coffin bone and the base of the triangle located at the back of the hoof; the base is composed of two lumps of fibrocartilage known as the bulbar cushion (i.e., heel bulbs).
The digital cushion also acts as the hoof’s expansion joint, an assembly designed to allow a structure to safely absorb expansion and contraction, by pushing out against the lateral cartilages upon loading and receding upon unloading.
Lining the bottom of the digital cushion is another corium; this corium is known as the sensitive frog. Like all of the other coriums we have learned about in previous blogs, the sensitive frog corium is covered in many, tiny papillae which grow tubules—frog tubules! We also already learned in previous blogs that the different types of tubules have different characteristics to best serve their specific purposes for the hoof; frog tubules are soft, elastic, and strong with a rubbery quality—the ideal material to protect the digital cushion and aid in additional shock absorption.
The emerging frog tubules are known as the “active frog” and grow perpendicular to the sensitive frog corium.
Since all of the emerging tubules grow perpendicular to the sensitive frog corium, the tubules in the area of the bulbar cushion end up growing towards each other—forming the area known as the central sulcus. The active frog tubules are very tightly packed which prevents dirt and moisture from entering. Once frog tubules grow to a length beyond the length of the active frog region they begin clumping together and folding like an accordion—becoming compressed against the active frog forming a thick, protective callus known as the frog padding or frog callus. The tubules become more loosely packed in the frog padding which can allow dirt and moisture to enter. This frog padding provides ideal protection to the sensitive frog corium.
All of the frog tubules combined form the area of the hoof capsule known as the frog—the final piece of the imaginary hoof we are building! It is now obvious why the frog is triangular in shape since we know the the frog tubules grow from the sensitive frog corium located on the bottom of the triangularly shaped digital cushion.
You will notice there is a distinct arch on each side of the back portion of the frog. These arches are formed by the bars and digital cushion; they are flexible and function like leaf springs on each side of the frog, lowering on loading and returning to their natural shape on unloading. One more characteristic of the frog area that aids in absorbing impact.
Now that we have all of the pieces to our imaginary hoof, the next blog will cover how all of the pieces connect!
***All photos courtesy of Cheryl Henderson—The Oregon School of Natural Hoof Care***
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