Emily Decker, DVM

July 2006

The horse world is famous for producing anatomic terms that are confusing.  It is not uncommon to hear of several names for the same anatomic structure, or hear of structures that you may not have even known existed.  For example, what many people call the knee on the front leg of the horse is actually known as the carpus.  The true “knee” of the horse is actually on the hind leg above the hock and is known as the stifle.  The following is a list of terms that often surprise or perplex horse owners when they are mentioned.

Ergots and Chestnuts

Ergots, often mistaken for scar tissue, are small, hard protrusions found just below the fetlock joint.  These protrusions are made from material similar to the hoof sole and grow from the surface of the skin downward.  Your horse may not even have an ergot, while other horses may have ergots that grow several inches in length. 

Chestnuts are made of the same material, but are located on the inner side of the leg above the knee in the front limbs and below the hock in the hind limbs.  Chestnuts are scab-like and vary in size and shape.  They continue to grow throughout the horse’s life and can be peeled off during grooming.  Removal of chestnuts is non-painful, but may be easier if baby oil or a moisturizer is first applied to soften them.

Coffin Bone and Lamina

The coffin bone, or distal phalanx, is the lowest bone in the equine leg and is the same bone that we have in the tip of our finger.  It sits inside the hoof capsule in the horse and has evolved into a shape very similar to the hoof.  Humans can get along quite easily without their distal phalanx, but in the horse this bone is essential. 

The coffin bone is suspended inside the hoof capsule by a very strong tissue known as lamina.  It is actually a much stronger version of the tissue that holds our fingernail to our finger.  In the horse, this tissue becomes very important, as it is responsible for suspending the coffin bone and therefore supporting the entire weight of the horse.  Laminitis or founder is inflammation of this tissue.  With the entire weight of the horse bearing down and a strong hoof capsule to build up pressure, this inflammation can be extremely painful.  In addition, inflamed lamina becomes weak, and in some horses can lead to severe rotation or sinking of the coffin bone.

Navicular Bone

This bone is very important to the horse and often discussed in the horse world, but is difficult for the average horseman to point out.  The navicular bone, known medically as the distal sesamoid bone, is a small, oblong bone that lies behind the lowest joint in the equine leg, the coffin joint.  Hidden beneath the hoof, the navicular bone cannot be seen with the naked eye, but plays a very important role in the soundness of your horse.  This bone is in contact with the coffin joint and a very important tendon known as the deep digital flexor.  In some horses, the navicular bone undergoes arthritic changes and becomes weak and roughened.  Once these changes have occurred, movement can become very painful, and in some cases, the deep digital flexor ruptures, leaving a horse permanently lame.  These arthritic changes and pain in the horse are often referred to as "Navicular Syndrome".

Sesamoid Bones

The sesamoid bones are a pair of triangularly shaped bones that can be felt directly behind the fetlock joint of the horse.  The medical term for these bones is the proximal sesamoid bones and they come in contact with some very important structures in the horse; the fetlock joint, the suspensory ligament, and the deep digital flexor tendon. 

Arthritis in these bones, just like the navicular bone, can lead to lameness issues.  More commonly, however, these bones are involved with the suspensory ligament.  The suspensory ligament is attached to both sesamoids and if under excessive strain, can actually fracture the sesamoids.  This is a common injury in racehorses.  These fractures, in turn weaken the suspensory ligament, which is responsible for holding the bone structure of the lower limb in place.

Wolf Teeth

Wolf teeth are remnants of the first premolar in horses.  While normal molars in the horse are a few inches long and have 4 roots, wolf teeth are relatively small and have just one root.  Often, people mistake the canine teeth for wolf teeth, which are located near the front of the mouth.  True wolf teeth are actually difficult to see without opening the mouth and viewing the premolars of a horse.  Wolf teeth begin erupting at 6 months of age and some horses may have up to 4, while others never develop wolf teeth.  Many horsemen believe that wolf teeth interfere with bitting and have the teeth removed before training.  The tooth is easily removed by your veterinarian with a little sedation and patience.

Guttural Pouch

The guttural pouch is a large, air-filled space that sits behind the jaw and underneath the ear in the horse.  It is an enlarged pocket of the auditory tube.  The auditory tube runs from the ear to the back of the mouth and is know as the eustachian tube in humans.  Horses have two guttural pouches, one for each auditory tube.  Although the guttural pouch is filled with air only, many important arteries and nerves run along its surface.  Many theories exist as to the function of the guttural pouch, but the most recent and accepted theory is that the air in the guttural pouch allows blood running through these important vessels to cool before it reaches the brain. 

Although it is not often seen or thought of when looking at our horses, disease in the guttural pouch can have serious consequences.  Fungal infections called guttural pouch mycosis can weaken the walls of the arteries near the guttural pouch and can cause a horse to bleed to death.  In addition, bacterial infections of the guttural pouch can lead to nasal discharge and serious illness. 

Finally, some foals are born with a treatable condition known as guttural pouch tympany.  These foals have guttural pouches that are overly distended with air and can be seen from the outside of the foal's neck.