Cannon Bone

Horses have totally lost their thumbs and their pinkies, and run on an enlarged third finger called the cannon bone while the ring and index fingers have diminished in size, and now simply hang on either side of the cannon bone as splint bones.

Katie, Rory, and Gay gathered around a formalin-stiffened horse positioned upright within a sling, winched up in a chain, so the legs were at eye-level.

Removing the skin from the right front and rear legs allowed the students to follow the limbs’ nerve and vessel pathways. Today Gay would be the reader while the other two followed the anatomy she described.

“What are we reviewing today?” Rory asked.

Gay flipped the pages in her study guide.“Bones and vessels of the lower limb,” she replied. “Oh, and nerve blocks, we need to memorize anatomical landmarks for the different blocks.”

One of the ways to isolate a problem in a lame horse is to anesthetize the nerves systematically, starting at the hoof working up. If deadening an area allows the horse to lose the lameness, the veterinarian can more precisely locate the problem. Through this approach, one can see if the lameness is in the foot, the pastern, the cannon bone area or higher up. However, before a vet can start injecting lidocaine, he/she must first know the nerve’s location, which comes with anatomy reviews and by looking at the nerve tracts themselves on specimens in the anatomy lab.

“Okay, let’s start with the bones. Rory, point out the cannon bone.”

He did.

“It’s also called the third metacarpus,” Katie added.

“Rory, are you flipping me off?” Katie asked when she noticed he was making ‘the bird’ with his right hand, turning it back and forth, looking at it as though for the first time.

“The bone that makes this finger evolved into the cannon bone,” he marveled. “And the splint bones are short versions of our index finger and ring finger. Did you know that?”

“Sort of, but not with the epiphany you’ve reached,” Katie teased. “Go on.”

“Okay, horses have lost their thumbs and their pinkies. They walk on an enlarged middle finger, which is now called the cannon bone. The ring and index fingers have diminished in size and simply hang on either side of the cannon bone as splint bones, and the hoof is an enlarged fingernail that entirely surrounds the last bone on the end of that finger.”

They liked his analogy and helped him improve on it, pointing out the correspondences between their hand and the equine limb.

“The fetlock is our knuckle, the cannon bone runs from the knuckle to the wrist, and our wrist is the same as the horse’s knee,” Katie realized.

The equine leg is made up of the same hand bones we have. Here is a human hand pointing down. Horses have lost P1 and P5, while P2 and P4 have shrunk and become splint bones.