Monday, June 30, 2008

Your Inner Fish and the Elbow Joint

I am on chapter 3 of Neil Shubin's book Your Inner Fish. The writing is interesting and engaging, but I came across one peculiar section dealing with the elbow joint and how it has evolved from the latest fish to the earliest tetrapods and beyond. He writes:

We humans, like many other mammals, can rotate our thumb relative to our elbow. This simple function is very important for the use of our hands in everyday life. Imagine trying to eat, write, or throw a ball without being able to rotate your hand relative to your elbow. We can do this because one forearm bone, the radius, rotates along a pivot point at the elbow joint. The structure of the joint at the elbow is wonderfully designed for this function. At the end of our upper-arm bone, the humerous, lies a ball. The tip of the radius, which attaches here, forms a beautiful little socket that fits on the ball. The ball-and-socket joint allows the rotation of our hand, called pronation and supination. (p. 42)

Now, say you got an itch on the back of your left arm. How would you scratch it? Easy, you would reach over with your right arm and scratch it. But, why couldn't you scratch it with your left arm? Because the joint at the end of the humerus is, in fact, not a ball-and-socket joint, it is a hinge joint. The process at the distal end of the humerus is not a ball, as Dr. Shubin suggests. It is a cylinder. When you extend your arm, the radio-humeral joint locks! This prevents the joint from bending at more than a 180-degree angle. Further, the pronation and the supination doesn't occur at the elbow joint, it occurs at the scapulo-humeral joint. If you rotate your arm laterally, you will discover that your humerus rotates as well.

I am quite certain that Dr. Shubin knows the difference between a ball-and-socket joint and a hinge joint. That is what makes this error so peculiar. Is this the way the joint works in fish? It sure doesn't work this way in humans. How very odd.

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