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Re: Palms in



Patrick Norton writes:
I'm still not clear about the relationship between the radius/ulnar and the
semilunate carpals with respect to the "palms in" attitude of the hands and
the "sidways" motion Luis refers to above, although that's my general
question.

The best way to do this is to use your own arms as a model. Hold your arms out in front of you so that your elbow is facing down (not out to the side) and your hands are turned with their palms in. In this configuration, your radius bone is up and your ulna bone is down. To be more clear, your radius bone is aligned with your thumb, and your ulna bone is aligned with your fifth finger. In sideways profile, your radius is on top of your ulna, and they are uncrossed and run parallel to one another.


Keeping your elbows still facing down, turn your hand palm-side down. You will notice that the motion that causes this is largely your radius bone rolling over your ulna bone. Hold your elbow still with the other hand if you like to make sure you are not also bending your elbow out to the side. Got it? Okay.

Now, you can do this because your radius has a rounded head (the top portion of it in your elbow) that articulates with a rounded groove on the ulna. When two muscles in your forearm, called the pronator teres and the pronator quadratus, contract, they pull on the radius, it turns in the groove on the ulna, and your hand is rotated downward.

Now, imagine that you no longer have this nice joint. Immobilize your forearm (the best place would be right before the wrist) so that you can no longer rotate your radius about your ulna. Again, make sure your elbow is pointed down when you do this. Now, some motions at your wrist are possible -- there are some twisting, sliding motions and your hand can start to face palm-side down, but it cannot completely be made to do so.

Imagine further that you cannot flex your hand in or out, only up and down along the axis of your thumb and fifth finger. This is what it would be like to be a typical theropod.

Where does the semi-lunate carpal come into this? Well, imagine, since you cannot flex your hand in or out, that you instead had a rounded hinge joint between your forearm and hand that allowed it to move up and down along the thumb-fifth finger axis easier. This bone is flat on one end and it caps the ends of the radius and ulna, further immobilizing them. The other end of the bone is smooth and U-shaped, with the rounded part of the U facing the other wrist bones and the hand. The more distal wrist bones of the hand articulate with the semi-lunate carpal such that they can only slide up and down, not side to side. Thus, the semi-lunate carpal bone further restricts motion side to side, but gives a great deal of smooth motion up and down (through the thumb-fifth finger axis) with the hand palm side in.

Now, keeping your hand and forearm stiff, lift your arms out to the sides. Notice now how your hand is rotated palm side down. It is in this way that 'raptors and other such dinosaurs could pronate their hands.

I apologize in advance for not knowing your level of anatomical knowledge. I have tried to stick to more simple terms to keep the aspects of the function clear. However, this was a response to clarify things to the whole list, so hopefully those without extensive anatomy can now join in the conversation, or at least understand what all of us are talking about! =)

Hope this little exercise helps.

Matt Bonnan
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