Re: Palms in

["Matthew Bonnan" <mbonnan@hotmail.com>]

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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