Re: Can people inherit an extra tendon ?

Sounds to me like the question is referring to palmaris longus, a 
muscle\tendon which most people have. I think 90% of people still have it  
but it is a muscle that is disappearing - we can live without it. 
So it's not a matter of inheriting an extra tendon - it's an issue of a 
muscle that is beginning to disappear down the evolutionary path. Perhaps 
it was important when we spent more time swinging in trees! 
So if you lack the muscle/tendon, you're supposedly more evolved!

"By bending the wrist forward" you raise flexor carpi radialis (which 
everyone has - one of the flexors of the wrist; the most prominent tendon 
in the flexor compartment of the forearm), and if you have it, palmaris 
longus will also be very evident medial to it at the wrist. 
You DO need to "touch the tips of the pinky and thumb together" while 
flexing the wrist to really see it.
Main action is to (weakly) flex the wrist.

Origin = medial epicondyle of the humerus (common flexor origin)
Insertion = palmar aponeurosis and flexor retinaculum.
Palmaris longus inserts into the palmar aponeurosis - the tendinous sheet 
of the palm which resists shearing forces on the hand. It thus serves to 
tighten up the aponeuorosis. Like flexor carpi radialis and flexor 
digitorum superficialis, palmaris longus has a short muscle belly and a 
long tendon which gives good leverage, without bulking up the wrist area. 
Hence the visible tendon at the wrist.

Most people have the muscle in both forearms. Of those that don't, some 
people have it in one forearm, some lack it in both.

One advantage of the palmaris longus tendon, is that it protects the 
median nerve which passes deep to it. This nerve is at risk in 'slashed-
wrist' suicide attempts.
Because we can function perfectly without it, palmaris longus tendon is 
sometimes harvested for tendon grafts.


Moore:  "Absent in 1 or both sides (usually the left) in approx. 14% of 
people, but its actions are not missed"
Lasts:  (Last's Anatomy, Regional and Applied  ISBN 0 443 05611 
0)   "Palmaris longus:  Muscle arises from the common origin. Is absent in 
13% of arms. Its long, flat tendon broadens as it passes in front of the 
flexor retinaculum, to which it is partially adherent.
Action:  It is a weak flexor of the wrist, and anchors the skin and fascia 
of the hand against the shearing forces in a distal direction.  The tendon 
can be used in transplant procedures."