Final answer:
Abduction is the movement that takes the thumb from a medial to lateral position, moving it away from the index finger across the palm.
Step-by-step explanation:
Abduction of the thumb is the movement that takes the thumb away from the index finger across the palm, essentially spreading the thumb away from the hand's midline. This action is opposed by adduction, which brings the thumb back towards the hand's midline, or in anatomical position, next to the index finger. These movements of the thumb occur at the first carpometacarpal joint, which is a saddle joint formed between the trapezium carpal bone and the first metacarpal bone. When you perform abduction, the thumb moves anteriorly to a 90° angle from the palm.
This is distinct from the opposition, a combination of flexion and abduction, which allows the thumb to touch the fingertips, and is an essential functional movement for gripping objects. In summary, abduction is the motion involved when spreading the thumb and fingers apart, and is a key movement in the dexterity of the hand.