The Thumb Group of the Forearm
There are a number of muscles responsible for moving the thumb that originate on the forearm. The two most visible ones form what will be referred to here as the thumb group.
The thumb group emerges from between extensor carpi radialis brevis and extensor digitorum, which touch higher up on the forearm.
It consists of abductor pollicis longus, which originates on the ulna and radius, and extensor pollicis brevis, which originates on the deep part of the radius. Their tendons insert at the base of the metacarpal of the thumb.
These muscles form two curves on the dorsal side of the forearm. Extensor carpi radialis brevis seems to run in back of abductor pollicis longus as the latter crosses over it.
Extensor pollicis brevis forms a flat bulk on the wrist over the lowest end of the radius.
Abduction and extension of the thumb are similar motions, and it makes sense that an abductor and extensor of the thumb are so allied as to be fit for grouping.