Keep working the carving as a whole, refining those main shapes, finding the thumb etc. You'll be thinning the wrist as you extend the forearm down the block.
Anatomically, to all intents and purposes the muscles for the fingers all lie in the forearm; the wrist is full of their tendons, so relatively thin. When the wrist arrives at the hand it meets some big muscles for opposing the thumb, and this makes up the 'heel' of the hand.