The muscles of the hand can be divided into two groups: extrinsic and intrinsic muscles. The extrinsic muscles are long flexors and extensors that originate in the forearm, that is, originate external to the hand. Intrinsic muscles originate within the hand and wrist itself. The extrinsic muscles are primarily responsible for the powerful gripping force of the hand, and gross hand movements. The intrinsic muscles are primarily responsible for intricate finger movements, and fine motor control.
Muscles controlling the hand are capable of moving the fingers in four directions: flexion or bending, extension or straightening, abduction or moving sideways away from the body, and adduction or moving sideways towards the body.
Muscles controlling the fingers, such as bending at the finger joints, are located in the palm and forearm. These muscles include the flexor carpi ulnaris, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and the flexor digitorum profunfus. These muscles are connected to the finger bones by tendons. Contraction of a muscle pulls on the tendon to move the finger. This is similar to a marionette where you pull strings to control movement. There are no muscles inside the fingers themselves.
The only intrinsic muscles are connected to the thumb and small finger. These are the abductor pollicis longus, flexor pollicis brevis, opponens pollicis that control thumb movement. The adductor pollicis, opponens digiti minimi, flexor digiti minimi, and abductor digiti minimi are used to control movement of the little finger.