Wrist
The wrist is made up of two parts working together as one functional unit. It allows us to flex or extend the hand. The hand can also be tilted in the direction of the little finger or the thumb.
Carpus
The carpal bones are held together tightly by ligaments, and are more or less fixed in relation to one another. They form two rows: four carpal bones are in the first row closer to the forearm. Two of these, together with the radius form the inferior radioulnar joint, which is very important for the movement of the hand. The ulna is separated from the carpal bones by a cartilage disc. The other joint is located between the two rows of carpal bones.
Metacarpus
The metacarpus extends from the second row of the carpal bones. It has five elongated metacarpal bones. You can feel them quite well through the skin on the back of your hand. One of the metacarpal bones together with the thumb’s proximal phalanx makes up the carpometacarpal joint of the thumb, giving the thumb better freedom of movement compared with the other fingers.
Fingers
The five fingers make up the freely moveable part of the hand. Each has three individual bones – except for the thumb, which has only two. In each of these four fingers there are three joints, which can only be moved in one way (extension and flexion). The thumb, however, can also rotate owing to the carpometacarpal joint so that it can also be positioned opposite the other four fingers.
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Muscles
The hand and forearm, connected by the wrist, make up a functional unit. There are over 30 muscles here, working together in a highly complex way.
Long forearm muscles
Movements of the hand are mostly started by muscles in the forearm. Only the thin tendons of these muscles are directly a part of the hand: the extensor tendons used for extending the hand run through the back of the hand to the tips of the fingers, and the flexor tendons run through the palms to the fingers.
Short hand muscles
The short muscles of the hand lie between the individual metacarpal bones. They allow us to spread our fingers (abduction) and then press them back together (adduction). They also help us to flex the metacarpophalangeal joints and extend our fingers.
The thenar eminence and the hypothenar eminence muscles
Two groups of more powerful muscles in the hand itself make up the thenar eminence (at the base of the thumb) and the hypothenar eminence (controlling the little finger). The thenar eminence helps the thumb to move. This includes the essential movement of opposition, allowing the thumb and the tips of the remaining four fingers to touch. A separate muscle for flexing (adductor) can help move the thumb towards the palm. The muscles of the hypothenar eminence are mainly used for extending and bending the little finger, as well as for tightening the skin that covers the hypothenar eminence.