Bone · Hand

1st metacarpal

The thumb's metacarpal: the shortest and stoutest of the five, and the only one that is turned. In development it rotates so that its palm-facing surface looks toward the other fingers rather than straight down the palm, which is why the thumb sits in a different plane from the rest of the hand. Its base carries the saddle-shaped joint with the trapezium, and its head is broad and flattened with two small sesamoid bones lying in the tendons beneath it. Unlike the others it has no ligament tying it to its neighbour, leaving it free to move independently.