In humans, the carpus is a collection of eight small bones that form a joint between the forearm and the hand. It is more commonly referred to as the wrist. Despite lacking hands, quadrupeds have a similar arrangement of bones in their forelimbs, also called the carpus. The carpal bones are homologous to the cluster of bones in the human foot known as the tarsus, or ankle. Carpal bones give freedom of movement to the wrist and allow humans to effectively position their hands.
Carpal bones are arranged in two rows across the wrist. The upper row is called the proximal row, while the lower row is called the distal row. Starting with the bones on the thumb side, the proximal row is comprised of the scaphoid, lunate, and triquetrum or triquetral. From the same perspective, the bones of the distal row are the trapezium, trapezoid, capitate, hamate, and pisiform. The proximal row of carpal bones connects the bones of the forearm — the radius and ulna — to the bones of the palm called metacarpals.
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The wrist owes its flexibility to the sheer number of individual joints between each of the carpal bones. Wherever two carpal bones meet as a joint, their ends are covered in a slippery, white substance called articular cartilage. The purpose of this substance is to allow joint surfaces to move against each other without causing damage, thus making movement easier.
There are also four sets of ligaments in the area of the carpus. One group of ligaments connects the bones of the wrist to each other. A second set unites the carpus with the radius and ulna, while a third unites the carpal bones with the metacarpals. The metacarpals are connected to one another by the fourth set of ligaments. Ligaments in the wrist help to support the carpal bones and increase the wrist's range of movement.
On the palm side of the hand, the carpus forms an arch commonly known as the carpal tunnel. The carpal tunnel contains several muscle tendons, most of which aid in the movement or flexing of the fingers. Also contained in the carpal tunnel is the median nerve, which runs from the forearm into the hand and controls sensation in the palm area. If this nerve is squeezed or compressed, it can lead to the condition known as carpal tunnel syndrome (CTS). Individuals with carpal tunnel syndrome may suffer from pain, numbness, or weakness in the affected wrist and hand.