saddle-joint

Century Dictionary and Cyclopedia

1. n. A joint made by turning up the edges of adjacent plates of tin or sheet-iron at right angles with the bodies of the sheet (one margin so turned up being nearly twice as wide as the other), and then turning down the broader margin snugly over the other so that the margins interlock.
2. n. In anatomy, a joint where the articular surfaces are inversely convex in one direction and concave in the other, admitting movement in every direction except axial rotation. This joint occurs between all saddle-shaped vertebræ, as notably in the necks of all recent birds and of many reptiles. It is exemplified in man in the carpometacarpal joint of the thumb. Also called reciprocal reception joint.