Carbocations are species bearing a formal + charge on carbon. The have sp2 hybridization and trigonal planar geometry, with an empty p orbital on carbon, perpendicular to the plane containing the substituents (see diagrams shown to the right).
Aortic cross-clamp. An aortic cross-clamp is a surgical instrument used in cardiac surgery to clamp the aorta and separate the systemic circulation from the outflow of the heart. An aortic cross-clamping procedure serves, for example, in the repairing of coarctation of the aorta.
The typical clamp is a vertical bar or bars, hinged on a pivot attached to or passing through the bench/seat top. The top of this bar is enlarged into the horse -, dog - or dumb head, the part that holds the wood. Some clamps are worked via string or rope, rather than mechanical leverage.