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.
cross tie. a common method of restraining a horse for simple procedures such as grooming. The horse is tied to a pillar on either side, the shorter and tighter the better and preferably from the cheek dees of a hackamore. The head should be kept high to avoid the horse lashing out with both feet at once.