Decussation is used in biological contexts to describe a crossing. (In Latin anatomical terms the form decussatio is used, e.g. decussatio pyramidum.) In anatomy the term chiasma or chiasm means nearly the same as decussation.
Examples include:
In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin. See for examples Decussation of pyramids and sensory decussation. Decussation describes the point where the nerves cross from one side of the brain to the other, and typically the nerves from the left side of the body decussate to the right side of the brain and the nerves from the right side of the body decussate to the left brain, however depending on the function of the nerves the level of decussation is variable.
In botanical leaf taxology, the word decussate describes an opposite pattern of leaves which has successive pairs at right angles to each other (i.e. rotated 90 degrees along the stem when viewed from above). In effect, successive pairs of leaves cross each other. Basil is a classic example of a decussate leaf pattern.
In tooth enamel, where bundles of rods cross each other as they travel from the enamel-dentine junction to the outer enamel surface, or near to it.
In taxonomic description where decussate markings or structures occur, names such as decussatus or decussata or otherwise in part containing "decuss..." are common, especially in the specific epithet.
After the origin of the nervous system, in the course of evolution from invertebrate to vertebrate, the nerve cord moved from the ventral side (side towards the belly) to the dorsal side (side towards the back). It is believed that in this process, the decussation arose with a 180° shift with respect to the brain.
