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Section of the medulla oblongata at the level of the decussation of the pyramids.

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 much the same as decussation.[1]

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 example Decussation of pyramids. 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 phyllotaxis when an opposite pattern of leaves has successive leaf pairs that are at right angles to each other, it is called decussate. In effect, successive pairs of leaves cross each other.
    Decussate phyllotaxis of Crassula rupestris
  • 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.

Evolutionary significance[edit]

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.[2][3]

See also[edit]


  1. ^ Stedman, Thomas Lathrop. A practical medical dictionary. Publisher: New York, W. Wood 1920. May be downloaded at
  2. ^ Kinsbourne, M (Sep 2013). "Somatic twist: a model for the evolution of decussation.". Neuropsychology 27 (5): 511–5. doi:10.1037/a0033662. PMID 24040928. 
  3. ^ Dixon, A. Francis (31 May 2014). "Why are the great motor and sensory tracts of the central nervous system crossed?". The Dublin Journal of Medical Science 124 (1): 1–4. doi:10.1007/BF02972358. 
Further reading