Some afferent decussations. 160px|Pyramidal decussations. Anatomically, the contralateral organization is manifested by major decussations ( latin: the Latin notation for ten, 'deca ',is an uppercase 'X') and chiasmas (after the Greek uppercase letter 'Χ ', chi). A decussation denotes a crossing of bundles of axonal fibres inside the central nervous system. As a result of such decussations , the efferent connections of the cerebrum to the basal ganglia, the cerebellum and the spine are crossed , and the afferent connections from the spine, the cerebellum and the pons to the thalamus are crossed. As a result , motor, somatosensory, auditory, and visual primary regions in the forebrain represent predominantly the contralateral side of the body. Two of the cranial nerves show chiasmas:  the chiasm of the optic tract (  cranial nerve II)  which originates from the eyes and inserts on the optic tectum of the midbrain , and the trochlear nerve (  nerve IV)  which originates in the ventral midbrain and innervates one of the six muscles that rotate the eye (  superior oblique muscle). Although the forebrain of all vertebrates shows a contralateral organization, this contralaterality is by no means complete. Some of these exceptions are worth mentioning: Olfaction (  smelling sense) : Each olfactory lobe connects to the ipsilateral centers of the frontal cerebrum. In chondrichthyans (  sharks and skates)  the thalamus does not retrieve a branch from the optic tract but only from the contralateral optic tectum, so that the optic path decussates twice  and the forebrain represents the ipsilateral eye. In large brains, some functions tend to be strongly lateralized. E.g. , the language regions (  Broca's and Wernicke's area) are  in the left side in most humans. Most afferent and efferent connections of the forebrain have bilateral components, especially outside the primary sensory and motor regions. As a result, a hemiplegia that is acquired at very young age can be completely compensated over time.
Some afferent decussations. 160px|Pyramidal decussations. Anatomically, the contralateral organization is manifested by major decussations ( based upon the Latin notation for ten, 'deca ,' as an uppercase 'X') and chiasmas (after the Greek uppercase letter 'Χ ,' chi). A decussation denotes a crossing of bundles of axonal fibres inside the central nervous system. As a result of such decussations : The efferent connections of the cerebrum to the basal ganglia, the cerebellum and the spine are crossed ; and the afferent connections from the spine, the cerebellum and the pons to the thalamus are crossed. Thus , motor, somatosensory, auditory, and visual primary regions in the forebrain predominantly represent the contralateral side of the body. Two of the cranial nerves show chiasmas: (1) the chiasm of the optic tract ( i.e., cranial nerve II) , which originates from the eyes and inserts on the optic tectum of the midbrain ; and (2) the trochlear nerve ( i.e., cranial nerve IV) , which originates in the ventral midbrain and innervates one of the six muscles that rotate the eye ( i.e., the superior oblique muscle). Although the forebrain of all vertebrates shows a contralateral organization, this contralaterality is by no means complete. Some of these exceptions are worth mentioning: Olfaction ( i.e., smelling sense) is a noteworthy exception. Each olfactory lobe connects to the ipsilateral centers of the frontal cerebrum. In chondrichthyans ( e.g., sharks and skates) , the thalamus does not retrieve a branch from the optic tract but only from the contralateral optic tectum, so that the optic path decussates twice , and the forebrain represents the ipsilateral eye. In large brains, some functions tend to be strongly lateralized. For example , the language regions ( i.e., Broca's and Wernicke's area) are situated in the left hemisphere of most humans. Most afferent and efferent connections of the forebrain have bilateral components, especially outside the primary sensory and motor regions. As a result, a hemiplegia that is acquired at very young age can be completely compensated over time.
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