The two halves of the brain and spinal cord are interconnected by numerous fibres that cross the middle line. In some situations such fibres form recognisable bundles that are called commissures. Strictly speaking commissural fibres are those that connect corresponding regions of the two sides. Many of the fibres passing through the socalled commissures do not fulfill this criterion as they connect different regions of the two sides. Such fibres are really association fibres. We have seen that several tracts passing through the spinal cord and brainstem cross from one side to the other. These crossings are decussations, but collections of such fibres are sometimes loosely referred to as commissures e.g., the ventral white commissure of the spinal cord.
The corpus callosum is the largest commissure connecting the right and left cerebral hemispheres.
The fibres passing through the corpus callosum are generally believed to interconnect corresponding regions of the entire neocortex of the right and left sides. However, some important exceptions are now known.
a) The greater parts of the visual areas are not interconnected. Only those parts of visual areas that receive impulses from a narrow strip along the vertical meridian of the retina are interconnected. The band of cortex concerned lies at the junction of areas 17 and 18. Similar bands are also present in relation to other visual areas.
b) The parts of the sensorimotor areas (SI and SII) concerned with the hands and feet are not interconnected.
The corpus callosum can be congenitally absent. As the two cerebral hemispheres are not connected there is a split brain effect. If one hand is trained to perform an act the other hand may not be able to do so.
Other commissures connecting the two cerebral hemispheres are the anterior commissurethe posterior commissure the hippocampal commissure or commissure of the fornixand thehabenular commissure.
In close relationship to the optic chiasma a number of commissures carrying fibres not concerned with vision are described. These include the ventral supraoptic commissure (of Gudden), the dorsal supraoptic commissure (of Meynert) and the anterior hypothalamic commissure (of Ganser). The constitution of these commissures is controversial and will not be considered. Commissural fibres also interconnect the two halves of the cerebellum, and the colliculi of the midbrain.