ABSTRACT
Pure disorders of asymmetry are rarely looked for in clinical practice. There is an enduring puzzle concerning the adaptive significance of there being two brain hemispheres rather than one. Solutions to this puzzle are usually sought in terms of natural selection and some authorities have suggested that an animal with two brain hemispheres would be more likely to survive, and consequently pass on its genes, if one hemisphere is damaged. Evidence from the different sources combine to reveal that nearly all of us who are right-handed have language housed in the left brain hemisphere. Disconnection between the two hemispheres may occur through naturally occurring lesions such as those due to stroke and the surgical treatment of tumours. The connections between the hemispheres and the neuronal projections to the upper limbs decussate below the commissures at the level of the brain stem.
