Tapetum

The tapetum of the corpus callosum is a thin sheet of commissural fibres forming part of the callosal radiation. Its fibres arise predominantly from the posterior parts of the corpus callosum, particularly the posterior body/isthmus and splenial region, and sweep laterally and then inferiorly within the white matter of the cerebral hemispheres.

The tapetal fibres curve around the posterior part of the lateral ventricle and extend into the temporal lobe. They contribute to the white matter forming the roof and lateral wall of the atrium and posterior horn of the lateral ventricle, and continue along the lateral wall and roof region of the inferior horn. In this position, the tapetum forms a thin callosal layer immediately external to the ventricular ependyma.

The tapetum is therefore not a separate commissure; rather, it represents a specialised component of the radiation of the corpus callosum, carrying interhemispheric fibres into posterior and temporal regions.