Bending a slab of neural tissue

In comparative and developmental neuroanatomy one encounters questions regarding the deformation of neural tissue under stress. The motivation of this note is an observation (Barbas {\it et al}) that at cortical folds or gyri, the layers of neural tissue show relative thickening or thinning of upper or deep layers. In general, the material properties of a slab of neural tissue are not known, and even if known, would probably lead to a difficult problem in elasticity theory. Here a simple argument is presented to show that bending an elastic slab should produce a relative thickening of the layers on the inside of the bend. The argument is based on the incompressibility of the material and should therefore be fairly robust.