  We investigate experimentally and model theoretically the mechanical
behaviour of brain matter in torsion. Using a strain-controlled rheometer we
perform torsion tests on fresh porcine brain samples. We quantify the torque
and the normal force required to twist a cylindrical sample at constant twist
rate. Data fitting gives a mean value for the shear modulus {\mu} = 900 $\pm$
312 Pa and for the second Mooney-Rivlin parameter $c_2$ = 297 $\pm$ 189 Pa,
indicative of extreme softness. Our results show that brain always displays a
positive Poynting effect; in other words, it expands in the direction
perpendicular to the plane of twisting. We validate the experiments with Finite
Element simulations and show that when a human head experiences a twisting
motion in the horizontal plane, the brain can experience large forces in the
axial direction.
