  Discontinuous changes of the lattice parameters at the Mott metal-insulator
transition are detected by high-resolution dilatometry on deuterated crystals
of the layered organic conductor $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br.
The uniaxial expansivities uncover a striking and unexpected anisotropy,
notably a zero-effect along the in-plane c-axis along which the electronic
interactions are relatively strong. A huge thermal expansion anomaly is
observed near the end-point of the first-order transition line enabling to
explore the critical behavior with very high sensitivity. The analysis yields
critical fluctuations with an exponent $\tilde{\alpha} \simeq$ 0.8 $\pm$ 0.15
at odds with the novel criticality recently proposed for these materials
[Kagawa \textit{et al.}, Nature \textbf{436}, 534 (2005)]. Our data suggest an
intricate role of the lattice degrees of freedom in the Mott transition for the
present materials.
