  The Witt group of skew hermitian forms over a division algebra $D$ with
symplectic involution is shown to be canonically isomorphic to the Witt group
of symmetric bilinear forms over the Severi-Brauer variety of $D$ with values
in a suitable line bundle. In the special case where $D$ is a quaternion
algebra we extend previous work by Pfister and by Parimala on the Witt group of
conics to set up two five-terms exact sequences relating the Witt groups of
hermitian or skew-hermitian forms over $D$ with the Witt groups of the center,
of the function field of the Severi-Brauer conic of $D$, and of the residue
fields at each closed point of the conic.
