  In this paper we fully describe the cuspidal and the Eisenstein cohomology of
the group $G=GL_2$ over a definite quaternion algebra $D/\Q$. Functoriality is
used to show the existence of residual and cuspidal automorphic forms, having
cohomology in degree 1. The latter ones turn out to be CAP-representations,
though $G$ satisfies Strong Multiplicity One. A non-vanishing result on
intertwining operators of induced representations will serve as a starting
point for further investigations concerning rationality of critical $L$-values.
