  We investigate the magnetic properties of the Cu-O planes in stoichiometric
Sr$_{n-1}$Cu$_{n+1}$O$_{2n}$ (n=3,5,7,...) which consist of CuO double chains
periodically intergrown within the CuO$_2$ planes. The double chains break up
the two-dimensional antiferromagnetic planes into Heisenberg spin ladders with
$n_r=\frac{1}{2}(n-1)$ rungs and $n_l=\frac{1}{2}(n+1)$ legs and described by
the usual antiferromagnetic coupling J inside each ladder and a weak and
frustrated interladder coupling J$^\prime$. The resulting lattice is a new
two-dimensional trellis lattice. We first examine the spin excitation spectra
of isolated quasi one dimensional Heisenberg ladders which exhibit a gapless
spectra when $n_r$ is even and $n_l$ is odd ( corresponding to n=5,9,...) and a
gapped spectra when $n_r$ is odd and $n_l$ is even (corresponding to
n=3,7,...). We use the bond operator representation of quantum $S=\frac{1}{2}$
spins in a mean field treatment with self-energy corrections and obtain a spin
gap of $\approx \frac{1}{2} J$ for the simplest single rung ladder (n=3), in
agreement with numerical estimates.
