Smale’s $h$-cobordism theorem

Let $(N,M,M^{\prime })$ be an $h$-cobordism (http://planetmath.org/HCobordism) of smooth manifolds with $N$ (and hence $M$ and $M^{\prime }$) simply connected, and $dim(M)\ge 5$. Then $N\cong M\times [0,1]$ and $M$ and $M^{\prime }$ are diffeomorphic.