Consider the Borel-Serre compactification of the quotient of hyperbolic 3-space $\Hy$ by a finite index subgroup $\Gamma$ in a Bianchi group, and in particular the following question which Serre poses on page 514 of the quoted article. Consider the map~$\alpha$ induced on homology when attaching the boundary into the Borel-Serre compactification. How can one determine the kernel of $\alpha$ (in degree 1) ? Serre uses a global topological argument and obtains the rank of the kernel of $\alpha$. But in the quoted article, he kept asking what submodule precisely this kernel is. With a local topological study, we can decompose the kernel of $\alpha$ into its parts associated to each cusp.