def find_vertex(vertices, connection):
    """
    Find which vertex of *vertices* corresponds to *connection*.

    Parameters
    ----------
    vertices: DataFrame
        Dataframe of the vertices.
    connection: Series
        Row of the dataframe *connections* obtained with the function
        *dhd.connect.second_selection*.

    Returns
    -------
    Series
        Series of the dataframe *vertices* corresponding to *connection*.
    """
    vertex = vertices.loc[vertices['idE'] == connection['idE']].iloc[0]
    return vertex
