ball of her index finger

今日とりあげる "ball" は誰でも知っている単語ですが、指の一部を示すのに使われることは知りませんでした。WILDERNESSからの抜粋です。
Janet looked at him without speaking. She ran the ball of her index finger around the rim of her glass.
"ball of her index finger" の "ball" が示している個所は辞書を見なくても分かりますが、辞書に説明があるか確認します。
・Macmillan Dictionary: the part of your foot, hand, or thumb that is slightly round and sticks out: ball of: He balanced on the balls of his feet
・Merriam-Webster's Online Dictionary: a roundish protuberant anatomical structure (as near the tip of a human finger or toe or at the base of a thumb); especially : the part of the sole of the human foot between the toes and arch on which the main weight of the body rests in normal walking
・Cambridge English Dictionary: the rounded part of your foot or thumb where the toes join the foot and the thumb joins the hand
人指し指の場合はやはり指先を示すのでしょう。