An excerpt from "The Four Foundations of Mindfulness" by U Silananda, Wisdom Publications, Boston, 1990:

WHEN YOU OBSERVE YOUR breaths, you must try to see all the breaths clearly. "Making clear" means making the breaths known, making them plain, trying to see them vividly. In the original Pali text, the word for "the entire in-breath body" is sabbakaya, which literally means the entire body. But kaya or body here does not mean the entire physical body. It means the breath body. The Pali word kaya can mean the physical body as well as a group. It is similar to when you talk about a body of members. Here it means not the entire physical body but just the breath, and "entire" means, the beginning, the middle and the end. So, meditators must try to see thoroughly the beginning, the middle, and the end of each breath. You must also not forget that this section is on mindfulness of breathing so that the object of this meditation must be the breathing and not the entire physical body.