The Word of the Buddha The Effort to Maintain

¶ 54.) And what, bhikkhus, is striving to protect? Here, bhikkhus, a bhikkhu protects an arisen excellent object of collectedness: the perception of a skeleton, the perception of a worm infested corpse, the perception of a livid corpse, the perception of a dismembered corpse, the perception of a bloated corpse. This, bhikkhus, is called striving to protect.

¶ 7Restraint and abandonment,
cultivation and protection;
These four strivings were taught,
by the kinsman of the Sun;
By these means, a bhikkhu with continuous effort,
arrives to the end of suffering.”