All My Bones Shall Declare

The Four Species also allude to the body. When one takes them on the Festival and recites the blessing, it is as if he is subjugating his heart, his limbs, his faculties of sight and speech, to God alone. It is as if he declares to God: "All of me is given to You alone and all my joy in this Festival is in You alone."

All traces of sin or transgression in a person's limbs, senses, or powers are atoned for in the merit of this mitzvah which is performed out of love and fear of God.

The Sages expounded:
The verse states: "All my bones shall declare: God, who is like You?" (Psalms 35:10). The verse is referring to the mitzvah of lulav [i.e., the four species]. The spine of the lulav (palm tree), is like man's spine; the hadas (myrtle), is like the eye; the aravah (willow), is like the mouth; and the etrog (citrus), is like the heart. David said: There are no more important parts of the body than these and they are equivalent to the entire body. Thus [when one takes the four species, he is saying,] "All my bones shall declare" (Vayikra Rabbah 30).

Anaf Yosef quotes another midrash on this theme:
Why do we take the etrog? Because it is similar to the heart, and [thus] atones for the heart's evil thoughts.
Why do we take the hadas? Because it is similar to the eyes, and [thus] atones for the evil sights which the eye seeks, as the verse states: "And you shall not turn after your hearts and your eyes" (Bemidbar 15:39).
Why do we take the aravah? Because it is similar to the mouth, and thus atones for the expressions of the lips.
Why do we take the lulav? Just as the lulav has only one heart [i.e., a single central stem], so too does Israel have only a single heart for their Father in Heaven.