Rav Hirsch: Prayer IV: Selicha & Mechilla
In the third group we first note the kidneys, which in Scrip[ture is used as a metaphor for the “lower” physical urges and feelings. In this sense also the kidneys are prominent in the Chatat – Mistaken Sin Offering – and are offered with a part of the liver, as well as in our Tamid offering.
The Sages teach that the liver bile is the seat of anger. Anger, resentment against opposition, against that which is offensive or repugnant, is directed against the impulses which the kidneys represent.
Klayot – Kidneys – Kaleh – yearning and desire. “And long for them,” – “v’chalot,” (Deuteronomy 28:32). “My eyes long – ‘kalu’ – for Your words.” (Psalm 119:82)
Altogether desires and anger may well form the positive and negative poles of our wishes. “Ukatz” is another part of this group. The word is synonymous with “atzeh,”, thus, “l’umat ha’atzeh,” is the region of the kidneys. Thus conceivably the leading right hind leg is brought together with these symbols of the lower instinvts. The potential conflict between the above mention human trends, movement and impulses justify the urgent need for Selicha and Mechillah as expressed in the third of the middle blessings.