The second half of the tenth perek (=chapter) of Masechet Menahot focuses on the laws of semikhah – laying of hands on the sacrifice. The Mishnah on today’s daf (=page) teaches that no communal sacrifices include semikhah, except for the unique se’ir ha-mishtale’ah – the goat sent off to Azazel as part of the Yom Kippur service – and the par ha’alem davar shel tzibbur – the sacrifice brought by the Sanhedrin when they mistakenly misled the people with an erroneous ruling, leading the community to sin. Rabbi Shimon adds another communal sin-offering – the one brought when a mistaken ruling leads the community to commit an act of avodah zara – of idol worship.
Semikhah as part of the sacrificial service in the Temple, was an important part of the process of atonement of the individual who brought the sin or guilt offering, as is indicated in the Torah (Vayikra 1:4). Semikhah was performed as follows: The sacrificial animal is positioned in the northern part of the Temple courtyard with its head facing to the west. The person doing semikhah places both of his hands on the head of the animal, between its horns. He then recites viduy, that is, he confesses the sins for which the sacrifice is being brought, as appropriate for a hatat, an asham or an olah (for neglecting positive commandments).
The semikah and viduy serve to clarify the connection between the person bringing the korban and the atonement that is sought by means of the sacrifice. In the cases of communal sacrifices where there was semikhah, the kohen gadol (=High Priest) acted as the representative of the community on Yom Kippur and three members of the Sanhedrin played that role when the par ha’alem davar shel tzibbur was brought. Some suggest that the 24 ma’amadot of ordinary Jews who came to the Temple throughout the year were established to serve as representatives of the people in communal sacrifices that do not ordinarily have these elements. See Ta’anit 26a for a discussion of ma’amadot and their role in the sacrificial service.