Middot, Chapter Three, Mishnah Two
At the southwestern corner [of the foundation] there were two openings like two small nostrils through which the blood which was poured on the western side of the foundation and on the southern side flowed down till the two streams became mingled in the channel, through which they made their way out to the Kidron wadi.
The blood of all sacrifices was either drained on the western side of the foundation (see Zevahim 5:1-2) or on the southern side (Zevahim 5:3). The blood would drain out through two holes shaped like nostrils, and then flow down to the channel that flowed through the Temple Courtyard. From there the blood would be flushed out to the Kidron wadi that flows below the Temple Mount.