Middos, Chapter 3, Mishnah 2. This chapter is devoted to the mizbeach and its measurements, and having described the altar itself, the Mishnah now turns to a small but remarkable detail of its construction: two tiny openings built into the base at the southwest corner, through which the leftover blood of the korbanot drained away.
The Mishnah opens with the location: "uv'keren ma'aravit d'romit," at the corner where west meets south. Built into the yesod there, the ledge of stone running around the bottom of the mizbeach, was a pair of openings, in the Mishnah's words "hayu shenei nekavim." How narrow were they? The Mishnah supplies an image: "k'min shenei chotamin dakin," the size of two slender nostrils, small apertures pierced into the upper surface of that base.
Why Two Holes and Not One
The Mishnah itself supplies the reason, opening with "shehadamim hanitanin," the bloods that are placed. Blood reached this spot from two directions. Some korbanot required their remaining blood to be spilled on the western stretch of the base, "al yesod hama'aravi," while others required it on the southern stretch, "ve'al yesod d'romi." Since the corner in question is exactly where west and south converge, each of those two sides was given an opening of its own.
And so, "yordin bahen umit'arvin ba'ama," the blood descended through these openings and mingled together in the ama, the water channel. The two streams, arriving from two different sides of the mizbeach, met and combined below.
The Order of the Service
Which blood is under discussion? After a korban's blood service had run its course and every required application had been placed on the mizbeach, the surplus was not thrown away. Halachah directed that it be spilled at the foot of the altar, on the yesod. That surplus is what the Mishnah is following here.
The two perforations were not shallow depressions. They passed all the way through the thickness of the yesod and emptied into a channel of water that began near the mizbeach and ran the length of the courtyard until it reached the Water Gate, the Sha'ar HaMayim. The various bloods of the various korbanot blended into that flowing water, and the water carried them out beyond the Beit Hamikdash.