We are looking at Mishnah 9 in the second chapter of Tractate Bava Basra, which continues the discussion of keeping things away from the city when they disturb its residents. The focus of this Mishnah is on things that smell bad.
"Marchikin es haneveilos ve'es hakevaros ve'es haburseki min ha'ir chamishim amah" - we must distance carcasses, graves, and a tannery from the city by fifty cubits:
"Haneveilos" - carcasses. A place where animal carcasses are placed, dead animals that were thrown out (for dog food and the like). Since they are left outside, they eventually rot, and this decomposition produces a terrible smell.
"Hakevaros" - graves. This refers to the burial area. Often, bodies were left to decompose before a secondary burial in a niche, and here too the smell is severe.
"Haburseki" - a tannery. A place for processing leather, which has the worst smell of all: both because of the rotting organic material of the hides themselves, and because of the materials used to process them in the various stages - chemicals and animal feces.
Because of this, it was established that all of these must be kept at least fifty cubits away from the city, so that their smell does not disturb the residents.
The location of the tannery:
"Ein osin burseki ela lemizrach ha'ir" - we only establish a tannery to the east of the city. Furthermore, the smell of the tannery is so severe that it must be placed specifically on the eastern side of the city. The wind usually blows in the opposite direction, and even when it comes from the east, eastern winds tend to be weak and warm and do not carry much smell with them. Therefore, this is the optimal place to position it.
"Rabbi Akiva omer: lechol ruach ya'aseh chutz mima'aravah" - Rabbi Akiva says it may be established in any direction except for its west. According to Rabbi Akiva, it can be located on any of the four sides of the city, except for its western side. The reason for excluding the west is not because of the wind, but because people praying face west, as the Divine Presence is there, and it is inappropriate to have a bad smell in that direction. "Umarchik chamishim amah" - and he must distance it fifty cubits. Wherever it is placed around the city, it must be kept fifty cubits away.
The root of the dispute:
The Tanna Kamma argues with Rabbi Akiva. In his view, the Divine Presence is not specifically in the west, and this is not the issue. Rather, he believes that winds blow from all directions and carry the bad smell with them, and therefore when placing the tannery, it must be positioned specifically in the east, to prevent the bad smell from entering the city with the winds.
In summary: In this Mishnah, we learned that carcasses, graves, and a tannery must be distanced fifty cubits from the city, due to their bad smell. For a tannery, a special rule is added: according to the Tanna Kamma, it must be positioned specifically to the east of the city, because the winds coming from other directions carry the smell with them; whereas Rabbi Akiva permits it in any direction except the west, because people praying face toward the west.