Eruvin, Chapter 10, Mishnah 5. This mishnah deals with a person standing in one domain while his bodily needs are discharged into another domain, such as urinating or spitting from one domain to another.
The text of the Mishnah:
"Lo ya'amod adam bir'shus ha-yachid vi'yashtin bir'shus ha-rabbim, bir'shus ha-rabbim vi'yashtin bir'shus ha-yachid, v'chein lo yarok" - a person may not stand in a private domain and urinate into the public domain, and likewise the reverse: stand in the public domain and urinate into a private domain. In the same way, he may not spit from one domain into another.
The required size of the place and the role of intent:
The Gemara clarifies the basis of this prohibition in light of the usual rule regarding carrying from one domain to another:
The usual rule: A person is liable for carrying and transferring from one domain to another only if he picked up the object from a place where it was resting securely, that is, from a solid, defined surface of at least four handbreadths by four handbreadths. A place smaller than that is not a significant place, and one is not liable for carrying out from it.
The power of intent: The Gemara explains that a person's thought and intent, when he deliberately sends the object out from a particular place, render that place as though it measured four handbreadths, and he is therefore liable. The exemption was stated specifically about someone who incidentally picked up an object from a place smaller than four by four, but one who does so deliberately, as in our case, is liable.
For this reason a person may not spit or urinate from one domain into another.
"Af mi she'nitlash roko b'fiv":
Rabbi Yehudah adds: "Af mi she'nitlash roko b'fiv - lo yehaleich arba amos ad she'yarok" - that is, someone in whose mouth a complete lump of saliva has gathered, not the moisture that the glands release as he walks along, but a formed, ready lump, may not walk four cubits in the public domain until he spits it out. If he does walk, he becomes liable for transferring a defined object four cubits in the public domain.
In summary: Ordinarily a person need not be concerned about the saliva or moisture in his mouth, but once a complete lump of saliva or phlegm has gathered there, it has the status of a defined object, and he may not walk four cubits in the public domain before spitting it out of his mouth.