Eiruvin, chapter 1, part 2

In my last post about Eiruvin, I mentioned some of the technical details of the use of a "lechi", "korah" or "tzuras ha-pesach" to encompass an area. Any of these - a wall fragment, a firm crossbar, or a doorway - can halachically have the same status as a wall, as long as certain requirements are met. Once an area is surrounded by 4 "walls" of height 10 tefachim - around 2.5 feet - (as per the chachamim who added one to the original Torah law of 3), it has the status of a private domain, a "reshus ha-yachid", in which ha'avara, intradomain transportation, is not restricted (If the area enclosed by the walls is less than 4 tefachim by 4 tefachim, or approximately 1 square foot, the area is considered a "m'kom p'tur", an area of exemption, in which even the laws of "hotza'a", interdomain transportation, do not apply. On another note, if the walls are less than 10 tefachim, the area has a status of a "karmelis", a neutral area, which is subject to Rabbinical restrictions similar to those mandated by the Torah for a public domain, a "reshus ha-rabbim". If the walls are less than 3 tefachim, approximately 10 inches, they are not considered walls at all, but rather the enclosed area is considered to be part of its surrounding domain).

Eiruvin 1:7 is an extremely interesting Mishnah that begins by discussing the use of a living animal as a "lechi" - for example, fastening a live cow to a pre-existing 3-wall structure to use as a 4th wall fragment. According to the accepted majority opinion in the Mishnah, it constitutes a valid eruv! The mishnah also quotes majority opinions that hold that a live animal can be used as writing material for divorce get and, most interestingly, that a live animal used for a "golel" (it's a machlokes whether it's a grave covering or a coffin covering) becomes tamei and can be m'tamei people and utensils, even when it is no longer being used as such. Being the fan of wacky trivia that I am, this greatly fascinated me, as, to my limited knowledge, it is the only way that a living animal can become tamei - dead animals can become tamei either as food or because of their own status as a n'veila, an animal that died on its own, whichever is applicable.