The Gemara investigates the hypothesis that the Cow's Mouth is to be considered the "Reshus" of the cow's owner, which hypothesis - were it accepted - would exempt the owner from paying damages.
This hypothesis is rejected.
Where does the hypothesis come from? What initial rational is the basis for such a strange hypothesis?
I thought perhaps the investigation is justified in light of the statement of Rava in Eiruvin 99A: If one threw something and a dog caught it in its mouth - one is liable for a sin offering. >From this statement we see that the mouth of a domestic animal is considered a separate Reshut. If we apply this ruling here - the question arises: Whose Reshut is it? Can it be that this is the source of this investigation? Or is another source suggested by some authority?
The Gemara in Eruvin is only saying that the mouth of the Kelev has a Din of Makom Dalet which is necessary for making an Akirah or a Hanachah. As far as which Reshus it is, it is whichever Reshus the dog is in at the time and not a separate Reshus.
See Tosfos, that when he actually took it from the Reshus of the Nizak he is Chayav. Tosfos also writes that although there is no Sevara, still there is a Gezeiras ha'Kasuv that the Nezek has to be carried out in the Reshus of the Nizak.
D. Zupnik