Reference for Eruvin 40:16

אלא זמנין דמתקן ליה והדר מעייל ליה וקא מעייל ליה מרה"ר לרשות היחיד

In the case of a human being it had definitely been laid down that it was enough if his head and the greater part of his body [were in the domain from which he drinks]. Is it enough, however, in the case of a cow, that its head and the greater part of its body [should be in the domain from which it drinks] or not?

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