Tosefta for Pesachim 148:15

A drop of milk that fell onto a cut [of meat], Rabbi Yehuda says, [it is forbidden] if it imparts its taste into that cut, and the Sages say, [it is forbidden] in the same cooking pot. Said Rebbi, the words of Rabbi Yehuda appear preferable in a situation where [the mixture] has neither been stirred nor covered, and the words of the Sages [appear preferable] in a situation where it is stirred or covered. Hot [milk that fell] inside a hot [meat dish], or cold [milk that fell] inside a hot [meat dish] -- these are forbidden. Hot [milk] inside cold [meat], he may rinse it off and eat it. The udder of [an animal] nursing [offspring]: he must tear it open and extract the milk [inside the udder]. [But] if he does not tear it open he does not violate [the prohibition]. The heart: he must tear it open and extract the blood. If it is cooked [with the blood inside], he may tear it open after it is cooked [and extract the blood].