Commentary for Kiddushin 49:8

Rabbi [Yehudah Hanasi] says that the slave goes free if he is castrated, but he does not seem to agree with Ben Azzai that he goes free if his tongue is cut off. But there is a baraita that seems to contradict this. This baraita is about sprinkling a person with the red heifer waters in order to purify him. According to Rabbi, if he is sprinkled in his mouth, which we at first interpret as on his tongue, then the sprinkling is considered valid. This implies that the tongue is an exposed organ and that a slave would go free if the master cut it off.

The Talmud solves the difficulty by saying that the water fell on his lips, not on his tongue. But if it falls on his lips, then it should be obvious that he is pure—after all the lips are an external part of the body. The answer is that sometimes when his lips are tightly closed, the lips are not exposed to the outside. Therefore Rabbi needed to say that if the water falls on the lips, he is pure.