A person who had a nesek the size of a gris that [then] covered his entire head is tahor. The head and the beard do not prevent one another [from becoming tahor, these are] the words of R’ Yehudah. R’ Shimon says: They do prevent one another. Said R’ Shimon: It is a logical argument: The skin of the face and the skin of the body, which have something separating them, prevent one another [from becoming tamei; it therefore follows that] the head and the beard, which have nothing separating them, certainly prevent one another [from becoming tamei]. The head and the beard do not combine with one another, nor do they spread from one to another. What is [the definition of] the beard? From the joint of the jaw until the protrusion at the top of the trachea.

[Negaim that appear on] a karachas or [on] a gabachas become [confined] tamei for two weeks, [and are confirmed tamei] with two symptoms: living skin or expansion. What is karachas? [If] he ate neshem, he applied neshem [to his skin], [or he received] a scar on which hair cannot grow. What is karachas? From the crown, sloping backward until the vertebra of the neck. What is gabachas? From the crown, sloping forward until the hairline. Karachas and gabachas do not combine with one another, nor do they spread from one to another. R’ Yehudah says: If there is hair between them, they do not combine; if not, they do combine.