QUESTION: The Gemara earlier (37b) quotes the Mishnah in Ohalos (2:3) in which Beis Shamai and Beis Hillel argue about the degree of damage or deficiency a body part must have in order for it not to be Metamei b'Ohel. With regard to a skull with a hole, Beis Shamai says that a skull with a hole is not Metamei b'Ohel when the hole is the size of the hole made by a surgeon's drill. Beis Hillel says that a skull is not Metamei b'Ohel when it has a hole that would cause a living person to die. Rav Tachlifa bar Avodimi in the name of Shmuel explains that a hole the size of a Sela coin would cause a person to die.
Rav Chisda there questions Rav Tachlifa's explanation, because we learn from the Mishnah in Kelim (17:12) that a hole the size of a Sela and a hole the size of a drill-hole are the same size, and yet we know that Beis Shamai and Beis Hillel are arguing with each other! According to Rav Tachlifa, what is their argument?
Rav Nachman answers that a large drill makes a hole the size of a Sela Nironis (a Sela of Nero), which is larger that the size of a regular Sela. Thus, Beis Shamai requires the hole to be larger in order for the skull not to be Metamei.
RASHI (DH b'Eizeh Makde'ach) explains that the surgeon uses his drill in order "to pierce the head to heal a wound." If the drill is used to pierce skulls for curative purposes, then it is obvious that a person can live with a piece of his cranium he size of that drill's hole missing from his cranium! How, then, can Beis Hillel say that a hole smaller than the size of a drill-hole cause a person to die?
In addition, the Gemara in Eruvin (7a) teaches that the Shi'urim that Beis Hillel and Beis Shamai give apply not only to the laws of Tum'as Ohel, but also to the laws of Tereifah. Beis Shamai, who says that a large hole in the skull renders the skull unable to be Metamei b'Ohel, also maintains that a large hole must be missing in order for an animal to become a Tereifah. The Gemara in Eruvin implies that the Shi'ur that Beis Shamai describes as the size of a surgeon's drill is the amount of missing cranial bone that will render an animal (or a person) a Tereifah. How can the surgeon's drill then be used to heal a person by piercing his skull, if that very hole is large enough to make the person a Tereifah?
(a) Perhaps we may suggest that when Beis Hillel mentions the Shi'ur of a hole that will cause a living person to die, he means that it will cause a person to die if it is not filled or covered. The surgeon immediately covers up the hole he makes after the procedure, thereby preventing the person from dying. (Y. Tavin)
However, this answers only how the surgeon's hole can be larger than the hole that causes a person to die. It does not answer how the hole can cure a person while at the same time it renders a person a Tereifah. The Gemara in Chulin (57b) teaches that when a hole renders an animal a Tereifah, medical intervention does not prevent the animal from dying. Therefore, covering the hole that makes a person a Tereifah should not prevent him from dying.
(b) RAV MOSHE SHAPIRO shlit'a suggests that perhaps the surgeon's drill is not used in the same place on the skull where a hole will render a person a Tereifah. A hole will only render him a Tereifah in certain parts of the skull. When Rashi mentions that a surgeon's drill is used to cure a person, he does not mean that it is used on the same part of the skull that Beis Shamai is discussing in the Mishnah.
(c) Rav Moshe Shapiro suggests further that perhaps a hole that normally renders an animal or person a Tereifah will not render him a Tereifah when it is made under controlled conditions during a surgical procedure.
When Beis Hillel mentions a hole large enough to cause a living person to die, he is referring to a hole that is made without medical supervision. However, a surgeon can remove even large amounts of the skull without killing the person, because he does it under surgical conditions.