There are four stages to a circumcision — circumcision, in which the excess foreskin is cut uncovering, in which the remaining foreskin covering the corona is separated from the glans by means of the mohel’s fingernail, sucking, in which the mohel must suck the blood flowing from the sexual organ by means of his mouth, and placing a compress in which the wounded sexual organ is bandaged. The first two stages, circumcision and uncovering, are the essence of the ceremony demanded by Halacha. Sucking and bandaging are medical procedure and are not the essence of the ritual. The early sages ruled that each of the above four stages are to be done even on the Sabbath; circumcision and uncovering because the action of the ceremony supercedes the Sabbath and sucking and bandaging because the saving of lives supercedes the Sabbath. One of the sages, Rav Papa, said that a mohel who did not suck the blood from the infant’s sexual organ endangers the infant, and therefore he is removed from his position as mohel. The scholars asked: The words of the sage Rav Papa are not new, and we already know this from the words of the early sages who permitted sucking the blood on the Sabbath, meaning that the sucking is a necessary step in healing the infant, and so it supercedes the Sabbath. Answer: From the words of the early sages we do not necessarily see that the sucking is necessary and dangerous. It is possible that the circumcision blood which is sucked from the infant’s sexual organ is not forbidden on the Sabbath because it is stored in the organ and is not like the blood of a wound which has damaged the blood vessels. Therefore the sage Rav Papa told us something new, that sucking the blood of the circumcision is like damaging the blood vessels and would be forbidden on the Sabbath, yet it is permitted after a circumcision because medical dangers superceded the Sabbath.
(Babylonian Talmud, Tractate Shabbat 133a-b)