Bechoros Chapter 7, Mishnah 2. The Mishnah continues listing anomalies that cause a person to look so unusual, unlike the rest of the descendants of Aharon, that he cannot function as a kohen in the Beis Hamikdash.
"Hakeireach":
The Mishnah states that "hakeireach" is invalid for service, and defines: "Eizehu keireach? Kol she'ein lo shitah shel sei'ar makefes me'ozen le'ozen" - who is the bald person? Anyone who does not have a row of hair spanning from ear to ear. His hair has completely receded, and not even a single row of hair spanning from ear to ear at the back of his head remains.
The Gemara explains that if he is left with an unusual bald pattern - a strip of hair behind the ears and a strip of hair at the front of the head, such that a large circle is formed on his head - this appearance is even more unusual, and he is certainly not permitted to serve. However, the Mishnah is discussing a person who is completely bald, who does not even have the last hair remaining at the back of the head; and if he is left with even this row alone, he is valid and permitted to serve.
"O giben o dak o tevalul be'eino":
A person who is a "giben", or who has other blemishes in his eye, is too anomalous to serve in the Beis Hamikdash. The question is what the Torah means by the word "giben", and the Tanna'im disagree about this.
The view of the Tanna Kamma - Eyebrows:
According to the first view, "giben" refers to the eyebrows, and the word "in his eye" at the end of the verse refers back to the eyebrows above the eye. The basic understanding is exaggerated eyebrows, and in the words of the Mishnah:
"Ein lo gevinim" - he has no eyebrows. Since the verse says "or giben," the superfluous "or" comes to teach not only about overly large eyebrows, but even about one who lacks eyebrows completely.
"Ein lo ella gevin echad" - he has only one eyebrow. The simple meaning is: his eyebrows are connected to each other into one long eyebrow across the entire width of the forehead above the eyes, and instead of two he has one huge, continuous one. And certainly, one who has only one eyebrow and is missing the other, his appearance is even more unusual and he is invalid for service.
"Hu giben ha'amur baTorah" - the Tanna summarizes: this is the giben spoken of in the Torah.
"Rabbi Dosa omer: kol shegevinav shochvim" - Rabbi Dosa says: anyone whose eyebrows lie down. Rabbi Dosa adds to the list another form of exaggerated eyebrows: not only when they are connected into one eyebrow, but even when they are so long that they cover the eyes, and according to Rabbeinu Tam - cover the eyelashes. Out of their great length they enter into the eyes, and the appearance of someone with these bushy eyebrows is unusual, and therefore he is invalid for service.
The Bartenura explains that this person can trim his eyebrows, and then he will look normal and can return to serve; as long as his appearance is unusual, he is not permitted. This sharpens the distinction mentioned above: the things that cause a kohen not to look like the seed of Aharon HaKohen are not fundamental blemishes. If he undergoes plastic surgery, or in our case gets a haircut, he is repaired and his appearance is no longer unusual, and he returns to being valid for service. This is not a fundamental defect of a blemished person, but merely an anomalous appearance.
The Opinion of Rabbi Chananya ben Antigonos - A Back:
Rabbi Chananya ben Antigonos disagrees and maintains that the word "giben" in the verse does not refer to eyebrows. Rather, it is derived from the word "gav" (back): it refers to a person who appears to have two spinal columns or two backs. A severe curvature has completely distorted his back, making it look as though he has two backs. This person is the "giben" mentioned in the verse, and he is disqualified from the service due to his unusual appearance.
The Bartenura emphasizes that there is no practical dispute here. Everyone agrees that a person with highly unusual eyebrows, as well as one with a severe curvature of the spine, does not look like an ordinary person and is unfit for the service. The entire question in our Mishnah is regarding the simple meaning of the verse: what does the Torah mean by the word "giben". However, according to all opinions, these anomalies are so extraordinary that all of the individuals mentioned above are not permitted to perform the service in the Beis Hamikdash.