19a: Surely, a dead louse is a Chatzitzah (between the Kohen and Bigdei Kehunah).

Question: Is a live louse a Chatzitzah? Since it comes and goes by itself, it is like his skin. Or, since he does not want it there, it is a Chatzitzah?

Surely, dirt is a Chatzitzah.

Question (Mar bar Rav Ashi): Is hair (of his head) under a garment (other than the hat) a Chatzitzah? Is his hair like his body, or not?

(R. Ami): Tefilin are a Chatzitzah.

Question (Beraisa): If they wore Tefilin, it is not a Chatzitzah.

Answer: That applies to the Shel Rosh (head Tefilin). R. Ami discusses the Shel Yad (Tefilin on the arm).

Question: What is the difference between them?

Regarding the Kesones (tunic), it says "Yilbash Al Besaro." Also regarding the Mitznefes, it says "v'Samta ha'Mitznefes Al Rosho"!

Answer (Beraisa): The Kohen Gadol's hair was seen between the Tzitz and the Mitznefes. There he wore Tefilin. (The Shel Rosh is not a place of Bigdei Kehunah, but the Shel Yad is where the Kesones is worn.)

24a (Mishnah): If a Kohen was standing on top of an animal or another person's feet, his Avodah is Pasul.

(Tana d'Vei R. Yishmael): The floor of the Azarah is Kodesh, and Klei Shares are Kodesh. Just like a Chatzitzah between the Kohen and the Klei Shares disqualifies, also between him and the floor.

Had the Mishnah taught only standing on animals, one might have thought that this is Chotzetz, for it is not the same Min (species) as people, but standing on a person is not Chotzetz.

Menachos 37b (Beraisa - R. Eliezer): "V'Hayah Lecha l'Os" teaches that Tefilin is a sign to you (the wearer), not to others (for it is covered by clothing).

Ameimar had a wound. His shirt was torn (to avoid aggravating the wound). His Tefilin could be seen through it.

Rav Ashi: Don't you agree that "v'Hayah Lecha l'Os" teaches that it must be visible to you, but not to others?!

Ameimar: No. The verse teaches that Tefilin Shel Yad (of the hand) is worn in a place on the arm normally covered (not the palm).

Rosh (Hilchos Tefilin Sof Siman 18): Zevachim 19a says that the Kohen Gadol's hair was seen between the Tzitz and the Mitznefes, and there he wore Tefilin. This shows that one may not wear them on the Mitznefes.

Rosh (Teshuvah 3:4): "L'Os Al Yadecha" teaches that nothing may interrupt between Tefilin and his skin. This is like we expound about Bigdei Kehunah "Yilbash Al Besaro." L'Totafos Bein Einecha" teaches that nothing may interrupt between Shel Rosh and his (forehead) between his eyes.

Rashi (19a DH Yatza): The Gemara asked about hair of the head that extends to the Kesones.

Rashba (827): Presumably, one may wind the strap on one's garment. We say that nothing may interrupt between Tefilin and his skin regarding the Batim, which have the law of Tefilin. Rav Ashi asked Ameimar why his Bayis (of the hand Tefilin) was visible. The strap is wound on the finger. Rav Ashi would not expect that it should be (totally) covered! I say so l'Halachah, but in practice, 'when the Halachah is shaky (unclear), follow what people do.'

Shulchan Aruch (OC 27:4): Nothing may interrupt between his skin and Tefilin, both of the head and of the arm.

Magen Avraham (4): Zevachim 19a asked whether a live louse or hair is a Chatzitzah for Bigdei Kehunah. It seems that this is why the Roke'ach says that one should shave the hair on the neck, lest it be a Chatzitzah. The Agur (55) says that this is not the custom. One must be careful about a dead louse or dirt, for this was not a question (surely it is a Chatzitzah). The Shlah says that one should wash the place of Tefilin.

Machatzis ha'Shekel (DH v'Lo): Perhaps the custom is not to shave the hair on the neck because the Gemara did not resolve whether hair is a problem, therefore, we rely on the Rashba who says that Chatzitzah does not apply to the head, even though he did not say to rely on this in practice.

Eshel Avraham (4): The Magen Avraham is difficult. Rashi explained that we asked if hair is a Chatzitzah when it is not in its place. If it is in its place between the eyes or on the back of the neck, it is not a Chatzitzah!

Machatzis ha'Shekel: Rashi explains that the Gemara asked about hair of the head that extends to the Kesones. The Gemara did not resolve the question. The Rambam is stringent. Perhaps one may be lenient about short hairs on the face, for this is normal. This is why the Gemara did not ask about hair of the underarm. Also, the Kohen Gadol used to cut his hair once a week, and he did not totally shave it, and it was not a Chatzitzah between his head and the Tefilin or Mitznefes (his turban). However, on the back of the neck the hairs are long and extend past their place (from where they sprout). There, they are a Chatzitzah. I vehemently disapprove of those who grow long hair. Aside from haughtiness (see YD 178), it is a Chatzitzah regarding Tefilin. Since they grow it long, we cannot say that it is (a normal) part of the person. The Rashba says that Chatzitzah does not apply to the head, but in such a case perhaps he would agree that it does. He forbids putting Tefilin on a thick cap, for it must be where a baby's head is soft. Long hair is like a thick cap.

Mishnah Berurah (15): Even without the problem of Chatzitzah, since the hairs are long, one cannot make the Tefilin taut and in the proper place.

Mishnah Berurah (14): Sometimes the strap interrupts between the Tefilin and his skin. This is Min b'Mino (the same kind), which is not a Chatzitzah. The Levushei Serad holds that l'Chatchilah one should be stringent about this.

Kaf ha'Chayim (19): The Ari Zal says that the strap may not interrupt between the Tefilin (Bayis) and his skin, for it is a different level from the Tefilin.

Rema: This refers only to the Tefilin (Batim). We are not particular about the straps.

Taz (4): The Rashba concluded 'when the Halachah is shaky, follow what people do.' Why is the Rema lenient? The Levush says that the custom is to be stringent about the part of the strap needed to tie the Tefilin on the arm and head. This is proper.

Magen Avraham (5): The Rashba concluded 'follow what people do' regarding wrapping the straps over a garment. However, one need not be concerned for a small Chatzitzah between his skin and the straps. Perhaps this comment of the Rema belongs in Sa'if 5, which permits one prone to get sick to put Tefilin on a thin cap.

Mishnah Berurah (16): Acharonim wrote that one may be lenient only in the windings, but one must be stringent even about the straps where they tie the Tefilin to him, both regarding the head and hand. This is an open rebuke to those who put Tefilin over a toupee, even if only the strap is on it. However, the Magen Avraham and Chayei Adam connote that if one has a wound on his head, one may not put the Bayis on a cloth over the wound, but the straps may be on the cloth, or on a thin cap. He may bless, since the Chatzitzah is not in the place of the Bayis. The same applies to a wound on the arm, even in the place (on the circumference of the arm, even with the Tefilin) where the strap ties the Tefilin on the arm. However, he should be careful to cover it above to fulfill "Lecha l'Os Al Yadecha", and not a sign for others.

Kaf ha'Chayim (20): Mishbetzos Zahav (4) says that one should be careful about the windings, but he need not be careful about what hangs from the Shel Rosh on his garment. Yad Eliyahu (3) says that surely one should not have a Chatzitzah without need even regarding the straps, but if there is a reason one should not be concerned about the straps.

Teshuvos v'Hanhagos (2:26): It is good (but not a real obligation) to remove one's watch from the arm on which he wears Tefilin.