The beginning of the nineteenth perek discusses tumah as it relates to beds and their various attachments and accessories. One attachment is the mizran. The mizran is a strap that is placed tightly around the bed frame to provide support.

The fifth Mishnah discusses a bed that was an av ha'tumah -- a source of tumah -- specifically one that is tameh midras. One example of this would be if a zav sat or lay on that bed. The Mishnah teaches that if the mizran was then added to the bed, the mizran would also be a tameh midras -- also an av ha'tumah -- since it is now part of the bed. If the mizran was subsequently removed, it status changes. Having had been in contact with an av ha'tumah, the mizran is now treated as an object that has come into contact with something that is tameh midras -- a maga midras -- and is therefore a rishon le'tumah.

The Mishnah Achrona questions the necessity of this Mishnah. This is especially puzzling considering that in the previous perek (18:7) we learnt the more novel case, of a bed leg that was tameh midras and how it effects a bed if it was attached to the bed and then detached after. Note that a bed leg is a small part of the bed. Nevertheless, when it is attached it makes the entire bed tameh midras. Consequently, it should be obvious that if one connects the mizran to a bed that is tameh midras, it will also become tameh midras.

The Mishanh Achrona answers that this Mishnah is needed to teach that even if the mizran was attached when the zav sat on the bed, if the mizran was then removed it becomes a maga midras. This understanding is derived from the fact that the next Mishnah (19:6) discusses where the bed became tameh while the mizran was attached. The tumah discussed in that Mishnah is tumah that originated from a sheretz or corpse. In those cases, when the mizran is removed, it continues to share the same level of tumah as the bed. Tumat midras however is not discussed in that Mishnah, implying that the law is different. Why?

The Mishnah Achrona explains that the mizran itself is a belt that is narrower that three tephachim. Consequently it is not, in and of itself, fit for sitting or lying upon. That being the case it is not fit for tumat midras¹. When removed from the bed, the tumat midras would disappear leaving the mizran at the level of maga midras, since it was in contact with the bed.

The Tifferet Yisrael has a different explanation of why the mizran is treated differently. He explains that the mizran had many uses. The use as a bed belt was not one of the primary uses. Consequently, once removed, it would have the same status as other keilim that, whilst one could sit on it, if someone were to sit on it, they would be told to stand up, since the kli is need for some other use. Therefore it is not susceptible to tumat midras.

From a different comment of the Tifferet Yisrael (Boaz 24:4) we can find another difference between the case of the bed leg and mizran. He explains that the reason why in the case of the bed leg that was tameh midras and then attached to the bed, the entire bed becomes tameh midras, is because even though the bed leg is a "minor" part of the bed, it is necessary to complete the bed itself. A tameh addition that is not necessary for the completion or function of the kli would not make the kli share the same tumah status as the attachment. Similarly in this case, if something that was tameh midras was attached to the bed to function like a mizran, it would not make the entire bed tameh midras.

¹ This article is assuming the explanation of the Rambam. The Rash however has a different understanding of a mizran. The Mishnah Achrona explanation of the juxtaposition of the two Mishnayot supports the Rambam's understanding of the mizran.