Midrash sur Les Nombres 31:20

2) And thus is it written in respect to dead-body (tumah, Bamidbar 31:20): "and every work of goats." I might think (to include) ropes and cords (as acquiring tumah). (But) it follows (by reason that they should not be included), viz.: A dead body confers tumah and a sheretz confers tumah. Just as a sheretz confers tumah only on what is reticulated, so a dead body confers tumah only on what is reticulated.

3) But if Scripture was lenient with a sheretz, a lesser (form of tumah, should we, therefore, be lenient with dead-body (tumah), an acute (form)? But (by reason alone a dead body) should confer tumah on ropes and cords. It must, therefore, be written "a garment … skin" (Vayikra 11:33, in respect to sheretz) — "a garment … skin" (Bamidbar 31:20, in respect to dead-body tumah), as an identity (gezeirah shavah) — Just as a sheretz confers tumah only on what is reticulated, so a dead body confers tumah only on what is reticulated; and just as a dead body confers tumah on "every work of goats," so a sheretz confers tumah on every work of goats.

7) R. Akiva says: "And what touches their carcass shall be tamei": ("what") to include a vessel of bone. (Why is a verse needed for this?) Should it not follow a fortiori? viz.: If wood, whose "father" (the tree) is clean, the vessels made of it acquire tumah, then bone, whose "father" (flesh) is tamei, how much more so should the vessels made of it acquire tumah! — No, this may be true of wood, which is instrumental in (the tumah of) a plague-stricken house. Would you say the same for bone, which is not instrumental in (the tumah of) a plague-stricken house? Since it is not instrumental in (the tumah of) a plague-stricken house, then the vessels made of it should not acquire tumah. It is, therefore, written "And what touches their carcass shall be tamei," to include vessels made of bone. R. Yishmael b. R. Yochanan b. Broka says: (This verse is not needed for this inclusion.) It is already written (Bamidbar 31:20) "and all work of goats," to include (as acquiring tumah) everything that is made from goats, even from their horns (i.e., bones!) and from their hooves. Whence do I derive the same for other beasts and animals? From "and all work." If so, why is "goats," (specifically,) written? To exclude (vessels made from the bones of) birds.