פירוש על גיטין 112:15

Arzafta: A hammer.

And picked at his brain for seven years: He did not become a tereifa (one who has a wound or condition that will lead to his death within twelve months) with this. For it entered through his nostril and did not pierce the membrane [of his brain]. Moreover (alternatively), there is a distinction between [what causes the condition of being a tereifa in] people and animals, as I explained in the first chapter of Eruvin (Tosafot on Eruvin 7a, s.v. Kegon).

It was silent: That gnat [was silent] from picking because of the sound of the hammer.

Since it became accustomed, it became accustomed: From when the gnat get used to the sound of the hammer, it recognized it and did not leave its picking.