Nostromo, of course, was in the lighter along with Hirsch, and would make just as tempting a target for Sotillo's tortures. Since Dr Monygham has just expressed his absolute callousness about throwing lives away for the sake of decoying Sotillo, Nostromo would perceive himself to be in some danger here. At any rate, he has clearly resolved not to mention the location of the silver, which translates symbolically into him holding tight to his endangered integrity.