The bacteria that usually inhabit the large intestine are prevented from backing up into the ileum by the action of the

The bacteria that usually inhabit the large intestine are prevented from backing up into the ileum by the action of the ileocecal sphincter. 

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.