According to chapter 9 in fast food nation, how does manure get into the meat at slaughterhouses and hamburger grinders?

Since they pull out the cow's innards by hand, the stuff in it's digestive tract can spill out and get on the meat.

"The pathogens from infected cattle are spread not only in feedlots, but also at slaughterhouses and hamburger grinders. The slaughterhouse tasks most likely to contaminate meat are the removal of an animal's hide and the removal of its digestive system. The hides are now pulled off by machine; if a hide has been inadequately cleaned, chunks of dirt and manure may fall from it onto the meat. Stomachs and intestines are still pulled out of cattle by hand; if the job is not performed carefully, the contents of the digestive system may spill everywhere." From improperly cleaned animal hides, and improperly digested foods contaminated with harmful bacteria.