They’re called “Fistulated” cows.
A section of the cow’s abdomens are removed to expose their stomachs. The mutilation has a four- to six-week recovery period, in which there’s no doubt that the animals are uncomfortable.
Microbes in fistulated cows’ stomachs are studied and sometimes transferred to other animals. Fistulated cows are also put on display at events in which patrons are invited to reach inside a live cow. The federal Animal Welfare Act does not extend to animals used in agricultural experiments, so cows have no legal protection from cruelty.