Like those cows with the portal in their side that they bring around to elementary schools to show kids how chambered stomachs work! (Just now realizing this might not be a universal experience and may have had to do with my elementary school's proximity to UC Davis (Agriculturally focused college).
It is. No glass, they just take the lid off when they want to look inside, and it’s a flexible material . The thing on top of the cow here is the lid. Search Fistulated Cow for more info.