When reflecting on the aftermath of Myrtle's party, Nick mentions a bloated dog biscuit floating around in a bowl of milk on the floor. What does this image symbolize?

This occurs as a bleak image on the aftermath of the party. The idea that the dog biscuit is bloated intimates that the dog too, as did the other party goers, engaged in such excess that it didn't eat the biscuit. It is ultimately a reflection of the excess of the 1920's which was one of the underlying social commentaries Fitzgerald was making on the society he was living in.