Chullin 47 - Adhesion of the Lung
A lung has five lobes, three on the right and two on the left. If two lobes are found to adhere to each other, there is no point in the examination - the animal is terefah even if the lungs are airtight. Rashi says that the adhesion is the result of a hole in the lungs, from which viscous fluids leaked, causing it to close. It is nevertheless terefah, because such healing is impermanent. Tosafot say that there was no hole as yet, but that the adhesion will cause it to happen. Since a puncture will inevitably occur, the animal is terefah immediately. This is true only for two lobes that are not in proximity of each other, for otherwise this is the way they naturally grow.
Rava says that an extra lobe in the lungs makes an animal a terefah. Rav Acha was sitting in front of Mereimar's house when a butcher brought a case of an extra lobe for examination. Mereimar declared it kosher. Rav Acha said to the butcher, "Go ask Mereimar again." Mereimar said to the butcher, "Tell the one sitting in front of my house that the law is not like Rava."
Art: Claude Oscar Monet - The Blue House At Zaandam