(Mishnah): If a person sold the head of a large animal, it does not include the legs; if he sold the legs, it does not include the head.

If he sold the lungs, it does not include the liver; if he sold the liver, it does not include the lungs. (Note: Although the literal translation of the word kaneh is trachea, it is translated here by many Meforshim to be lungs.)

However, for a small animal, if he sold the head, it includes the legs; if he sold the legs, it does not include the head. If he sold the lungs, it includes the liver; if he sold the liver, it does not include the lungs.

(Gemara) - Question (R. Yitzchak): (According to the Mishnah, if he sold the head of a small animal, it includes the legs as well etc.) If he sold a half of the head, does it include half the legs? If he sold half the lungs, does it include half the liver?

Concerning the case of selling half of the lungs, let us learn it from the following...

Answer (Tosefta in Chulin): If a person sold an animal's arm or its head or its intestines, without it depending on its weight, he must the gifts (the arm, the cheeks and the stomach) to the Kohen and he cannot reduce the price paid for them as it was not sold by weight and the seller has the upper hand. (Here also, when he sold half of the head, he can refuse to also give half of the legs, as he has the upper hand.)

But if he sold them by weight, the buyer must still give the gifts to the Kohen but the price he pays the seller should be reduced.