Chapter 7, Mishnah 11. This mishnah deals with the question of which parts of the pesach offering are considered fit to be eaten, and which parts are considered bone. The question matters, because there are parts of an animal that begin as cartilage, and as the animal grows older they harden and become bone.
The rule the mishnah establishes:
"Kol hane'echal beshor hagadol ye'achel bigdi harach" - any part that is still fit to be eaten in a full grown ox, after it has matured and its bones have hardened, is fit to be eaten in a tender kid as well. In a young animal there are parts that are presently soft and cartilaginous, but are destined to harden as it matures. Therefore even the tender kid is judged according to its condition at maturity: if in the mature animal that part would be fit to be eaten, it counts as something eaten from the pesach offering; and if not, it is not eaten.
The mishnah goes on to specify: "Vera'shei kenafayim vehaschusim" - the tips of the shoulders, which are apparently of the cartilage type, and likewise the other soft cartilage parts. These are included among the things eaten even in a full grown ox, and are therefore considered fit to be eaten.
It is worth noting: although the mishnah speaks of a "full grown ox," a pesach offering is not brought from an ox at all, but only from lambs or goats in their first year. The ox serves here merely as an example, illustrating what remains cartilage even in a large, mature animal, and from it we learn what may be eaten in the young pesach offering.
The view of the Rambam:
This is how Rashi and the Bartenura explained the mishnah. The Rambam, however, holds that the subject of the mishnah is not the definition of what is fit to be eaten and what is not, but rather the prohibition of breaking a bone of the pesach offering, for it is forbidden to break the bones of the pesach offering.
According to the Rambam, the question before us is what counts as cartilage that may be broken and what counts as bone that may not be broken. The determination is made according to the condition of the mature animal: a part that is presently cartilage in a kid or a lamb, but will eventually harden, counts as bone and may not be broken, even though right now it appears to be cartilage.
The prohibition of breaking a bone:
The mishnah continues with the matter of breaking a bone, and here even according to Rashi's explanation: "Hashover et ha'etzem bapesach hatahor - harei zeh lokeh arba'im" - one who breaks a bone of a pesach offering that is tahor receives lashes, since this is one of the prohibitions of the Torah.
In contrast, the mishnah rules: "Aval hamotir batahor vehashover batamei - eino lokeh et ha'arba'im". There are two cases before us:
One who leaves over from a tahor offering - one who leaves meat of the pesach offering beyond the time in which it may be eaten. Although this is forbidden, and one who leaves it over is commanded to burn the remainder, no lashes are given, for two reasons: first, this is a prohibition that involves no action, a warning unaccompanied by any deed; and second, it is a prohibition that is remedied by a positive command, a warning that can be rectified through a mitzvah of action, the command to burn the remainder.
One who breaks a bone of a tamei offering - one who breaks the bones of a pesach offering that is tamei. Here we learn from the verse that the prohibition of breaking a bone of the pesach offering was stated specifically regarding a pesach offering that is tahor, and not one that is tamei.
In summary: in this mishnah we learned the rule "kol hane'echal beshor hagadol ye'achel bigdi harach," which includes the tips of the shoulders and the cartilage. According to Rashi and the Bartenura the rule concerns the definition of what is fit to be eaten in the pesach offering, while according to the Rambam it concerns the definition of a bone that may not be broken. We also learned that one who breaks a bone of a pesach offering that is tahor receives forty lashes, whereas one who leaves over from a tahor offering and one who breaks a bone of a tamei offering are exempt from lashes.