Comentario citado sobre Números 18:1

מאת זובחי הזבח, “from the animals fit to be eaten;” the priest claims his respective portion from the slaughterer of the animal. אם שור אם שה, “be it an ox or a lamb;” to exclude free-roaming animals from claims by the priests. הזרוע, “the front leg,” of the body, the two upper parts of that leg to the ankle. הלחיים, “the cheekbones,” including the tongue. והקבה, “and the stomach;” some of the interior parts of the animal, i.e. the fourth of the four stomachs. All of these parts are totally devoid of sanctity. This is the reason that these gifts were not listed in Numbers 18,1 where the priests’ shares are listed. In that chapter the Torah discussed matters which had some degree of sanctity, especially animal parts which were part of sacrificial animals. Even though the items listed here were not mentioned specifically in that chapter in Numbers, we can find allusions to them even there. We read in Numbers 18,12: ראשיתם אשר יתנו לה' לך נתתים, “their first which they give to Hashem, to you I have given them.” Sifri Korach 117 understands the word ראשיתם in that verse as applying to the first shearing of the wool of the lambs, whereas the additional words אשר יתנו, “which they are to give,” is interpreted as referring to the parts of the animal mentioned in our verse here; the word לה' is understood as referring to חלה, the part of the dough given to the priests. The significance of just these parts of the animal being given to the priest is as follows: the foreleg represents the hand. The Torah, when describing Pinchas’ deed (Numbers 25,7) which saved the people of Israel, writes ויקח רומח בידו, “he took a spear in his hand.” The hand which performed the deed qualifies for the foreleg of the animal. The cheekbones symbolise the organ used for prayer. Pinchas prayed as we know from Psalms 106,30. Finally, the stomach is the organ which suffered the fatal blow as described in Numbers 25,8. (compare Sifri Korach 117).

ושרתו אתו [BRING THE TRIBE OF LEVI … BEFORE AARON] THAT THEY MINISTER UNTO HIM — And wherein consists their ministering?

ושרתו אותו. Seeing that the ultimate responsibility for anything connected with the Tabernacle was Aaron’s, as we know from Numbers 18,1 אתה ובניך ובית אביך אתך תשאו את עון המקדש ואתה ובניך אתך תשאו את עון כהנתכם, “you and your sons and the house of your father with you will have to carry the sin or guilt of the Temple; and you and your sons with you will have to be responsible for any sin/guilt involving the priesthood you have been appointed to.” As a result of these verses, the service performed by the Levites was in the nature of being assistants to the priests, comprising standing guard over the Tabernacle, protecting it against the entry of people not in a fit state for one reason or another. They had to ensure that the vessels of the Tabernacle remained not only in good condition but were not stolen, etc. The second half of that verse in chapter 18 refers to no non-priests being allowed within the sacred domain reserved for either the priests or even only the High Priest.

ושמרו את משמרתו THAT THEY SHALL KEEP HIS CHARGE — Because the care of the Sanctuary, viz., that no stranger should approach it, was committed to his charge, as it is said, (Numbers 18:1) "Thou and thy sons, and thy father’s house with thee, shall bear the iniquity of the sanctuary”, (which Rashi explains to have reference to the iniquity committed by a “stranger” in approaching the holy things), and these Levites aided them — this therefore, was their ministering [alluded to here].

and their iniquities he would bear He would bear, in the manner of all the righteous, as it is said (Num. 18:1): “You and your sons shall bear the iniquity of the sanctuary.”