פירוש על אהלות 1:8

פיסת הרגל (the foot to the ankle) – the sole of the foot is the foot.

There are two hundred and forty-eight limbs in a human body: Thirty in the foot, six for every toe, Ten in the ankle, Two in the shin, Five in the knee, One in the thigh, Three in the hip, Eleven ribs, Thirty in the hand, [that is] six to every finger, Two in the fore-arm, Two in the elbow, One in the upper arm and Four in the shoulder, [For a total of] one hundred and one on the one side [of the body] and one hundred and one on the other. Eighteen vertebrae in the spine, Nine in the head, Eight in the neck, Six in the key of the heart, And five around the genitals.
Each one [of these] can defile by contact, carriage or overshadowing.
When is this so? When they have upon them the appropriate amount of flesh, But if they do not have the appropriate amount flesh upon them, they can defile by contact and carriage but cannot defile by overshadowing.
Today's mishnah defines "limbs." Obviously, the modern use of the word limb is not the same as the rabbinic use of the term. The word "limb" mostly seems to refer to bones, as we shall see.
Section one: The count of 248 likely refers to bones and perhaps a few sinews and ligaments as well, and not what we would call limbs. It does not include the teeth. When I googled how many bones are in the human body, most answers seemed to be 206. The rabbinic answer seems to include some parts that modern anatomists do not consider bones. In any case, the count is not that far off.
Section two: Each of these "limbs" can cause defilement in any of the three ways that a dead body causes defilement: by contact, by being carried (even without contact) and by overshadowing (ohel). However, for it to defile as a "limb" it must have enough flesh such that if it was attached to the body it could heal.
If it does not have this amount of flesh, it still defiles by contact and carriage, but it doesn't defile through overhanging. We shall see more concerning this is 2:3.

ששה בכל אצבע -for five fingers, which are thirty.

קרסול (ankle) – It is the Aramaic translation of כרעיים/leg, (see Leviticus 11:21): “[But these you may eat among the all the winged swarming things that walk on all fours:] all that have, above their feet, jointed legs [to leap with on the ground,” קרסולי. It is the place of the connection of the foot and the foreleg.

קטלת ([club-shaped], the thigh with the knee-pan)- above the top of the knee.

בקנה (forearm) – he bone that is connection to the hand. From the language (Ezekiel 40:5): “The rod that the man held [was six cubits long, plus one handbreadth for each cubit],” that with it they measure the cubit.

מרפק (elbow) – KODO in the foreign language.

מאה ואחד מזה – that all that he counted until now was from one side from the body, such as for example, one hand and one leg and one rib.

מפתח של לב (valve of the heart) -this is the chest. That because of the movement of the chest, the lung causes to blow on the heart, it is found that the chest opens the path that from it the air enters in and goes out to the heart.

בנקוביו (the perforations/organs of the extremities) – the testicles and the membrum virile.

בשר כראוי (when there is an appropriate amount of flesh) – in order that it will rise up in length and be healthy (see Tractate Kelim, Chapter 1, Mishnah 5), if it is connected to a living person.

ואין מטמאין באוהל – for defilement from a tent is not other either with a complete person, as it is written (Numbers 19:14): “When a person dies in a tent,” or with a limb that is similar to a person, as it states (Numbers 19:16): “or human bone,” just as a human being that has flesh and sews and bones and defiles in a tent, even a limb that has flesh and sinews and bones defiles in a tent.