Commentary for Oholot 1:8
מָאתַיִם וְאַרְבָּעִים וּשְׁמֹנָה אֵבָרִים בָּאָדָם, שְׁלשִׁים בְּפִסַּת הָרֶגֶל, שִׁשָּׁה בְכָל אֶצְבַּע, עֲשָׂרָה בַקֻּרְסָל, שְׁנַיִם בַּשּׁוֹק, חֲמִשָּׁה בָאַרְכֻּבָּה, אֶחָד בַּיָּרֵךְ, שְׁלשָׁה בַקַּטְלִית, אַחַת עֶשְׂרֵה צְלָעוֹת, שְׁלשִׁים בְּפִסַּת הַיָּד, שִׁשָּׁה בְכָל אֶצְבַּע, שְׁנַיִם בַּקָּנֶה, וּשְׁנַיִם בַּמַּרְפֵּק, אֶחָד בַּזְּרוֹעַ, וְאַרְבָּעָה בַכָּתֵף. מֵאָה וְאֶחָד מִזֶּה וּמֵאָה וְאֶחָד מִזֶּה. וּשְׁמֹנֶה עֶשְׂרֵה חֻלְיוֹת בַּשִּׁדְרָה, תִּשְׁעָה בָרֹאשׁ, שְׁמֹנָה בַצַּוָּאר, שִׁשָּׁה בַמַּפְתֵּחַ שֶׁל לֵב, וַחֲמִשָּׁה בִנְקָבָיו. כָּל אֶחָד וְאֶחָד מְטַמֵּא בְמַגָּע וּבְמַשָּׂא וּבְאֹהֶל. אֵימָתַי, בִּזְמַן שֶׁיֵּשׁ עֲלֵיהֶן בָּשָׂר כָּרָאוּי. אֲבָל אִם אֵין עֲלֵיהֶן בָּשָׂר כָּרָאוּי, מְטַמְּאִין בְּמַגָּע וּבְמַשָּׂא, וְאֵין מְטַמְּאִין בְּאֹהֶל:
There are two hundred and forty eight limbs in the body. Thirty in the foot - six in each toe, ten in the ankle, two in the shin, five in the knee, one in the thigh, three in the hip, thirteen ribs, thirty in the palm - six in each finger, two in the forearm, two in the elbow, one in the upper arm, and four in the shoulder. One hundred and one of this [side of the body], and one hundred and one of that. And eighteen vertebrates in the spinal chord: nine in the head, eight in the neck, six in the openings of the heart, and five around its cavities. Each of these impurifies through touching, carrying, or sharing quarters. When is this true? When the limbs still have an appropriate amount of flesh on them. But if they do not have an appropriate amount of flesh on them, they will impurify through touching and through carrying, but not through sharing quarters.
Bartenura on Mishnah Oholot
English Explanation of Mishnah Oholot
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.