The Mishna says:
"Man comprises 248 'אברים':
30 in one foot, 6 in each digit (5 x 6 = 30); 10 in the ankle, ... etc."
According to the Mishna, then, each foot comprises 40 (30 + 10) "אברים".
Obviously "Eivarim" cannot mean BONES, because the number of bones in each foot is 26. (7 in the ankle, and 19 more more distally.) The Mishnah's total count is 14 more than the foot's number of bones!
Below the ankle, the Mishnah says "6 in each digit" (5 x 6 = 30). It cannot mean bones because anatomically the 5 digits comprise 19 bones. The Mishnah's count exceeds this bone count by 11!
Each ankle has 7 tarsal bones; Not 10. The Mishnah's count is 3 more!
Looking into the MUSCLE groups involved also does not match the Mishnah's count of "אברים".
We can begin to flesh out an answer by the definition of אבר
as given by the Bartenura:
צריך שיהיה בהם בשר וגידים ועצמות ובכך הוא נחשב אבר
That is, an "Eiver" must have bone, sinew and skeletal muscle, as a composite, to be called אבר.
Perhaps, then, אבר means a group of these tissues that collectively subserves a function. The foot therefore must have 30 such functions. If so, different functional units (אברים) probably overlap in their anatomical components with other such functional collective units.
Now consider this: In order for a skeletal muscle to work, it has to cross a joint. Which means that two bones are a minimum necessary to achieve function! And of course two muscles and two tendons are also the very minimum required for the joint to move. So notice the accuracy of the Bartenura's use of nouns in the plural!
By function I mean movement; Which implies these movements take place across joints! So in deciding what an אבר is, we ought to concentrate on the joint(s) involved!
Here's what one sports medicine site writes about "Foot and Toe Movements".
Toe movements take place at the joints. These joints are capable of motion in two directions: plantar flexion or dorsiflexion. In addition, the joints permit abduction and adduction of the toes.Thus we have 1) plantar and 2) dorsal flexion, as well as 3) inversion, 4) eversion, 5) adduction and 6) abduction of the toes, in conjunction with the ankle. That's 6 movements - for 5 toes = 6 x 5 = 30, a number we find in the Mishnah. Is this significant? Probably not because I cannot account for 10 movements of the ankle as per the Mishnah. And the idea that "joints" are central also presents a problem because the foot and ankle have 33 joints!
The foot as a whole (excluding the toes) has two movements: inversion and eversion. All the joints of the hindfoot and midfoot contribute to these complex movements that ordinarily are combined with movements at the ankle joint.
More perplexing is the incongruity between "Eivarim" and the anatomy of the knee. The KNEE joint gets a count of FIVE in the Mishnah. Are 5 different movements possible at the knee joint? The following chart suggests YES!
Here's a chart I found on the knee's movements (and muscles involved). Note that there are FIVE - just as the Mishnah states!
Does the rest of the Mishnah appear to coincide with the above "movements hypothesis"? I'd appreciate your input to determine the real English equivalent of an "Eiver"?
One thing for sure: The 248 “Eivarim” relate to the 248 Positive Commandments of the 613 set! It may well be, therefore, the behaviors involved in these positive commandments can clarify the answer for us.