This is a great article. I have a suggestion. And that is to put an asterisk (*) before any sentence that is wrong. This is commonly down in linguistic texts. For example, if

‘Sto muovendo entrambi i bracci’

is wrong, than write:

* ‘Sto muovendo entrambi i bracci’

That way it is easier to see that the sentence is incorrect, especially for fast readers who may be browsing the article...

Many of the strange forms for body parts are because many of the terms were originally neuter nouns in Latin, which were dropped for the most part from Romance languages. In Latin, the neuter plural always ends in -a-. for example, -bracchium> bracchia. Other Latin neuter body parts are:

-labrum-lip

-genu-knee

-os-bone

Vocabulary for body parts in Latin spans 5 different nouns groups and therefore a variety of possible plural endings. Body parts are common vocabulary, but when spoken by most people the plural forms were confusing and that is why they are all over the place in most Romance languages.

Thank you for the very useful article. What is the reason for the grammar going so awry over body parts?

Nice article indeed, however, I believe it's necessary to correct a statement. When you talk about "piede" and "mano", you say that they are an exception of the rule according which masculine words end in "o". Well, masculine words mostly end in "o", but one other possible ending is "e" and it's not an exception at all. There is a great amount of masculine words with an "e" at the end (il generale, il maiale, il giornale, l'ente, il mare, il sole, il sale... and so on).