Curious use of the definite article with body parts in dictionary definitions

Question

I've come across the following definition of plectrum in the Oxford Dictionary of English, which can be found at https://en.oxforddictionaries.com/definition/plectrum:

A thin flat piece of plastic, tortoiseshell, or other slightly flexible material held by or worn on the fingers and used to pluck the strings of a musical instrument such as a guitar.

This isn't an isolated use of the definite article with parts of the human body in this context. Other examples include definitions of shoe,

A covering for the foot, typically made of leather, having a sturdy sole and not reaching above the ankle.

thimble, hat, and so on.

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What's the reason behind the use of the definite article in worn on the fingers, covering for the foot, and so on?

From what I know, the definite article is used when the reader is expected to know and recognize its referent – but I don't know what or whose fingers the definition makes reference to; the fingers don't seem unique or well-known, and neither they nor their possessor (the person or very adroit animal) is mentioned or implied – something that would otherwise help infer their existence. The same goes for the foot, etc.

Body parts are an example of something usually inalienable (courtesy of snailplane), the fact which might be of help when answering this question.

Answer 1

As described here, it is idiomatic to use the zero article in some possessive phrases which refer to a thing as representative of sub-class within a class:

• this type of guitar
• that make of car
• what sort of plectrum

It is also idiomatic to use the definite article with the singular or plural noun when that noun refers to a thing as representative of an entire class of things.^†

Examples abound:

• The capo d'astro is affixed to the neck of the guitar.
• The computer has changed our world.
• The Dodo is an extinct pigeon-like bird.
• An example of the order Rosales is the rose.
• This pathogen infects the rose tree through the cut surface of the graft.¹
• A shoe is a covering for the foot.
• The unicorn is a mythical beast.
• Multiple plectra plectrums are worn on the fingers.
• "The rose looks fair, but fairer we it deem..." ^‡

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^†_{Note that this is true of all nouns, not just those which refer to parts of the body.}
^‡ _{William Shakespeare, Sonnet LIV}
1_{GEMES-ROSES - Disease and pest control}

Answer 2

The definite article is sometimes used to indicate that the noun refers to a type rather than to a specific person or thing. For example:

The beginning writer faces many frustrations.

Unless there is a character called "the beginning writer" in the context, this is a generalization about the problems people who decide to become writers face. It does not refer to any specific person.

In contrast "A beginning writer faces many frustrations." is probably about a specific writer. We might find this phrase in the summary of the plot of a film.

Sentences such as this will sound odd to the English speaker:

Electric lights, telegraphs, and phonographs are great 19th century inventions.

The electric light in my garage is not an invention. The electric light (the technology) is. Instead we would say:

The electric light, the telegraph, and the phonograph are great 19th century inventions.

Answer 3

The dictionary is combining two definitions into one:

... held by or worn on the fingers

When a single plectrum is used, it is normally held not worn. When multiple plectrums are used, they are "worn on the fingers".

A single plectrum can be worn on a finger.

Several can be worn on the fingers.

Which fingers?

Why, the ones not on the foot but on the hand.

Which hand? The hand that members of the species homo sapiens tend to have two of.