intro- vs. intra-

I recently found out that “extrovert” is a misspelling and that it’s actually written extravert. That makes sense, because other words use the same prefix, e.g. extraordinary, extradite, etc., but nothing I can think of begins with extro-⁽¹⁾.

However, intro- and intra- seem to be two different prefixes. There is introduce and introspect on the one hand, and intravenous and the more recent neologism intranet on the other hand.

What is the difference between intro- and intra- and what are their origins?

(⁽¹⁾ Even the end-credit section of a movie, the opposite of the intro, isn’t called an extro :) )

The prefix extro- means ‘outwards’. The Oxford English Dictionary has entries for both extrovert and extravert, with no difference in meaning. The only other word in the OED beginning with extro- is ‘extrospective’. Its Latin origin is the preposition extra which means ‘outside’. The ‘o’ appears to have replaced the ‘a’ of ‘extra’ by analogy with the Latin adverb intro.

Intro itself means ‘inwards’, ‘to the inside’, and is used in that sense as a prefix in English verbs and adjectives. Intra is a Latin preposition meaning ‘inside’ and is the origin of the prefix in English adjectives, typically those found in biological contexts.