INTERJECTION.
Interjections, so called because they are thrown in between the parts of a sentence without making any other alteration in it, are a kind of Natural Sounds to express the affection of the Speaker.
The different Passions have for the most part different Interjections to express them.
The Interjection O placed before a Substantive expresses more strongly an address made to that person or thing; as it marks in Latin what is called the Vocative Case.