What is this dash-like punctuation mark?


Here is a piece of text from 「ノルウェイの森」by Haruki Murakami:

>
> この寮の唯一の問題点はその根本的なうさん臭さにあった。寮はあるきわめて右翼的な人物を中心とする正体不明の財団法人によって運営されており、その運営方針は――もちろん僕の目から見ればということだが――かなり奇妙に歪んだものだった。

How are these things before `もちろん` and `かなり` called and what function they
serve?  
Is it a mark equal to `、` or something informal like `~~~` and expressing a
short speech pause in a middle of the sentence? In any case, what is its name?




It is called a "dash", ダッシュ in Japanese, according to this
[wikipage](http://ja.wikipedia.org/wiki/%E3%83%80%E3%83%83%E3%82%B7%E3%83%A5_%28%E8%A8%98%E5%8F%B7%29).

Several usages of the symbol are listed in the page. The usage here is to
insert an extra explanation to the sentence (or word before the first dash).
You can see that if you take off everything between the two dashes, the
sentence is perfectly correct and meaningful. The portion between the two
dashes is only to add an extra explanation that the statement is purely the
writer's own opinion. You can treat it as a pair as bracket "()" here.




In English typography, there is a practice to substitute two short dashes in
order to simulate a long one -- like this -- with or without surrounding
spaces, when the stock character is not available in the given typesetting
system.

What you have there looks like an imitation of the practice, using kanji style
dashes. The writer probably thought, "I want to use this neat thing seen in
Western writing, in my native writing".

It looks like a Western influence, akin to the main stream use of roman
numerals, or punctuation like exclamations and question marks.

