The Gradual for Monday in Holy Week: Exsurge Domine

I found this video while looking for something else; after some searching, I learned that Exsurge Domine is the Gradual for Monday in Holy Week:



Here's a page from the St. Gall codex that contains this chant:

I found the chant text and a link to the codex above at the Académie de Chant grégorien.  The text comes from Psalm 35:23:

Exsurge Domine,
et intende iudicium meum, Deus meus,
et Dominus meus, in causam meam.

Awake, and rise to my defense!
Contend for me, my God and my Lord.