This passage gives a metaphor, a hungry wolf prowling outside a sheepfold, wanting to get to the sheep safe within. This is a metaphor for an attacking army, prowling outside the walls of a defended city, trying to find a way to penetrate its defenses.
ac veluti pleno lupus insidiatus ovili
cum fremit ad caulas ventos perpessus et imbris
nocte super media; tuti sub matribus agni
balatum exercent, ille [the wolf] asper et improbus ira
just like a wolf lurking in ambush outside a sheep fold
when he howls at the rails enduring wind and rains
in the middle of the night; the lambs safe under the mothers
sound their bleatings; savage and greedy he [the wolf]
[ so far I understand; the trouble starts here;]
saevit in absentis; collecta fatigat edendi
ex longo rabies et siccae sanguine fauces.
he rages against the lack of food for so long
the compacted madness wearies [him] and so do [ his ] jaws
dry from want of blood. [missing the blood he craves]
What I need is a grammatical commentary on the two lines where the trouble starts, with word by word parsing.