"Then inch by inch out of the grass rose up the head and spread hood of Nag." Actually, it is an inverted sentence. If we reverse it back, it would be like "the (cobra's) head rose up and hood of Nag spread out of grass inch by inch." (Note: 'spread' is a past tense verb.)
But I am not sure if hood is some part near the head of the co
"spread" is indeed an adjective. The cobra spread its hood out wide, the way that type of snake typically does. The hood was spread. It was a spread hood. The head and spread hood rose up out of the grass.