PSAT Writing and Language Practice Question 243

Question: 243

(34) The tent was hot and dry, while the night was silent. Sleeping on the dirt floor and (35) knewing that the stars and moon above provided me a surreal sense of being out in the desert. Even with another camper fast-asleep inside the limited space of our (36) flimsy fortress, I felt a complete sense of peace. It wasn't until I heard a huff to my left that I began to stir.
The strange occurrence about the noise was that it had a faint snarl to it. (37) On my usual camping trips, the crickets of the night and rustle of tree leaves usually (38) alleviates the panic mode of strange sounds. Being out in the middle of a foreign desert, I was not afforded the comforting sounds of night bugs. So when I (39) lied still and (40) herd another huff, I tapped my fellow camper. By the time his eyes opened, or at least I could only assume that (41) they had, the huff had gone.
I whispered to him, "Something is outside, but I don't know what it is."
(42) Before he had time to respond, the huff reappeared, but on his side of the tent. His shift to my side of the floor showed me that there was a massive thing (43) kneading the polyester wall which bordered the tree stump anchoring our rooftop.
(44) Lion he whispered back.

What function does this sentence serve as the opening sentence?