The clouds grow in the chest, and the returning birds enter the canthus. These two sentences are about looking closely. Seeing endless clouds in the mountains, my heart was filled with excitement; my eyes felt like they were about to burst from staring at them for a long time.
Therefore, "dang chest" means that the mind is rippling. The poet stared at the mountains and rivers, saw the wind and clouds surging, his mind was agitated, and he entered a state of selflessness. His thoughts in his heart were like the rising clouds between the mountains and rivers.
And "in" means "to see", that is, the returning bird has entered the field of vision. Some of our discussions understood that the poet's eyes were so wide open to see the traces of the returning birds; some understood that the long gaze made the poet's eye sockets seem to burst, and at this time the returning birds appeared in his field of vision. The figures of birds (birds return to their nests at sunset, which means they spend a very long time watching).
(The translation takes the latter view)