Enjambment carries this first line on into the second, which has three iambs before the caesura pause, a commaa trochee and then another iamb. Specific metaphors: dying animal - the physical body. Visits to such places as Ravenna and Rome in Italy gave him the opportunity to see some of the Byzantine art at first hand, in particular some of the early Christian mosaics with their refined golds and exquisite craftsmanship.
Caesura When a line has punctuation roughly midway and the reader has to pause - for example: Whatever is begotten, born, and dies. The young In one another's arms, birds in the trees, —Those dying generations—at their song, The salmon-falls, the mackerel-crowded seas, Fish, flesh, or fowl, commend all summer long Whatever is begotten, born, and dies.
Traditional symbols of Christianity are recrafted, taking on new significance in this voyage, and the orthodox is dismissed as self-congratulatory and materialistic.
Yeats wrote: I have read somewhere that in the Emperor's palace at Byzantium was a tree made of gold and silver, and artificial birds that sang.
Stanford, Calif. From here, he sails to a civilisation outside time and, confusingly, almost unreachable by sea.
The speaker is clear - he's sick of the flesh and blood existence, he wants to be like a god and live forever. The reader is encouraged to carry on as if there is no line break at all. This is quite an image, a mix of Byzantine art gold mosaic and occult philosophy gyre - a burning away of the dying animal flesh and blood, corruptible in order that the eternal can be reached.
Again a pure iambic pentameter line precedes an eleven syllable line that alters the steady plod of the iambic with an anapaest and a pyrrhic.