The poem uses alliteration throughout. Alliteration helps to intensify the horror of the poem, with the relentlessness of the poem's sounds bombarding the reader much like the men's traumatic memories bombard them in the hospital. By essentially turning up the volume on the poem, alliteration also makes the grotesque images at hand stand out all the more starkly.
Lines 4 and 5, for example, use alliteration of sharp /t/, hissing /s/, and plosive /p/ sounds:
Baring teeth that leer like skulls’ tongues wicked?
Stroke on stroke of pain,—but what slow panic,
These sounds lend the lines a popping, sickly feel (especially when also taking into account the hard /k/ consonance throughout). The sounds here help capture the poem's shocking portrayal of violence and psychological torture. It's almost as though the sounds here are attacking the reader!
In the second stanza, /m/ alliteration occurs throughout the first three lines:
—These are men whose minds the Dead have ravished.
Memory fingers in their hair of murders,
Multitudinous murders they once witnessed.
These lines, not coincidentally, are about all the "murders" that replay in the men's minds. The alliteration itself—all those /m/ sounds—reflects that abundance. In the same stanza (line 14), "lungs" and "loved laughter" alliterate, connecting the men's joy to their delicate physical bodies—to those blood-leaking lungs.
In lines 20 and 21 of the final stanza, /b/ and /s/ sounds once again create an aural barrage on the listener:
Back into their brains, because on their sense
Sunlight seems a bloodsmear; night comes blood-black;
As best it can, the poem tries to bombard the reader with sound in a way that gestures towards the sensory onslaught experienced by the soldiers. The /b/ sound continues in the next line, while three /h/ sounds in line 23 evoke the sound of cackling laughter—"heads," "hilarious," and "hideous" create a kind of "ha-ha-ha." In lines 25 to 27, "plucking" alliterates with "picking," while the /s/ of "scourging" echoes in "snatching" and "smote." Again, the poem creates a mixture of abrasive and threatening sounds to evoke its atmosphere of torment and horror.