The central idea running throughout chapters 15 to 17 is that Ishmael's PTSD debilitated him psychologically, emotionally, and physically, and as if from the inside out. For example, in chapter 15, Ishmael says that he "felt no pity for anyone" and that "it seemed as if [his] heart was frozen." Both of these quotations suggest that Ishmael's experiences of war were emotionally debilitating. He lost his capacity for empathy and compassion. This is especially evident in the metaphor in the second quotation, in which Ishmael's "frozen" heart indicates an emotional numbness.
In chapter 16, Ishmael describes the severe migraines that he suffered from. He uses the rhetorical device of simile when he says that "it was as if a blacksmith had an anvil in [his] head." The idea of a blacksmith relentlessly hammering an anvil inside Ishmael's head, producing endless and "unbearable sharp sounds," emphasizes how agonizing these migraines must have been. These migraines would have been physically debilitating.
Also in chapter 16, Ishmael says that he would often wake up in the middle of the night, "sweating and throwing punches in the air." He would then "run outside to the middle of the soccer field and rock back and forth, [his] arms wrapped around his legs." This image of Ishmael "rocking back and forth" makes him seem like a young, scared child. Indeed, with his "arms wrapped around his legs," we have an image of Ishmael in a fetal position. The implication is perhaps that Ishmael was infantalized by the war. He was reduced, psychologically and emotionally, to a helpless, frightened child.
In chapter 17, again describing one of his physically debilitating migraines, Ishmael says that the pain would become "so severe that [he] couldn't walk." These migraines were of course the culmination of the extreme stress that Ishmael was subjected to during the war. The migraines physically debilitated Ishmael, taking away his ability to walk, but they also took their toll psychologically. This is evidenced when Ishmael states, "I began to cry out loud." This last quotation is also written as a simple, declarative sentence, emphasizing perhaps the inevitable outcome and the simple truth of Ishmael's psychological debilitation.