“My Heart Why Dost Thou Throb So in My Breast?” is one of Pulter’s brief devotional lyrics, in this case chastising her own heart and soul for their “sighs and sobs”, and comparing them to various creatures—the lark, the lamb, and the phoenix—who offer themselves up willingly for death. The “ail[ments]” and “unrest” of line 2 may be the generic woes that typically afflict the devotional speaker, who endures the earthly life, and who both longs for union with the divine and is unwilling to give up the human body. But the poem may also reflect more specifically on aging, as “thy sorrows with thy years increase” (line 8). The speaker “wouldst have the course of nature turn”, rather than offering herself up to the more sudden sacrifices of the lark, the lamb, and the phoenix. The lark, the lamb, and the phoenix all have associations with the praise of God or with Christ himself, willingly sacrificing themselves to death. Pulter’s use of these images owes much to the emblematic mode of thinking that is evident in the emblem poem series later in the manuscript.
Attempting to persuade the soul of the miraculous glory that would follow death, the concluding five lines rely on a cluster of images and words that recurs in Pulter’s devotional poetry: obliviation, calcination, and refinement. Her emphasis here is on the necessary calcination of the impure flesh, in death, and its “infinite” refinement, to achieve glory beyond. These lines are almost certainly indebted to George Herbert, who evokes a similar devotional concept in the opening stanza of “Easter”. Here, he describes the salvation of the believer made possible through Christ’s sacrifice: “as his death calcined thee to dust, / His life may make thee gold, and much more just” (lines 5-6). Herbert’s devotional lyrics are a clear influence on Pulter’s but in this poem, as is usual in the comparison between them, Pulter’s emphasis is more material and Herbert’s is more metaphorical and explicitly doctrinal.
— Leah Knight and Wendy Wall