The imagery in this poem by Dickinson is used to describe the process of moving from great suffering towards death and the final moments of a human life. Dickinson makes great use of a range of stylistic devices in order to capture this final stage of life. For example, she uses a simile, describing the nerves as if they were "tombs" in their ceremony and as they and the rest of the body prepares for death. The precise choice of the word "tombs" adds to the sense of formality as death approaches. Tombs are by their nature a more formal form of a grave, as they are often highly elaborate and decorative burial places.
The imagery in the second stanza serves to describe the last moments of life as if they lack any spontaneity or emotion. The way that the feet are described as being "mechanical" presents an image of the body merely going through the motions of life, without purpose or aim, as it slowly winds down. This image of life slowly ending is also captured in the image of a "quartz contentment" an the "hour of lead" that is an excellent metaphor to describe the slow process of hardening as the body lets go and embraces death, captured perfectly in the final stanza:
This is the hour of lead
Remembered if outlived,
As freezing persons recollect the snow--
First chill, then stupor, then the letting go.
The three stage process of the final line helps explore the body's reaction to death and dying, with the feeling of cold, then numb insensitivity before finally the body relinquishes its hold on life and succumbs to death.