There are some excellent examples of alliteration being used to give emphasis to the poem's themes in the second stanza. Writing about snakes very frequently uses alliteration on the "s" sound to excellent effect: there is something inherently sneaky and sibilant about this consonant which is associated with snakes—naturally, because they emit this sound when hissing. As such, we are better able to imagine the "narrow fellow" parting the grass, as if with a comb, because of Dickinson's frequent use of the "s" sound here. It isn't simply at the beginnings of words—"spotted shaft" certainly creates a sound picture which underscores the idea of snakelike movement, but we can also hear the sound repeated at the end of such words as "divides," and within the word "closes." The hissing sibilant sound pervades this paragraph, and brings alive the image of the slithering snake approaching.

The word "narrow" contains short letters and the word "fellow" contains three tall letters and this connotes the short side-to-side movement of the snake while the long letters describe his straighter movement. The alliterative use of the 's' also resembles the hissing sound the snake makes. The letter 'S' itself resembles a snake. The repetition of "L" and long "O" sounds literally sounds "mellow." The image of the snake is calming but also surprising ("His notice sudden is"); so, the image of the snake makes the speaker relaxed but also a bit uneasy. 

Viewing the poem as figuratively sexual, the snake is clearly a phallic symbol. And as the snake comes through the grass, it "divides," with the letter 'V' in "divides" appearing as a vaginal symbol. So, it is not just the sounds of the words and letters; it is also the appearance of the letters that adds to potential meaning. 

The final two lines have more disjointed sounds. Disjointed sounds are less graceful, less calming than the alliterative phrases. This illustrates the speaker's fear of the snake. (Consider this in the literal or figurative interpretations of the poem.) The phrase "Zero at the bone" does use assonance (repetition of the long "O" sound), but it comes across as coldness. It is an icy feeling of fear which means being chilled (zero degrees) to the bone. The "O" sound is in the word "zero" and the letter "O" is nearly identical to the number "0."