Kabbalah zu Bereschit 49:17

And there is a dog, and a snake and a braying donkey which are ridden by a person. And immediately when a person recognizes which one that s/he rides, s/he is liable. And about that person it is written "and its rider will fall backwards" (Genesis 39:17). And the secret of the word/expression is "when the fallen person falls from it" (Deut. 22:8). And regarding this Yov said "I did not fall lower than you" (Job 12:3). And a tzadik will ride on it - he will tie it with the strap of the tefilin - the letter of the tefilin is the letter yud of Shaday [symbolized/connected to] the [top] vertebrae of the back of neck; shin of the tefilin [symbolized/connected to] chain of vertebrae of the back of the neck.