BC...In a human, the spinal cord runs through the entire spine and ends in the coccyx, or tailbone...So to answer your question, if a deer's spine is like a human's spine, the cord will run down to within about 3-4" of the tail.

BTW...Whenever you hit a deer in the spine, and the cord is severed or severly damaged, the deer will lose serious function of everything from there back.

As a kid, I shot a small doe with a smokepole. I dang near blew its tail off, and it dropped on the spot. I must have had enough shock to damage the spinal cord bad enough so that it was down...Not a pretty story, but sometimes hunting just isn't pretty.