Hilchos Shechita 9:3

The spinal cord starts behind the two glands at the start of the neck; it goes until the second division (defined in the next halacha). Nothing remains after this except for the third division, which is near the beginning of the tail.

Hilchos Shechita 9:4

There are three divisions, which are three bones that cling to one another below the vertebrae of the spine. A bird’s spinal cord reaches to in between its wings. We are unconcerned with any cord that reaches beyond these spots, even if the skin is severed or the marrow deteriorates.