The structure indicated is the gray ramus communicans (known collectively as the gray rami communicantes).

The spinal nerves at each level receive this branch, the gray ramus communicans, conveying postganglionic nerve fibres from the sympathetic nervous system. In the same picture, parallel to the gray ramus communicans, the branch which conveys the preganglionic sympathetic fibres into the paravertebral sympathetic ganglia is known as the white ramus communicans.

The preganglionic nerve fibre which passes through the white ramus communicans has three potential outcomes:

1. Synapse in the paravertebral ganglion.
2. Travel up or down a level in the sympathetic chain to synapse with a paravertebral ganglion at a higher or lower level.
3. Travel straight through the paravertebral ganglion without forming a synapse, to form a synapse at a distant ganglion anterior to the vertebral column, in a plexus.

Learn more about the structure of the sympathetic nervous system in this anatomy tutorial.