commissure

noun

1. The joint between two bones.

   “...that round about the commissure of all our joints...”

2. A band of nerve tissue connecting the hemispheres of the brain, the two sides of the spinal cord, etc.
3. The line where the upper and lower lips or eyelids meet.

   “There is a third inner eyelid, highly developed and of beautiful mechanism: this is the nictitating membrane, or "winker" (nictito, I wink), a delicate, elastic, translucent, pearly-white fold of the conjunctiva. While the other lids move vertically and have a horizontal commissure, the winker sweeps horizontally or obliquely across the ball, from the side next the beak to the opposite.”