noun

• (anatomy) The joint between two bones.
  "...that round about the commissure of all our joints..."
• (neuroanatomy) A band of nerve tissue connecting the hemispheres of the brain, the two sides of the spinal cord, etc.
• (anatomy) 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."