Roger Bacon says:
Therefore the eye has three tunics, or little garments, and three humors, and one web like a spider's web. And its first tunic comes from the innermost tunic of the nerve, which comes from the pia mater according to all authors. And it expands from the extremity of the nerve where it enters the fissure of the bone, and it branches out like a concave net (rete) into its first part. Therefore it is called the rete or retina, according to Avicenna in his third book on medicine and also according to Constantine, and it has veins, arteries, and fine nerves. Then its second part is coarser, as Avicenna says, and it expands spherically to the front of the eye, having a fissure in the middle of its front part, so that the species of light and color and of other visible things is able to go through the middle of the eye to the nerve that comes from the brain.