Answer: The blind spot is the part of our retina that lacks light-sensitive receptor cells.
Region a: The blind spot
The back of our eye is called the retina. The retina is packed with photoreceptor cells that respond to incoming light. These cells do not exist in the blind spot.
After light hits a photoreceptor cell, a chemical signal is sent through a pathway of cells. The signal can be processed all along this route by horizontal cells and amacrine cells. Finally, the retinal ganglion cells are responsible for sending the signal into the brain. These axons form a bundle of nerve fibers (optic nerve) and exit the eyeball through the blind spot.
The blind spot itself is about 15 degrees temporally (to the right of center for the right eyeball, and to the left of center for the left eyeball.) Images in the blind spot may seem to disappear. Alternatively, the brain may complete an empty space automatically by filling it in the brain to complete the image.