The innermost layer of the eye is the internal tunic, or retina. The stratified nature of the retina can be seen in the diagram of slide 1 and also in the photographic representation in slide 2. Note the "stacking" arrangement of the layers in both slides. The upper right corner of slide 2 shows a dark, thick line, the pigment layer of the retina, which lies tightly adhered to the choroid. The layer adjacent to the pigmented layer consists of columnar receptors for sensing light: the layer of rods and the cones. The layers below the rods and cones consist of two other principal nerve cell types: the bipolar cells, which transfer the nerve signals to the ganglion cell layer, and ganglion neurons themselves, which summarize all the inputs from bipolar calls and translate them into coded nerve impulses. The ganglion neurons contain axons that leave the eye and comprise the nerve fibers of the optic nerves, optic chiasma, and optic tracts. Not shown in the diagram in slide 1 are the amacrine and horizontal cells which conduct nerve signals laterally in the retina and help sharpen the edges of images that we see.