Inside of a Dog_ What Dogs See, Smell, and Know - Alexandra Horowitz [52]
Closer still, and we begin to see serious species differences. Dogs manage to gather more light than we do. Once light enters the eye of a dog, it travels through the gel-like mass that holds nerve cells to the retina (we'll get to that in a moment), then through the retina to a triangle of tissue, which reflects it back. This tapetum lucidum, in Latin "carpet of light," accounts for all the photographs you have of your dog with brilliant light shining out where their eyes should be. Light entering the dog's eye hits the retina at least twice, resulting not in the redoubling of the image but in a redoubling of the light that makes images visible. This is part of the system enabling dogs to have such improved night and low-light vision. While we might make out a match being brightly struck in the distance on a dark night, the dog could detect the gentle flame on the lit candle. Arctic wolves spend a full half of the year living in utter darkness; if there is a flame on the horizon, they have the eyes to spot it.
EYES OF THE BALL-HOLDER
It is inside the eye—at that retina receiving light twice—that, one by one, characteristic habits of dogs can be traced to their anatomy. The retina, a sheet of cells on the back of the eyeball, translates light energy into electrical signals to our brains, which leads to our feeling that we've seen something. Much of what we see is given meaning only by our brains, of course—the retina just registers the light—but without the retina, we would experience only darkness. Even slight changes in the conformation of the retina can radically change vision.
There are two slight changes in the canine retinae: in the distribution of photoreceptor cells and in the speed with which they operate. The former leads to their ability to chase prey, retrieve a tossed tennis ball, to their indifference to most colors, and to their inability to see something right in front of their noses. The latter leads to their being uninterested in daytime soap operas left on for them when their owners are out of the house. We'll look at these in turn.
Go get the ball!
Some of the most important things for humans to see are any other humans situated within a few feet of their face. Our eyes face forward, and our retinae have foveae: central areas with an extra abundance of photoreceptors. Having so many cells in the center of the retina means that we are very good at seeing things right in front of us in high detail, great focus, and strong color. Perfect for identifying that blob of color and form coming at you as your boyfriend or your mortal enemy.
Only primates have foveae. By contrast, dogs have what is called an area centralis: a broad central region with fewer receptors than a fovea, but more than in peripheral parts of the eye. Things directly in front of a dog's face are visible to him, but they are not quite as sharply in focus as they would be for us. The lens of the eye, which adjusts its curvature to focus light onto the retina, doesn't accommodate to nearby sources of light. In fact, dogs might overlook small things right in front of their nose (within ten to fifteen inches), because they have fewer retinal cells committed to receiving light from that part of the visual world. You need no longer puzzle at your dog's inability to find the toy that he is nearly stepping on: he's not got the vision to take note of it until he takes a step back.
Breeds of dogs differ so much in their retinae that they see the world differently. The area centralis is most pronounced in those breeds with short noses. Pugs, for instance, have very strong areas centralis—almost fovea-like. But they lack a "visual streak," which dogs with long noses (and wolves) have. In Afghans and retrievers, for instance, the area centralis is less pronounced, and the photoreceptors of the retina are more dense