If you ever get the chance, examine a cow's eye. It will be just as honey brown and benevolent as you'd expect, but look closer and you'll see that it's pupil forms a long thin, horizontal slit. The grain of the eye, the small drifts and flecks of colour, seem to disappear into this dark line, almost as if it were an opened flaw in a thick liquid. And, of course, the bovine eyes, in situ, are placed at the sides of the head. Both these facts mean that, although a cow or a bull will turn its head inquisitively to face a movement, it doesn't actually see that movement in anything like the way we do. As far as research and observation can tell, any bovine, including the toro de lidia, will perceive anything more than roughly ten feet away rather poorly. The left and right fields of vision do not integrate, and so don't produce the kind of stereoscopic vision that humans or primates have. This leads many theorists to agree that toros have a large blind area or 'anticone of [matador] immunity', directly to the front of their heads. Research seems to show that animals without binocular vision are less stimulated by movements which run towards their temples. This makes a lot of sense. A toro, for example, pushing its head forwards, eyes largely passive, will see most of the world head towards it as a unified movement towards the back of its head - the things which stand out - like the flick of a cape - are the things move out of this mass drift. This may actually begin to explain why toros charge at threats - running forwards tones down the visual input from the background and heightens their perception of the errant target. It would also explain why a man standing still, unless he's directly in the bull's path, can be safe - he'll simply fit in with the general, less stimulating, drift of the landscape. And if you consider that movements going towards the nose of the bull - against the usual flow - are something immensely stimulating, it suddenly makes sense that the pase natural, a cape pass which moves the cloth across the toro's leading eye in a noseward direction, is regarded as the pass upon which all toreo is founded and is the pass which always leads the bull forward for the final sword thrust... The matador's experiential knowledge of how to play the toro, the aficionado's rules and regulations for who should stand where to get what result, are all based on simple anatomy. Or, to put it another way, the acres of poetic theorising and the yards of geometrical and technical theorising by observers of the corrida all come down to the simplicity of two different species responding to each other as best they can, the simplicity of a man trying to keep himself alive by thinking and feeling beyond the speed of his own and the bull's reflexes. The bull is also, of course, colour blind. So the red rag to the bull doesn't have to be red - although the magenta, yellow and red used in the torero's capes all have long wavelengths and are easy for the bull to see.
A.L. Kennedy, On Bullfighting