In its basic structure, the dog's eye is much like a human's but there are a few differences which mean that the dog has a different type and range of vision.
The Three "Coats" of the Eye
The dog's eye is made up of three layers. From front to back, these are the sclera, the uvea and the retina. The sclera incorporates the transparent cornea at the front of the eye. The uvea consists of three parts: choroid, iris and ciliary body. The coroid contains a reflective layer called the tapetum.
The iris is a muscular ring controlled by the nervous system. It moderates the amount of light entering the eye, like the aperture of the camera. The ciliary body, a ring of tissue behind the iris, is the point of attachment for the supporting ligament which holds and moves the lens. It also plays a part in focusing the image on the retina and secretes fluid for nourishing the cornea.
The retina is the light-sensitive inner layer of the eye. It contains light-sensitive cells of two types: rods and cones. Rods are very sensitive and work well in low light levels. They only appreciate black and white. Cones operate under good lighting conditions and can appreciate colors. In a dog's retina, only about 5% of the cells are cones and the remainder are rods, so a dog is probably largely color blind, seeing in black, white and shades of gray.
A dog's eyelids have a number of special features. Under the upper lid is the lacrimal gland which produces tears to keep the cornea moist and prevent it from drying out and becoming inflamed. To avoid tears flowing down the face continually, there is a special drainage system. Both top and bottom lids have a short duct at the inner corner. The two ducts fuse to form a single lacrimal gland for each eye, leading tears to the nasal cavity. Various problems can cause blockage of these ducts and it is important to treat such problems seriously.
Eyelashes are absent from the lower lid of carnivores. If these point the wrong way, they may hurt the eye. The eyelids deformities known as entropion and ectropion are fairly common and affect certain breeds more than others.
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Dogs have a third eyelid on each eye, known as the haw, or nictating membrane. This is mainly hidden under the lower lid and just a small part of the pigmented edge is visible in the corner of the eye close to the nose, although in some breeds it is very prominent. The membrane acts like a windshield wiper for the eye sweeping away foreign bodies. When the eye is suddenly drawn back into its socket or becomes sunken through age or disease, the third eyelid becomes more prominent. If it appears suddenly and remains visible, this may be an indication of disease or slight pain. Watch the dog for other symptoms.
Dogs see better in the dark than man, partly because a dog's retina is dominated by rods which are sensitive to low light, and partly because of the tapetum lucidum which lies underneath the rod cells and reflects "concentrated" light back through them. This is a help to wild dogs who are likely to be hunting in poor light conditions. As a further aid to hunting, dogs are particularly sensitive to seeing movement in the distance. They perceive stationary objects relatively poorly, though, which makes some individuals seem clumsy.
The way a dog's eyes are positioned on its head, combined with good muscles for moving them around, gives a dog a comparatively wide field of view. The variation in the shape of dogs' heads alters the placement of the eyes and modifies the field of vision between the breeds. In general, the brachycephalic (short-nosed) breeds like the Pug and Bulldog, have eyes situated on the front of the head, giving better overlap in the field of vision than longer nosed breeds.
The dolichocephalic (long-nosed) breeds tend to have obliquely placed eyes with only a small overlap, and the poorest stereoscopic vision of all but a narrow field of view directly ahead of them. This may partly account for the ease with which the elegant sighthounds run into ditches or trip over small obstacles when running at full tilt.
In man, fields of vision of the eyes overlap, so we have stereoscopic vision and a good appreciation of depth and distance. Although dogs have a wider field of vision, they're not so good at judging distances.