Are the eyes "windows to the soul," as the ancient proverb has it? Maybe. But they're also portals through which one can glimpse signs of health problems -- not only eye disorders like cataracts and glaucoma, but also systemic illnesses like diabetes mellitus and cardiovascular disease. Sometimes the signs of these diseases are visible in, on, or around the eyes long before symptoms appear.

"The eyes truly are unique real estate," says Andrew Iwach, MD, associate clinical professor of ophthalmology at the University of California at San Francisco and executive director of the Glaucoma Center of San Francisco. "They're the only place in the body where you can see a bare nerve, a bare artery, and a bare vein without doing any cutting. And the disease processes we see occurring in the eye are probably occurring in the rest of the body."

The list of systemic diseases that can have ocular manifestations is a long one; in addition to diabetes and cardiovascular disease, it includes aneurysms, HIV, cancer, and rare hereditary diseases. The list is one reason eye experts recommend periodic eye exams.

"Everyone should have a comprehensive eye exam by age 40," says Ruth D. Williams, MD, an ophthalmologist and glaucoma specialist in Wheaton, Ill., and, like Iwach, a spokesman for the American Academy of Ophthalmology. "People who have a family history of eye problems should be seen earlier, and anyone who is having eye trouble should see a doctor right away. But people shouldn't wait until they experience symptoms to see a doctor, because many eye problems are silent," meaning they cause no symptoms.

Some ocular manifestations of systemic disease can be seen only by a trained specialist during the course of an eye exam. Others are plain for all to see. Here are 10 of the most common eye signs and what they might be saying about your health:

Things Anyone Can See

• Bloody eye. The eye's transparent outer layer, called the conjunctiva, is nourished by numerous tiny blood vessels. If these burst, blood may pool on the white of the eye (sclera). A subconjunctival hemorrhage, as it is known among doctors, can be caused by a blow to the eye but in most cases has no obvious cause. In rare instances, a subconjunctival hemorrhage can be a sign of severe high blood pressure or a platelet disorder, which can interfere with clotting, Williams tells WebMD.
• Bulging eyes. Though prominent eyes may simply be a family trait, eyes that appear to bulge may be evidence of thyroid disease. Abnormal levels of thyroid hormone cause tissues surrounding the eye to swell, making it appear that the eye is bulging.
• Droopy eyelid. This condition, known among doctors as ptosis, can be simply a sign of aging. But in rare cases it is evidence of a brain tumor or a neuromuscular disease known as myasthenia gravis (MG), Iwach says. MG is an autoimmune disorder that weakens muscles throughout the body.
• Pupil abnormalities. The pupils of healthy people are usually (but not always) symmetrical. They're usually of the same size, and they usually show the same reaction upon exposure to light. If one pupil is bigger than the other, or if one pupil shrinks less, or more slowly, on exposure to light, there could be an underlying medical problem. Possibilities include stroke, brain, or optic nerve tumor, brain aneurysm, syphilis, and multiple sclerosis (MS). Finally, many medications -- including illicit drugs -- can cause the pupils to appear unusually small or large, says Peter Kastl, MD, PhD, professor of ophthalmology, Tulane University School of Medicine.
• Rings on the cornea. A rare hereditary disorder known as Wilson's disease can cause copper to accumulate in various tissues, including those in the brain and liver. Copper deposits sometimes form on the inner surface of the cornea (though they appear to a casual observer to be on the iris, the colored disk that surrounds the pupil). These "Kayser-Fleischer rings" are themselves harmless. But without appropriate treatment, Wilson's disease can be fatal.
• Thickened eyelid. In very rare cases, a thickening or deformation of the eyelid is a sign of neurofibromatosis, a rare hereditary disorder marked by the growth of tumors along nerve fibers (the tumors themselves are called plexiform neurofibromas). Joseph Merrick, the 19th Century Englishman known as the Elephant Man, was long thought to have had neurofibromatosis. But experts now believe that he suffered from another rare condition known as Proteus Syndrome.
• Yellow eyes. Diseases of the liver, including hepatitis and cirrhosis, can turn the scleras yellow. The color is caused by the buildup of bilirubin, a compound created by the breakdown of hemoglobin, the oxygen-carrying molecule inside red blood cells. The medical term for yellow eyes is scleral icterus -- even though it's not actually the scleras that turn yellow, but the conjunctiva.

Things an Eye Doctor Can See

• Arterial plaques. Atherosclerosis is the disease process that causes cholesterol plaques to form in arteries, including the carotid arteries in the neck and the coronary arteries. Bits of cholesterol (usually from the carotids) can break away from these plaques and travel via the bloodstream to the eye, where they lodge in small arteries in the retina, the delicate network of blood vessels and nerve cells at the back of the eye. Kastl explains that these minute yellowish blockages, known as Hollenhorst plaques, can be evidence of severe atherosclerosis. Ultrasound testing of the carotid arteries may be needed to pinpoint the plaque or plaques.
• Optic nerve abnormalities. The optic nerve, which transmits visual information from the retina to the brain, is visible at the rear of the eye. It's supposed to be pink. A pale optic nerve can be an early manifestation of MS, as well as evidence of a brain tumor or aneurysm, Williams says.
• Retinal defects. Various medical conditions, notably diabetes and high blood pressure, can damage the blood vessels and nerves in the retina (as well as elsewhere in the body). This retinal damage -- which can cause blindness -- can take several forms, including tiny hemorrhages, leaks of yellowish fluid, and puffy-looking whitish patches known as cotton wool spots, Iwach says. White patches on the retina can also be a manifestation of cytomegalovirus infection -- possibly a sign of AIDS. In some cases, the retina is overgrown with tangled and highly fragile blood vessels -- the result of a process known as neovascularization.

What if you notice one of these signs in your own eyes? Alert your doctor. "We look at the physical findings and then listen to the patient's story line and family history," Iwach says. "Then we decide what testing is necessary." He says the last thing you want to do is ignore one of these signs -- especially one that has shown up recently.