By looking through the cornea and eye lens, eye doctors are afforded the only look inside the body that can be attained without making an incision. This glimpse inside the eye offers a good look at the size and condition of blood vessels, which can provide valuable information. For instance, researchers from Duke University were recently able to come to some conclusions about brain function by looking at the small blood vessels at the back of the retina. They were able to do so because retinal blood vessels are similar in size, structure, and function to the blood vessels in the brain. Consequently, a comprehensive eye exam can provide a way of examining brain health.
P.S. A look at the blood vessels in the eye can reveal any damage brought by high blood pressure, which may provide clues about heart health.