Why do cats have an inner eyelid, and what does it do?

—S. HUANG BROOKLYN, N.Y.

Veterinary ophthalmologist Paul Miller of the University of Wisconsin–Madison offers this explanation:

The inner eyelid of cats is no biological curiosity—it plays a critical role in maintaining the health of the eye surface. In fact, this third eyelid—so called because it complements the upper and lower outer lids—is so important that among mammals and birds it is the norm. Species lacking one, such as humans and most other primates, are the true oddities in nature.

The third eyelid—more formally termed the palpebra tertia—is anatomically complex. It is a fold of tissue covered by a specialized mucous membrane (the conjunctiva). A dense population of lymphoid follicles populates the side in contact with the surface of the eye and the tear film, a thin layer of liquid. These follicles function as the lymph nodes of the eye, protecting the surface against invasion by microorganisms.

Between the two layers of conjunctiva is a dense T-shapedcartilage plate—the crossbar of which stiffens the free edge of the third eyelid—that is curved to conform to the surface of the cornea (the clear covering on the front of the eye). An accessory lacrimal gland, which produces a substantial portion of the tear film, surrounds the stem of the T.

When the cat is alert, the bulk of the third eyelid is hidden within the eye socket; only a small part is visible in the inner corner of the eye. When the feline is relaxed, asleep or blinking, however, a set of skeletal muscles retracts the eyeball, causing the third eyelid to move across the ocular surface and completely cover the cornea.

In doing so, the third eyelid acts much like a windshield wiper blade, removing debris from the surface and redistributing tears over the cornea. It is also believed to help protect the cornea from injury as cats move through tall grass or capture prey. The presence of an accessory tear gland allows for even greater rinsing of the ocular surface than occurs in primates, and the third eyelid also seems to hold the tear film against the cornea better than the outer eyelids do by themselves.

Although no one knows why humans lack a third eyelid, it is possibly because we do not typically capture prey by biting (as would a cat) or eat by rooting through vegetation (as would a horse). Thus, there may be no survival advantage for us in having this extra measure of protection for the eye.

Why do veins pop out when we are exercising?

Mark A. W. Andrews, professor of physiology at the Lake Erie College of Osteopathic Medicine, replies:

Bulging veins during exercise result not from some worrisome increase in venous blood pressure or volume but from processes pushing veins under the skin toward the surface.

At rest, the heart pumps blood from its left ventricle into the high-pressure arteries, where systolic blood pressure, the highest pressure exerted in these vessels, is normally recorded around 120 mmHg (millimeters of mercury), and diastolic pressure, the lowest such pressure, is recorded around 80 mmHg.

When exercise begins, the heart's rate and strength of contraction increase, quickly pumping blood into the arteries. Systolic blood pressure increases linearly with exercise intensity, rising to nearly 200 mmHg during high-intensity aerobic activity (and to more than 400 mmHg during weight lifting). Diastolic pressure and pressure in the veins, which return blood to the heart, actually tend to decrease during aerobic exercise (rising somewhat during weight lifting).

The rise in arterial blood pressure during exercise forces plasma fluid otherwise resting in the capillaries—the smallest blood vessels, which nourish and remove waste material from active cells—out through the thin vessel walls and into muscles and the compartments surrounding them. This process causes the swelling and hardening of muscles noticed during exercise. As a result of the muscle swelling, veins near the skin are pushed toward the surface, giving the appearance of bulging.