Introduction
Anyone who has observed birds at close quarters cannot fail to be struck by how frequently they blink. Why do they do this? I started to video birds to see if I could shed light on this curious phenomenon. Then I moved onto other creatures and more questions arose. Why do some creatures blink with their upper lids, others with their lower? Why do some have nictitating membranes? Why do some retract their eyeballs? Rabbits and guinea pigs hardly blink at all. In cats, you may get a glimpse of the 'third' eyelid, or nictitating membrane. Blinking can be so brief it can only be caught on video. It is in birds that blinking really comes into its own.
So many questions! I started to read what had been written on the subject but could find very little. There were tomes on vision and on eyes, but eyelids tended to be relegated to afterthoughts on 'adnexae' (the parts adjoining the organ of main interest). Much of the anatomical work had been done, not in the last century but the one before! This website catalogues the ways in which blinking has diversified since fishes left water behind and established themselves on land.
Blinking is something that we all do, an unconscious action of little apparent significance or importance. Yet blinking is not only essential for the health of our eyes, it played a key role in the emergence of our ancestors from the sea and in their adoption of a terrestrial lifestyle. The cornea and lens focus light on the photoreceptors of the retina. They are transparent and have no blood vessels passing through them, though curiously, light has to pass through layers of cells and blood vessels to reach the photoreceptors in the deepest part of the retina. In fish, oxygen dissolved in water diffuses into the cornea and then across the aqueous fluid in the anterior chamber of the eye, to the lens. Once fishes began to venture onto land, they faced the problem that the surface of the cornea dries out in air. Atmospheric oxygen cannot diffuse into the cornea unless it is wet. And so, anatomical structures - eyelids and secretory glands - evolved which allowed the cornea to remain moist at all times. These structures also assumed other functions like protection of the eyes, which were now more vulnerable to injury as particles tend to travel faster in air than in water. Eyelids, being soft structures, might shut out dust or wind but provide little protection from mechanical pressure or blows. Blinking in many species thus came to involve retraction of the eyeball into the relative safety of the orbit, where the bony skull could take the impact. In other species, prominent eyebrow ridges evolved under which the eyes could shelter.
Blinking involves transient eye closure. Sustained eye closure occurring during sleep poses a problem as the cornea, moist or not, no longer has access to atmospheric oxygen. This problem has been dealt with by providing both the conjunctival surface of the eyelids and nictitating membranes with a rich blood supply from which oxygen can diffuse into the cornea.
Upper eyelids, lower eyelids, nictitating membranes, globe retraction and provision of moisture are the available components of blinks. There are also fast blinks and slow blinks which are discussed in the section entitled 'Tonic and phasic blinks'. The method of blinking varies from species to species. Particular attention is given to birds, of which there are about 10,000 species, as these vary markedly in the way they blink.
Most of the blinks demonstrated are spontaneous - recorded while observing animals without in any way interfering with them. Reflex blinking, in response to the cornea being touched was observed in one bird. In some birds, blinking also occurs during pecking.
Discussion of the anatomy of blinking borrows heavily from the classic monograph of Gordon Lynn Walls, The vertebrate eye and its adaptive radiation (1943).
For about 30 years, I practiced as a clinical neurologist at Westmead, a Sydney teaching hospital. Neurology is a discipline which relies on observation of the clinical features and signs of patients and, when video cameras became available in the early 1980s, I began to video my patients, particularly those with interesting physical signs or those where there might be doubt about the diagnosis. Many of these videos are published online by Oxford University Press as a Manual of Neurological Signs, which I did with my colleague Paddy Grattan-Smith. When I retired in 2008, my interest turned to birds and other animals. I have tried to bring the same discipline I followed in neurological practice, that of careful observation coupled with a review of the literature, to the study of blinking.
John Morris