Sometimes when our worldview is wrong, we miss important things. The Darwinian point of view in particular may lead to false assumptions. A doctor, Alan B. Richards, who teaches at the Health Sciences Center at Louisiana State University, writes to us with an example. He describes a part of the eye that many consider to be vestigial, that is, an evolutionary holdover from the past that now supposedly serves no function. He points out that viewing the tissue in question as vestigial can lead to serious mistakes. Because it actually serves a purpose, surgeons who are ignorant of that purpose can inadvertently cause damage.
Writes Dr. Richards:
Here are a couple of observations from the world of medicine regarding the dangers of a purely Darwinist approach.
I have read several sources about the plica semilunaris in the eye, and even among intelligent design advocates, no one seems to explain this misunderstood part of the body.
I am a pediatric ophthalmologist and I teach residents how to perform eye muscle surgery. The plica semilunaris is the curvilinear pinkish tissue in each person’s eye nasally. According to neo-Darwinian advocates, the tissue is a useless holdover from evolution, a vestigial tissue of the nictitating membrane in other mammals. Residents, who are generally a bright bunch, routinely quote this “truth” to me each year. Thus, residents tend to be careless with this tissue unless taught properly.
When performing surgery for esotropia (“crossed eyes”), one must be very careful with the plica semilunaris. The tissue can easily be improperly attached too far temporally, as at the end of surgery the plica looks much like the normal conjunctiva that covers the eye (plica is modified conjunctiva).
I explain to the residents that the plica is needed to allow the eye to move outward or temporally, and sewing the plica in the wrong location can not only result in a dreadful red appearance to the eye, but the eye can be drawn inward.
In the first few years of my practice, I saw an unfortunate Vietnamese gentleman, who had immigrated to the USA during the upheaval in Southeast Asia (“boat people”). He had a benign growth on the nasal portion of his eyes (a pterygium). The operation to remove this lesion is usually straightforward, but whoever performed his surgery neglected the plica and sewed the plica semilunaris too far temporally, resulting in very crossed eyes and double vision. Understandably upset, I had to perform eye muscle surgery (strabismus surgery) to restore his vision to normal.
Since the residents may not consider me an authority on the subject of ocular anatomy, I give this this quote from Dr. Darlene A. Dartt, Associate Professor of Ophthalmology at Harvard Medical School in Duane’s Clinical Ophthalmology, a well respected standard text on all aspects of ophthalmology, the first comprehensive ophthalmology text in the USA. (Thomas Duane, the editor, was chairman of ophthalmology at Wills Eye Hospital in Philadelphia, where I trained, the second-ranked ophthalmology program in the USA. Also, he was a former chairman of the American Medical Association’s ophthalmology section and edited two standard reference works, Clinical Ophthalmology and The Biomedical Foundations of Ophthalmology.)
These words are from Chapter 2, “The Conjunctiva-Structure and Function“:
The plica semilunaris is a crescent-shaped fold of conjunctiva that is situated medially and conjoins the bulbar conjunctiva with the caruncle and lacrimal portion of the eyelids (Fig. 3). It is located in the superior fornix at the junction of extending downward, surrounding the limbus to end in the inferior fornix. It extends 3 to 6 mm laterally from the caruncle. The nictitating membrane present in some animals is the counterpart to the plica, and is a partial or complete third eyelid. Although humans do not have a nictitating membrane, occasionally smooth muscle fibers may be present that are innervated with sympathetic nerves. Goblet cells are present in the plica either singly or in clusters. In humans, the plica functions as the opposite of a fornix; that is, if the conjunctiva were to directly join the eyelids to the globe, the globe and eyelids would both be restricted in movement. The fornix provides for a fold of conjunctiva that may be extended or retracted as the globe moves. Extension occurs because of fibrous slips that connect the fornix to its extra ocular rectus muscle. As the muscle contracts, the globe rotates and the adjacent conjunctiva is retracted. This occurs above, laterally, and below the globe, but not medially, which would not allow the lacrimal puncta to drain the lacrimal lake. On abduction the plica tends to unfold and flatten, whereas on adduction it is drawn posteriorly and is unfolded by the fibrous slips that extend to the plica and caruncle from the medial rectus. While it never completely unfolds, extreme adduction of the plica causes it to form a true fornix. As this occurs, a small movement of the globe occurs as a result of the retraction of the medial canthal tendon. This keeps the lacrimal puncta properly positioned with the lacrimal lake. The puncta now dips into the lacrimal strip to allow continuous drainage despite the position of the globe. In addition, the plica helps to maintain the lacrimal lake in its proper position and location in the puncta.
The bold type and underlining are my emphasis: properly the nictitating membrane is the counterpart or analog to the plica; calling the plica vestigial ignores its function. (How does one prove an organ has no function, without very, very careful study?) While the plica is not as important as some other portions of the eye (e.g., the retina, the optic nerve), the structure still serves an important function.
Thus, a design approach (whether or not one accepts common ancestry, directed theistic evolution (highly recommend “Evolution of Living Organisms” by Pierre-P Grasse), or sudden or progressive creation) results in clearer thinking than the approach that man is a result of random accidents and full of useless organs.
Alan B. Richards, MD
Clinical Assistant Professor
LSU Health Sciences Center
Assuming non-function, or in this case, vestigiality, seems to be the default position of neo-Darwinism. When a biological structure seems useless, or it seems to be broken, and yet it appears to be derived from a similar structure in other organisms, the leap is made to say that the thing under study is evolutionary detritus that has not been completely removed by natural selection.
We have seen this viewpoint before. Junk DNA and pseudogenes, for example, have been considered to be evolutionary leftovers that are non-functional. You can even see the assumption in the names these genetic elements were given. But as time goes on, more and more functions are being discovered for junk DNA and pseudogenes.
The intelligent design intuition, in contrast, is to assume function, that things are there for a purpose, and are not merely some evolutionary holdover. I wonder how many “functionless” things like the plica semilunaris will be found to have function after all, once we begin to look more carefully using a design perspective.
Image: Plica semilunaris, by Henry Vandyke Carter [Public domain], via Wikimedia Commons.