The Plica Semilunaris

Stibbe, E. Philip.  “A comparative study of the nictitating membrane of birds and mammals.”  Journal of Anatomy 62(Pt 2):159–176.  1928.  A comparison by E. Philip Stibbe, an anatomist at the University of Liverpool, of the third eyelid in birds, which has its own muscles, with the same membrane in calves, sheep, rabbits and cats, which has no special muscle.  He also describes the human fold catching dust and foreign bodies, and doubts that it is the same structure as a bird's membrane.  The article uses him for the dust-catching role and for the muscle difference between birds and mammals.

1. 1a.

   Pages 169–170:  “If one goes out in a dust storm, one's eyes rapidly fill with dust, and as rapidly get rid of it.  Where does the dust go?  It will be found collected together into a little sticky mass at the inner canthus; and that mass is situated on skin, in the angle internal to the caruncula lachrymalis, in which situation it causes no irritation and does no damage.  There can be no doubt that this is brought about by the intervention of the plica semilunaris, which is not so vestigial a structure as the descriptions of it might imply; it is in fact a very respectable fold, with an underlying conjunctival fornix often a quarter of an inch in depth.” ↩ ↩ ↩ ↩

2. 1b.

   Page 170:  “An eyelash, for example, has been observed in a patient's eye; it was carried inwards by the tears and the action of the orbicularis, but slipped under the plica semilunaris; it was brought out from this position by an outward sweep of the eye abolishing the deep internal fornix; and after several trials it was finally picked up by the plica, and transferred to the skin at the inner canthus.” ↩ ↩

3. 1c.

   Page 170:  “(a) that the orifices of the canaliculi are very minute—about one-tenth of a millimetre in diameter”.  “Now the internal conjunctival fornix is bridged across by the plica semilunaris, which in man has a very thin free concave edge, like the lunula of a watch-pocket valve of the heart.  So the tears and foreign matter are diverted forwards across the caruncular region.” ↩

4. 1d.

   Page 164:  “The nerve supply of the pyramidalis and of the quadratus is from the sixth cranial nerve, which supplies the external rectus, and then turns inwards and backwards under the quadratus to end in the pyramidalis (fig. 2).” ↩ ↩

5. 1e.

   Pages 164–165:  “The writer has made numbers of dissections of the orbits of calves, sheep, rabbits and cats, all of which have a well-marked "nictitating membrane," and has failed to find any specialised apparatus for moving the membrane.  The examination of serial sections across the fold has revealed at the most a few scattered bundles of unstriped muscle fibres.” ↩

6. 1f.

   Page 175:  “In conclusion, it has been assumed that the plica semilunaris of man is the same as the plica of Mammals; cartilage has been found in the plica of negroes, and occasionally in Europeans.  The writer has as yet not found any cartilage in a number of sections of the plica semilunaris.” ↩

7. 1g.

   Page 175:  The paper's own summary of its conclusions.  “(a) That structurally and functionally the plica of man and Mammals differs entirely from the nictitating membrane of Birds.”  “(b) That there is reasonable ground for supposing that the plica of man and Mammals is morphologically different from the nictitating membrane.  But the morphology of the mammalian structures has yet to be worked out by a study of their ontogeny before any definite conclusion can properly be stated.” ↩ ↩ ↩

Wiedersheim, Robert.  The Structure of Man: An Index to His Past History.  Macmillan, London.  Translated by H. Bernard and M. Bernard.  Edited by G. B. Howes.  1895.  A translated textbook of comparative anatomy by Robert Wiedersheim, a professor at the University of Freiburg, that reads the structure of the human body as a record of descent from lower animals.  Its editor, G. B. Howes, suggested the translation for medical students and says the book is not an exhaustive treatise.  The article uses it for Wiedersheim's claims about the size and cartilage of the fold, and for the place of the fold on his list of vestigial organs.

1. 2a.

   Page 148:  “Great interest attaches to the fold of the conjunctiva which lies at the median angle of the eye, and is known as the plica semilunaris (pl., Fig. 91).  This corresponds with the third eyelid, the so-called nictitating membrane, of the lower animals.  In Birds, Anurous Amphibians [some Sharks], and in many Reptiles it is highly developed, and, by means of a special muscular apparatus, can be drawn across the eyeball.  It serves not only to cover, but to keep clean the surface of the eye, the upper lid [which in Man performs that function] being immovable, and the lower slightly movable or but little developed.  In Man, as in the Apes, in association with the absence of a retractor bulbi muscle, this third eyelid has undergone great degeneration, but it may still enclose (more frequently in Negroes than in Caucasians) a cartilaginous support.” ↩ ↩ ↩

2. 2b.

   Pages 148–149:  “Among sixteen pure Negroes this cartilage was found by Giacomini in twelve individuals.” ↩ ↩

3. 2c.

   Page 149:  “The plica semilunaris varies greatly in size at different ages and in different races.  In the new-born child, and during the early years of life, it is broader than later, when it does not exceed 1½ to 2 mm. in breadth.” ↩ ↩ ↩

4. 2d.

   Page 202:  In section B of the conspectus, under this heading:  “Retrogressively modified, the Organs having become wholly or in part functionless, some appearing in the Embryo alone, others present during Life constantly or inconstantly.  For the greater part Organs which may be rightly termed Vestigial”.  The entry for the fold reads:  “Plica semilunaris of the eye.” ↩

Gray, Henry.  Anatomy of the Human Body.  Lea & Febiger, Philadelphia.  20th American edition.  Edited by Warren H. Lewis.  1918.  A human anatomy textbook, in its 20th American edition, edited by Warren H. Lewis.  The article uses it for the standard description of the fold and of the caruncle beside it, and for an older report of smooth muscle in the fold.

1. 3a.

   Page 1027:  “at the medial angle it forms a semilunar fold, the plica semilunaris.” ↩

2. 3b.

   Page 1028:  “The caruncula lacrimalis is a small, reddish, conical-shaped body, situated at the medial palpebral commissure, and filling up the lacus lacrimalis.”  “Lateral to the caruncula is a slight semilunar fold of conjunctiva, the concavity of which is directed toward the cornea; it is called the plica semilunaris.  Müller found smooth muscular fibers in this fold; in some of the domesticated animals it contains a thin plate of cartilage.” ↩ ↩ ↩ ↩

3. 3c.

   Page 1028:  The passage is about the caruncle.  “It consists of a small island of skin containing sebaceous and sudoriferous glands, and is the source of the whitish secretion which constantly collects in this region.” ↩

Arends, G., and U. Schramm.  “The structure of the human semilunar plica at different stages of its development: a morphological and morphometric study.”  Annals of Anatomy 186(3):195–207.  2004.  Two anatomists at the University of Lübeck who sectioned the fold in the eyes of eleven fetuses, two newborns and an old man, to see how it develops.  They found the fold packed with immune cells, blood vessels and mucus-making goblet cells, but with no cartilage, and with muscle cells only in a thin layer around some vessels.  The article uses them for the fold's immune role and for its lack of cartilage.

1. 4a.

   Section Abstract:  “In this study development of the semilunar plica was examined histologycally by making sections through the eyes of eleven foetuses at different stages of gestation, two newborns and an old man.  We found that in the early stages of its development the semilunar fold covered a bigger part of the orbit and later did not keep up with the growth of the eyeball and the lids.”  "histologycally" is spelled that way in the original. ↩ ↩

2. 4b.

   Section Abstract:  “Beside poorly differentiated buds of the surface epithelium which can be classified as rudiments of the nictitating or Harderian gland, serous glands were detected which could be beginnings of Krause's glands.” ↩ ↩

3. 4c.

   Section Abstract:  “In general the palpebral side of the plica consisted of a higher number of epithelial layers and more goblet cells than its bulbar side.” ↩ ↩

4. 4d.

   Section Abstract:  “In the tight collagenous connective tissue no elastic or reticular fibres or cartilaginous structures could be found.”  “Later on, some of them showed an enlarged lumen and were covered by a thin layer of muscle cells.”  “Muscle cells could not be detected.” ↩ ↩

5. 4e.

   Section Abstract:  “A very dense concentration of leukocytes in the plica even in early intrauterine development was very striking and could be observed in this kind for the first time.”  “The discovery of dense infiltration with both specific and non-specific immune cells, abundant vascularization and secretory structures (goblet cells and surface enlargement) in the semilunar plica suggest that it plays an important role as a specialized organ in human eye protection.  Its anatomical position at the medial border of the eye supports this theory.  Its origin from the third eyelid of the mammals could be recognized to a certain extent.” ↩ ↩ ↩ ↩

6. 4f.

   Section Abstract:  “Additionally, a new kind of plica gland was identified in which ductular structures, with an onion skin appearance, could be discriminated from mucous acini.” ↩ ↩

Walls, Gordon Lynn.  The Vertebrate Eye and Its Adaptive Radiation.  Cranbrook Institute of Science, Bloomfield Hills.  Hafner reprint, New York edition.  1942 (edition of 1963).  A book by Gordon Lynn Walls, a researcher in ophthalmology, written for general readers rather than specialists.  It explains how the eyes of backboned animals differ and why, by asking what each animal does with its eyes.  The article uses it for the structure of the third eyelid in birds and in mammals, and for the fold as a leftover of that eyelid.

1. 5a.

   Page 38:  “In the inner canthus can be seen the plica semilunaris, a crescentic fold of conjunctiva which is a vestige of a third, sidewise-sweeping eyelid present in many animals, the nictitating membrane.  Neither the special muscles nor the special gland (Harder's) of the third eyelid are present, even as vestiges, in man.” ↩

2. 5b.

   Page 420:  “A third lid, the vertical 'membrana nictitans', has consequently evolved as a fold of the conjunctiva at the inner or nasal corner of the lid opening (Fig. 142).  Being transparent, it can sweep the cornea from the nasal to the temporal side, to clean and moisten it, without shutting out the light.” ↩

3. 5c.

   Page 420:  “In man, it is the heavy bony rim of the orbit, particularly the ridge bearing the eyebrow, which makes a retractor bulbi unnecessary.” ↩

4. 5d.

   Page 424:  The passage is about birds.  “Its inner surface is always covered by an epithelium whose surface cells are built like unicellular feathers, which improve its cleansing action.” ↩

5. 5e.

   Page 426:  “The nictitans has its ups and downs in the mammals.  Where it is well developed, it usually has a cartilaginous tarsus, but it never has a special musculature behind the globe as in lower vertebrates.  Hence, it slips over the eye only passively when the globe is slightly or markedly retracted.  Contrary to logical expectations, it is most rudimented in the lower mammals and has come back to greatest usefulness in some of the higher ones.  This probably explains the absence of its characteristic muscles, these having been discarded in early mammals to whom the nictitans was unimportant.  It is present in the duck-bill but lacking in the echidnas, and it is vestigial in rodents and others of the lower orders such as the insectivores, primates, and 'edentates'.” ↩ ↩ ↩ ↩ ↩ ↩

6. 5f.

   Page 426:  “All carnivores except the skunk, whose eye protrudes greatly like that of a mouse, have a nictitans.  All can move it, though not all ever do so.  In only a few could it possibly be drawn all the way over the cornea, the 'haw' of the domestic cat being a familiar example of this rare degree of development.” ↩

7. 5g.

   Page 427:  “Ungulates all have the nictitans, though with great interspecific variations, and usually with no apparent usefulness.  In the horse family, however, the nictitans is as extensive and as rapid in action as in many sauropsidans.  Its retention here is attributed to the need for special protection of the eye when feeding in deep grass”. ↩ ↩

Arao, Tatsuyoshi, and Edwin M. Perkins.  “The nictitating membranes of primates.”  The Anatomical Record 162(1):53–69.  Sep 1968.  A survey of the nictitating membrane, the third eyelid, in a sample of 22 non-human primate species, published in an anatomy journal in 1968.  In 21 of the 22 species the membrane was well developed, and only the chimpanzee's was vestigial.  The article uses it to test the claim that the membrane is vestigial across the primates, and to show that the human fold is not unusual among its nearest relatives.

1. 6a.

   Section Abstract:  “Of 22 species of subhuman primates examined, 21 have a well developed nictitating membrane; it is vestigial only in Pan satyrus/troglodytes.  The sizes of membranes range from those that cover only one‐tenth of the eye (Gorilla gorilla) to those that extend over the entire eye (Perodicticus potto and Arctocebus calabarensis).  Nictitans, best developed in the lower primates, have a free border that is generally heavily pigmented.  With the exception of Galago crassicaudatus, all prosimians studied have a membrane that contains a cartilaginous band; cartilage is also present in Cacajao rubicundus, Cebus albifrons, and Saimiri sciureus.” ↩ ↩ ↩ ↩

2. 6b.

   Section Abstract:  “The outer surface of the nictitans of all species of anthropoids has mucous glands; that of the prosimians has few of them.”  “Muscle fibers are present only in the nictitating membranes of Lemur monoz and Callithrix aurita.” ↩ ↩ ↩

Darwin, Charles.  The Descent of Man, and Selection in Relation to Sex.  Volume 1.  John Murray, London.  1871.  Charles Darwin's 1871 book on the descent of humans, here the first edition of its first volume.  The article uses it for Darwin's own words that the fold is "a mere rudiment" of the third eyelid, a claim about shared origin and reduction rather than about use.

1. 7a.

   Page 23:  “The nictitating membrane, or third eyelid, with its accessory muscles and other structures, is especially well developed in birds, and is of much functional importance to them, as it can be rapidly drawn across the whole eye-ball.  It is found in some reptiles and amphibians, and in certain fishes, as in sharks.  It is fairly well developed in the two lower divisions of the mammalian series, namely, in the monotremata and marsupials, and in some few of the higher mammals, as in the walrus.  But in man, the quadrumana, and most other mammals, it exists, as is admitted by all anatomists, as a mere rudiment, called the semilunar fold.” ↩ ↩

Scadding, S. R.  “Do 'vestigial organs' provide evidence for evolution?”  Evolutionary Theory 5:173–176.  May 1981.  A critical paper by a zoologist at the University of Guelph on the argument that vestigial organs, which have no function, are evidence for evolution.  It traces the argument back to Darwin, Haeckel and Wiedersheim, whose list of eighty-six vestigial organs in the human body it says mostly have some function, and it concludes that vestigial organs give no special evidence for evolution.  The article uses it as the 1981 reply to the claim about the fold's function, which Naylor later answered.

1. 8a.

   Page 175:  “The semilunar fold of the eye is simply that portion of the conjunctiva at the medial corner of the eye and as such aids in the cleansing and lubrication of the eye ball (Gray and Goss 1972, p. 1065).  The absence of a nictitating membrane in humans is of no significance to a discussion of the function of the semilunar fold.” ↩ ↩

2. 8b.

   Page 176:  “Since it is not possible to unambiguously identify useless structures, and since the structure of the argument used is not scientifically valid, I conclude that 'vestigial organs' provide no special evidence for the theory of evolution.” ↩

Naylor, Bruce G.  “Vestigial organs are evidence of evolution.”  Evolutionary Theory 6:91–96.  Sep 1982.  A reply to Scadding's 1981 paper by a researcher in vertebrate paleontology at the University of Alberta.  He argues that a vestige need not be useless, since many vestigial organs keep a reduced or changed function, and that the human coccyx is one such organ.  The article uses it as the 1982 reply that counts the fold as evidence for descent.

1. 9a.

   Page 92:  “Regardless of how definitions are altered to suit particular arguments, the fact remains that the coccyx, ear muscles, and semilunar fold of the eye provide powerful suggestive evidence for the evolution of man from "lower" primates.” ↩ ↩

Franks, Robert H.  “Vestigial organs: an idea destructive to medical science and our health.”  Creation 10(2):22–24.  Creation Ministries International.  Mar 1988 (edition of 2014).  A 1988 magazine article arguing that the idea of useless organs is destructive to medical science and to health, and that the plica has essential functions.  It was republished on the ministry's website on 3 December 2014.  The article uses it as an early creationist reply to the useless-organ claim, one that is right on the main point.

1. 10a.

   Section Semi-lunar membrane:  “The plica semilunaris is a little fold of tissue at the inner corner of the eye.  Some evolutionists feel that it is a remnant of the third eyelid of a lower form, such as the third eyelid in birds and reptiles.  But in man this tissue has several essential functions.  If you did not have the tissue for support at that location, the eyeball would sink.  This would cause double vision.  The tissue not only supports the eyeball, but the tearduct as well.  Without this tissue, tears would drain over the cheeks causing a cosmetic problem.  This area also serves to collect foreign matter.” ↩ ↩

2. 10b.

   Section Semi-lunar membrane:  “It is not true that this fold has no purpose.  It was designed.  And it does not represent the cleverly designed third eyelid of the bird which prevents the bird's eyes from drying out during flight.” ↩

Sanders, Lita.  “Badly designed arguments: 'vestigial organs' revisited.”  Creation Ministries International.  Nov 25, 2011.  A reply on a creationist ministry's website to a reader's request to answer another writer's list of supposedly useless human organs, taken one at a time.  The article uses it for the claim that the plica has a critical function for the health of the eye.

1. 11a.

   Section 'Third eyelid' (plica semilunaris):  “The plica semilunaris actually has a critical function for the health of the eye.  It secretes the sticky substance that hardens into the crust we sometimes find around our eyes when we wake up.  This substance surrounds any particles that find their way into the eye, making them less likely to scratch the eye, and making it easier for the tears and eyelids to remove the particles from the eye.” ↩ ↩

Guliuzza, Randy J.  “Evolutionary Vestigial Features: Worse Than Myth, a Scam.”  Acts & Facts 54(5):16–17.  Institute for Creation Research.  2025.  An article in Acts & Facts, published by the Institute for Creation Research, that calls the vestigial-organs argument a scam and sets out the steps it says biologists follow.  The article uses it for that list, which names the plica among the obscure parts.

1. 12a.

   “We can now begin to recognize the key steps of evolutionary biologists' vestigial organs scam.  Identify an obscure body part or action that most people have essentially no knowledge of (e.g., appendix, tailbone, adenoids, thymus and pineal glands, arrector pili muscles causing goosebumps, plica semilunaris or "third eyelid," etc.).”  “Declare that the part is a vestigial remnant of past evolution—since no one knows of a useful function.” ↩ ↩

Mitchell, Elizabeth.  “Vestigial Organ Arguments: Useful or Unnecessary?”  News to Know.  Answers in Genesis.  Oct 22, 2011.  A news commentary from Answers in Genesis that replies to a Fox News list of nine supposedly useless body parts, going through several of them.  It accepts that the plica is thought to be homologous with a third eyelid in some animals, but says it is far from useless.  The article uses it because Answers in Genesis accepts the homology and disputes only the claim of uselessness.

1. 13a.

   “The #1 item—the plica semilunaris (third eyelid)—according to Fox News, is "left over from what's known as a 'nictitating membrane,' which is still present in animals like chickens, lizards, and sharks."  While this fold of protective eye covering is thought to be homologous with the more complete protective covering in some animals, it is far from useless.  Secretions from this "#1 useless body part" mop debris from your eyes.” ↩ ↩

2. 13b.

   “Similarity of parts— homologies— between different creatures demonstrates that our common Designer used well-engineered designs to serve various functions.” ↩

Knop, Nadja, and Erich Knop.  “Conjunctiva-associated lymphoid tissue in the human eye.”  Investigative Ophthalmology and Visual Science 41(6):1270–1279.  2000.  A histology study of 53 complete conjunctival sacs and eight lacrimal sacs from donor eyes, prepared as whole mounts and sections.  It found diffuse lymphoid tissue in every specimen and organized follicles in about three fifths, mainly in the lid lining, and named the system conjunctiva-associated lymphoid tissue.  It shows that the immune cells in the plica are part of the eye surface's working immune system, and also that the fold is not its main site.

1. 14a.

   Section Abstract, Results and Conclusions:  “In all specimens, diffuse lymphoid tissue of lymphocytes and plasma cells, most of which were IgA positive, formed a thin layer in the lamina propria.”  “In approximately three fifths of the conjunctival sacs, organized follicular accumulations were embedded in this layer.”  “Human conjunctiva and lacrimal drainage system show an associated lymphoid tissue (suggesting the term conjunctiva-associated lymphoid tissue [CALT]) that contains all components necessary for a complete immune response.” ↩

2. 14b.

   Section Abstract, Results:  “Lymphoid tissue was mainly observed in the palpebral conjunctiva, more pronounced in the upper than in the lower lid.”  The palpebral conjunctiva is the part lining the inside of the eyelids. ↩

Sisson, Septimus.  The Anatomy of the Domestic Animals.  W. B. Saunders, Philadelphia.  2nd edition.  1914.  A book on the anatomy of domestic animals.  The article uses its description of the horse's third eyelid to show how the mechanism works.

1. 15a.

   Page 859:  The description is of the horse.  “The third eyelid (Palpebra tertia) is situated at the medial angle of the eye.  It consists of a semilunar fold of the conjunctiva, known as the membrana nictitans, which covers and partly encloses a curved plate of hyaline cartilage.”  “Numerous minute lymph nodules occur in the membrana nictitans, and the deep part of the cartilage is surrounded by a gland which resembles the lacrimal gland in structure”.  “Ordinarily the third eyelid extends very little over the medial end of the cornea, but when the eyeball is strongly retracted, the membrana is protruded over it so as to measure about an inch (ca. 2-3 cm.) in its middle.  This effect results from the pressure of the eyeball and its muscles on the fat which surrounds the deep part of the cartilage.” ↩ ↩ ↩

Thompson, J. W.  “The nerve supply to the nictitating membrane of the cat.”  Journal of Anatomy 95(Pt 3):371–385.  Jul 1961.  A laboratory study of the nerves that reach the cat's nictitating membrane, its third eyelid, from the sympathetic nervous system.  Thompson traced them in twelve dissected preparations and confirmed them by stimulating the nerves in five cats prepared as spinal animals.  He found that the medial and inferior smooth muscles each have their own sympathetic nerve, which reaches it along branches of the trigeminal nerve.  Thompson worked at the Institute of Basic Medical Sciences, Royal College of Surgeons, London.  The article uses it for the two muscles that pull the cat's membrane back, the counterpart it asks about in the human fold.

1. 16a.

   Page 371:  “The nictitating membrane of the cat is commonly used to study sympathetic transmission”.  “The membrane contains two thin sheets of smooth muscle.  Acheson (1938) showed that these smooth muscles, which he named the medial and inferior muscles, arise deeply in the orbit from the fascial coverings of the medial and inferior recti muscles, respectively, to be inserted into adjacent sides of a T-shaped piece of cartilage (Text-fig. 1 a, b).”  A figure interrupts the sentence after "medial and inferior muscles,". ↩

2. 16b.

   Page 384:  Reference list.  “PATON, W. D. M. & THOMPSON, J. W. (1953).  The muscles retracting the cat's nictitating membrane.  J. Physiol. 120, 55P.” ↩

Rehorek, Susan J., and Timothy D. Smith.  “The primate Harderian gland: does it really exist?”  Annals of Anatomy 188(4):319–327.  2006.  A microscope study of the orbits of newborn primates from twelve species, lemur and loris groups among them, and tarsiers, marmosets and their relatives.  The Harderian gland, a gland in the front of the eye socket, was present in numerous species at birth, and its size ranged from large to absent, with tarsiers the group that lacks it.  The authors say its role needs further study.  The article uses it to correct the claim that the gland is absent in humans, since it is present in the human fetus and newborn.

1. 17a.

   Section Abstract:  “The Harderian gland, an anterior orbital structure, is either absent or vestigial in primates.  This is based upon gross anatomical observations of scattered adult specimens.  Though largely absent in the adult human, it is present in the fetal and neonatal stages.” ↩ ↩

2. 17b.

   Section Abstract:  “The relative size of the neonatal primate Harderian gland can be subdivided into five separate categories, ranging from large to absent (tarsiers), with no apparent phylogenetic trends.” ↩ ↩

Coyne, Jerry A.  Why Evolution Is True.  Viking, New York.  2009.  A book about evolution by Jerry A. Coyne.  The article cites it only for his definition of a vestige: a trait can be vestigial and functional at the same time.

1. 18a.

   Page 58:  “Evolutionary theory doesn't say that vestigial characteristics have no function.  A trait can be vestigial and functional at the same time.  It is vestigial not because it's functionless, but because it no longer performs the function for which it evolved.” ↩ ↩

Bergman, Jerry.  “Do any vestigial structures exist in humans?”  CEN Technical Journal 14(2):95–98.  2000.  An article arguing that the list of vestigial organs in humans has shrunk from 180 in 1890 to 0 in 1999.  It rejects the newer "revisionistic" definition of a vestigial structure as too vague, since it would label almost every structure as vestigial.  The article uses it as the source that lists the plica among the 180 structures once called vestigial.

1. 19a.

   Page 96:  The caption to a figure listing "plica semilunaris" among other structures.  “A list of some of the 180 structures that were considered vestigial in the early 1900's.  It is now almost unanimously agreed that most of these structures have at least one function.” ↩

Sarfati, Jonathan, and Don Batten.  “'Vestigial' Organs: What do they prove?”  Creation Ministries International.  Feb 14, 2006.  A web article by two authors, published by Creation Ministries International, on what vestigial organs prove.  The article uses it as the reply from this literature that could meet the modern definition of a vestige.

1. 20a.

   “Second, even if the alleged vestigial organ were no longer needed, it would prove devolution not evolution.  The creation model allows for deterioration of a perfect creation.” ↩

Heralgi, Mallikarjun et al.  “Persistent unilateral nictitating membrane in a 9-year-old girl: a rare case report.”  Indian Journal of Ophthalmology 65(3):253–255.  Mar 2017.  A case report of one nine-year-old girl with a membrane covering one-third of the cornea of one eye, which was removed by surgery.  The authors say it is only the second human case of a persistent nictitating membrane that they found in the literature.  The article uses it as the modern human case of a full third eyelid, and says it adds little either way.

1. 21a.

   Section Case report:  “a translucent membrane arising from medial canthus and spreading out in a horizontal fashion over the bulbar conjunctiva and covering one-third of the cornea in the primary gaze”.  “The dimensions of the membrane were 20 mm horizontally and 18 mm vertically”. ↩

2. 21b.

   Section Case report:  “Histopathological report showed stratified squamous epithelium with underlying loose areolar connective tissue and nonspecific inflammatory cells around blood vessels.  No elastic, reticular fibers, cartilaginous structures, or muscle cells could be found”. ↩ ↩

3. 21c.

   Section Discussion:  “The presence of nictitating membrane in humans is rare and only one such case report by García-Castro and Reyes de Torres was found in the literature where they describe a newborn with features of trisomy 18 syndrome having bilateral nictitating membrane.” ↩