The Tapetum Lucidum

William Bradley

The History of Science

April 2011

The Tapetum Lucidum

The variation among different creatures in the animal kingdom is astounding. There are so many different methods and mechanisms used in evolution, and we can see that in many of the creatures alive today. The tapetum lucidum (commonly shortened to “tapetum”) is no exception. It is a thin reflective layer in the eyes of many animals that reflect a specific color when light is shone upon it. This information is learned only by observing, just as Goethe did during the formulation of his theory of colors. His entire theory was based solely on his observations through experimentation and, possibly, dissection. This is similar to what we wish to achieve in this study. Through dissection, observation, and an assimilation of research articles, we can come closer to fully understanding the purpose and functions of the tapetum. We can then use this information to formulate hypotheses on related questions. For example, why is the tapetum not a part of every animal’s eyes, specifically humans? This question among others may easily be answered after we have collected and analyzed the results of this research and dissection.

Purpose:

To analyze and experience, through dissection, the tapetum lucidum in a sheep’s eye and pig’s eye.

Experiment and Observations:

Sheep eye: I started the procedure by trimming the excess fat off of the eye. I decided to use my camera and the flash on it to take a picture of the front of the eye to see if the tapetum was visible through the translucent cornea. Surprisingly, it is possible to see a blue tint through the cornea. Next, I cut the eye in two halves, front and back. Once the eye was separated and the fluid removed, I was able to see the cornea immediately in the back of the eye. This may be the result of a damaged specimen, since the layer of photoreceptors should be in front of the tapetum. I then was able to peel the tapetum out of the eye, and view it up close. There is a tiny dark spot in the middle of the tapetum. This spot is where the optic nerve met on the outside of the eye, and where the visual sensory information would be sent to the brain for processing. I initially observed that near this point on the tapetum, the tapetum was much brighter, reflective, and white. But, once you look at the tapetum at a different angle, the blue areas move around as they reflect the specific wavelength (bright blue ~ 475 nm) directly at your line of sight.

Pig eye: I basically began dissecting the pig’s eye in the same way that I dissected the cow’s eye. I trimmed off the excess fat, and cut it open into the same halves. I attempted to take a picture of the front, but I didn’t get any results of color, which I was initially looking for. It didn’t take long for me to realize that pigs actually do not have a tapetum. At first I was disappointed, but I realized I could use the pig’s eye as a comparison for the cow’s eye and the tapetum in general. The inside of the eye looked almost exactly identical, but where the tapetum would have been in the cow’s eye, the pig’s eye only had the blood vessels and grey matter, which would have been red had there been blood flowing through the organ.

Conclusions:

Viewing the tapetum in the cow’s eye allows us to get a better idea of the structure itself, and how it functions. The color reflection changes if you view it at a different angle, and it is interesting to see the focal point of the eye where the information is sent through to get to the optic nerve. In humans, this is the “blind spot” or the range in our line of sight that cannot be seen. In the pig’s eye, we can still find this spot, but it is much harder since a reflective coating doesn’t surround it. We can also use the pig’s eye as a comparison between animals that do have the tapetum and animals that do not. The pig’s eye had some grey matter where the tapetum would have been, but we can speculate that it would have been visible or red, had there been blood flowing through it.

Unfortunately, there exists a limit on what we can observe with the naked eye even in a dissection. The importance of dissection is underlined in many different sources. In Goethe’s Color Theory, it is almost impossible not to assume that he has performed a dissection on an eye, whether the eye was from a human or not. He knows much about the retina and the cornea, both of which are structures that no one at the time could have really discussed as he did unless they were well educated and had seen the structures fully. Barbara Stafford, in an article written on dissection, discusses the importance of it when she quotes P. N. Gerdy’s Anatomie des formes extérieures, “ . . . anatomy functioned like an enlarging glass. It magnified the smallest detail, rendering distinct hidden morphologies” (1). The dissection proves to us that the tapetum exists, and gives us a historical perspective into the research that has been done on the tapetum over time. But by using existing research, we can get an idea of the actual function of the tapetum, as well as how it has changed over time.

Through our observations, we find that the tapetum reflects one wavelength of light (which changes depending on the species). Basically, as laid out in a comparative study by F. J. Ollivier, D. A. Samuelson, D. E. Brooks, P. A. Lewis, M. E. Kallberg, and A. M. Komáromy, the tapetum, “normally functions at low light levels to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity” (2). The tapetum reflects a specific wavelength of light back into the photoreceptors of the eye after the light has already passed through one time. This allows for greater sensitivity to light in low light conditions. We saw this in effect in our dissection when the light from the camera reflected off of the tapetum.

Now that we have an idea of what the tapetum does, we need to know the reasoning behind the specific color of different animals. A different study by Ivan R Schwab, Carlton K Yuen, Nedim C Buyukmihci, Thomas N Blankenship, and Paul G Fitzgerald shows that, “tapeta have a tendency to reflect wavelengths most relevant to the animal” (3). This study showed that the tapetum reflects the wavelength of light important to the animal. For example, the study discusses the fact that the tapetum in deep-sea fish almost all reflect the light wavelength of 475 nm (cyan-green) because this is the only wavelength that can ever reach those depths. We can, perhaps, speculate that a cat or dog may reflect a more greenish color to reflect light off of plants or grass.

The next question that is important to us is the reason why there exists no tapetum in many animals. The study by Ollivier, Samuelson, Brooks, Lewis, Kallberg, and Komáromy states that the animals that do not have the tapetum are primates, squirrels, birds, red kangaroo and pig. These animals have very few things in common, so this helps us in identifying how the tapetum has evolved over time, which we will discuss later. These animals are all, however, diurnal creatures. This makes intuitive sense, since a creature that is active during the daytime will have less of a need for an increase in light reception. As we saw in our dissection of the pig’s eye, these creatures have a “red or orange to pale gray fundus reflection” (4). In our pig eye, the area where the tapetum would be was a pale grey. This could, however, be solely from an absence of blood in the separated and treated organ. In a live pig, the eye could be red as well. By observing human eyes, we see the red blood vessels in the back of the eye when a light is shone upon it. This causes the undesirable “red-eye” feature of many pictures taken of humans. Primates, along with the other creatures listed above, are diurnal creatures, and thus did not have an evolutionary need for a tapetum.

Given the “random assortment” of the animals with the tapetum, it is safe to assume that the tapetum developed independently among specific species. The “parent creature” could not have been very early in the mammalian evolutionary chain, thus, the evolution happened on multiple occasions. In the study by Schwab, Yuen, Buyukmihci, Blankenship, and Fitzgerald, they conclude that, “the tapetum may have arisen independently in both invertebrates and vertebrates as early as the Devonian period (390 to 345 million years ago)” (5). They base this conclusion on evidence found in other organisms. They use the assumptions that vertebrates evolve from pikaia, an invertebrate that is ancestor to other organisms that do not have the tapetum. Also, both hagfish and lampreys do not have a tapetum and they separated from an ancestor fish in the period before the Devonian.

The tapetum allows nocturnal creatures to see better in the darkness, and through dissecting, we can see what it looks like up close, as opposed to viewing it in a live animal. In doing this, we must dissect the organ and view it piece by piece. Understanding all of the functions of the eye, as well as the positions and purposes of each individual part of the eye is also important in understanding the tapetum. The cornea protects the eye, the iris expands and contracts to let specific amounts of light in, and the retina captures the image and sends it to the brain for processing. The tapetum is important in this process in that is reflects light back onto the photoreceptors of the retina for a second viewing. The tapetum is a fascinating aspect of the animal world. So many artistic endeavors have been based around the glow of an animal’s eyes. We see pictures and paintings of wolves and other large mammals where the glow of their eyes is the focal point of the picture over and over again. The tapetum is an important structure of the eye, and it is of great interest to humans in many aspects of biology, evolution, and even art. Not only does the tapetum have a scientific value, the impression that it leaves on humans, who are without it, is everlasting.

Endnotes:

1) Stafford, Barbara, Body Criticism: Imagining the Unseen in Enlightenment Art and Medicine (Cambridge, Massachusetts: Massachusetts Institute of Technology, 1991), 54.

2) F.J. Ollivier et al., “Comparative morphology of the tapetum lucidum (among selected species).” Veterinary Ophthalmology 7, no. 1 (2004): 12.

3) Ivan Schwab et al., “Evolution of the Tapetum.” Transactions of the American Ophthalmological Society 100 (2002): 197.

4) F.J. Ollivier et al., 12.

5) Ivan Schwab et al., 197.

Bibliography:

Goethe, Johann Wolfgang. Goethe’s Theory of Colors. New York: Van Nostrand Reinhold Company, 1971.

Ollivier, F.J. et al. “Comparative morphology of the tapetum lucidum (among selected species).” Veterinary Ophthalmology 7, no. 1 (2004): 11-22.

Schwab, Ivan, et al. “Evolution of the Tapetum.” Transactions of the American Ophthalmological Society 100 (2002): 187-200.

Sepper, Dennis. “Goethe, colour and the science of seeing,” in Romanticism and the Sciences, edited by Andrew Cunningham and Nicholas Jardine, 189-198. Cambridge: Cambridge University Press, 1990.

Stafford, Barbara. Body Criticism: Imagining the Unseen in Enlightenment Art and Medicine. Cambridge, Massachusetts: Massachusetts Institute of Technology, 1991.