Until recently, scientists misunderstood much of the human anatomy. From wonky illustrations of organs to misplaced faith in phrenology – the supposed ability to predict criminality from a person’s skull size and shape – there have been a lot of missteps when it comes to understanding our own bodies. As may be expected, thinkers from every age have come up with some fascinating and downright bizarre explanations for how the human eye works.

The Fundamental Workings of the Eye: Extramission vs. Intromission

Model of the inside of the eye. Edina Eye.Nowadays, scientists have significant evidence that vision occurs when our body responds to the natural stimulus of light. When the light reflected off of objects enters our eye through the pupil, it is concentrated and stimulates the retina, which then sends signals via the optic nerve to our brain to interpret the image.Modern scientists do not believe that any substance of the body or eye is projected outward to create vision. However, as researchers noted in a 2002 American Psychologist journal article, many children and adults alike accept extramission as a part of the visual process.1

Perhaps it’s no surprise, then, that in the early days of eye history, there was quite a bit of debate over whether vision was a byproduct of something coming out of the eye, or something coming inward. These competing theories were known as extramission and intromission, respectively. Great thinkers were on both sides of the debate.

Pro-Extramission Thinkers

  • Plato, 420s-late 340s BC – In 360 BC, the famous Greek philosopher espoused his belief in his work, “Timaeus,” that flames emanate from the eye and combine with daylight to sense the objects around us.
  • Euclid, 325-mid-third century BC – In 300 BC, the Greek mathematician looked at sight from a geometric point of view in his work “Optics,” speculating that our eyes emit rays that encounter objects and tell us their properties.
  • Ptolemy, 100-170 AD – Unfortunately, Ptolemy’s works on optics only remain in partial form, and not in their original Greek. In his writings, he described a “cone” in the internal portion of the eye that concentrates light. He also believed the eye emits rays, like other thinkers before him.
  • Galen, 129-210 AD, – Galen is better known for his work concerning the anatomy of the eye, but he also held the position that pneuma – literally meaning “breath” in Greek and denoting a sort of life force – came from the optic nerve and outward through the eye to enable sight.
  • al-Kindi, 801-873 AD – The study of optics saw a revival in the Middle East after years of Greek thought. al-Kindi, an Arab philosopher and mathematician, supported Euclid’s views on extramission over Aristotle’s on intromission. Within the next few hundred years, other thinkers from this region would soon reject that train of thought.

Pro-Intromission Thinkers

  • Aristotle, 348-322 BC – The Greek philosopher rejected the teachings of Plato and surmised that the eye must perceive sight by taking in information. However, he did not agree with earlier atomists, like Epicurus and Democritus, who believed particles and images come off of objects and into the eye.
  • Avicenna, 980-1037 AD – Avicenna, a prominent Arab natural philosopher, agreed with the Aristotelian notion of sight over the previous extramission theories.
  • Ibn al-Haytham (Alhazen), 965-1040 AD – This brilliant Arab thinker wrote the “Book of Optics,” which was a major turning point in our understanding of vision. He blended the theories of all those who came before him, eliminating the weaknesses of each argument and presenting a new, much more accurate

    theory of intromission. He argued that every object reflects light, and that the eye catches any ray of light that comes in contact with it perpendicularly. Then, the light is concentrated by the cone shape of the eye and received by the center of the eye. After Alhazen’s more thorough explanation of intromission and complete rejection of extramission, intromission became the standard way to explain vision.

The Physical Anatomy of the Eye

Much of the early beliefs on the anatomy of the eye can be attributed to a Greek physician and anatomist named Anatomy of the eye. Edina Eye Physicians.Galen. Though he supported extramission theories, he rendered a surprisingly accurate anatomical study of the human eye that would influence scientists for years to come, including Alhazen. He accomplished this despite laws that forced him to perform his research through the vivisection and dissection of animals, rather than cadavers.

Notably, Galen believed the eye was composed of vitreous and aqueous humors and thought a crystalline lens was crucial for vision. He also successfully completed cataract removal surgeries much like we do today.

Receive Quality Eye Care From Edina Eye

Fortunately, medical professionals now have a much better understanding of the human eye and its inner workings. At Edina Eye, our highly qualified physicians and surgeons have been serving the Minnesota area for more than 50 years. We can provide expert eye exams and eyewear prescriptions as well as more complex procedures such as LASIK, cataract surgery, and glaucoma care. Call us at 952-832-8100 or go online to schedule an appointment today!

1 https://pdfs.semanticscholar.org/2fe9/016275946ab11e26a1b93f8039fab86a4d26.pdf

Additional Sources

https://web.stanford.edu/class/history13/earlysciencelab/body/eyespages/eye.html

https://www.princeton.edu/~cggross/neuroscientist_5_99_fire.pdf