Stephen G. Saupe - Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; (320) 363-2782;

Gink & Go Take Pictures

Setting:  Go is in his room, hunched over his biology book and notes.  Gink excitedly enters the room carrying a camera.

Gink: Hey Go, check out my cool new camera.
Go: Nice.  It reminds me of Saupe's lecture on the eye.
Gink: Oh no – why do you always have to mention that geek!
Go: I thought that a camera was a good analogy for the eye.  Let's see if we can remember what he said.
Gink: Do we have to?
Go: It'll be fun – and that way you can show me your camera while we study.
Gink:  So we're going to dispatch binary avians with a unitary projectile?
Go: Exactly – you show me your camera and then we can discuss the analogous structure in the eye.
Gink: This is gonna be boring.  To start, I have a nifty UV/haze filter that screws onto the lens to keep it from getting scratched.
Go: The cornea acts like the UV haze filter.  But it does much more.  It also functions to focus light onto the photoreceptive layer.
Gink: The lens of my camera focuses the image on the film.  
Go: The eye also has a lens, but it's main job is to help the eye to accommodate, or in other words, focus on objects whether they are close or near.
Gink: No way.  My eyes don't act like a toilet.
Go: That's accommodate, not a commode! 
Gink: Oh, right.  Is that when Saupe was trying to tell us that to see near objects the lens becomes rounder to bend the light more to focus the image?
Go: Yeah.  It happens because the ciliary muscles contract pulling the edge the eye closer to the lens.  This loosens the tension on the ligaments holding the lens causing it to become rounder.
Gink: So when we see an object further away the lens is flatter because the ciliary muscles are relaxed and the sclera moves away from the lens pulling on the lens causing it to flatten out.
Go: You've got it. 
Gink: My camera is much simpler than that – the lens just moves closer or further to the film to focus.
Go: True, but let's get back to our analogy.   How do you control the light getting into the camera?
Gink: See the diaphragm inside the lens?  It acts just like the iris of the eye.
Go: The opening into the eye, the pupil, is just like the F-stops on your camera.  
Gink: Is the camera body itself analogous to any part of the eye?
Go: Sure is – the sclera.  This is a tough layer of connective tissue the protects the eye
Gink: I've got some film in my camera – is that analogous to the retina?
Go: And it's dark inside the camera to avoid light scattering.  The choroid layer has lots of vascular tissue for nourishment and is pigmented to avoid light scattering.
Gink: Saupe talked about some gooey stuff inside the eye.  I'm glad my camera doesn’t' have any of that!
Go: That's the vitreous humor which helps to support the eye and transmit light to the retina.  Your camera has air inside of it instead of liquid.
Gink: Good thing or else my pictures might get wet.
Go: Yuck, yuck.  The aqueous body is the fluid inside the cornea – it helps to transmit light and maintain pressure in inside the eye.  Do you remember what the optic disk and fovea are?
Gink: The optic disk is where the optic nerve exits the eye.  It causes our  blind spot and the fovea is a densely packed area of cones.
Go: Wow – you never cease to amaze me!
Gink: I just wish Saupe would amaze me sometime - let's go take some pictures.
Go: First we should identify the parts of the eye and their function (Using the list of structures, label the diagram below.  Identify the function of each structure)


  • aqueous body 
  • cornea
  • choroid layer
  • iris
  • fovea
  • lens
  • optic disk
  • optic nerve
  • pupil
  • retina
  • sclera
  • vitreous body

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Last updated: April 03, 2007    � Copyright by SG Saupe