Eye Anatomy

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General Anatomy of the Eyeball

    We are figuring out the anatomy of eyes in lab...  But I want to be sure that you have the anatomy down cold.  On this page are some pictureseyeanat.gif (16646 bytes) of eye anatomy that I took off the web.  This first one is a partial cut-away view through an eyeball.  It shows how the retina surrounds the vitreous humor.  It also illustrates how the cornea and the sclera are continuous.



    A little more detail can be seen in this other picture of a section through an eyeball.  I also edited this picture to put in a few more labels, but it is somewhat similar to your textbook figure 12.27.eyeanat.jpg (15310 bytes)  In this picture, the two major fluid compartments stand out... there is an area filled with aqueous humor, and an area filled with the much thicker vitreous humor.  This also shows very nicely that the retina is continuous with the optic nerve... it has to be, since retinal axons (from ganglion cells) are the ones that form the optic nerve.

    What is unmarked in diagrams such as this one is how these parts of the eye fit into the classification system of the three tunics (although it is shown very nicely).  That's what I want to go over here.

    The three tunics are like concentric rings around the eyeball.  The outermost ring, illustrated by the pinkish colored-area with the dots in it in the above drawing, is the outer tunic.   The middle ring, illustrated above by the blue area, is the middle tunic, and the innermost ring (that actually doesn't go all the way around in a full ring), illustrated above by the red area, is the inner tunic.

    The outer tunic has to be tough.  The extrinsic eye muscles that you learned about in lab have to attach to the outer tunic and be able to pull on it to move the eyeball.  This outer tunic also has to shield the anterior aspect of the eye from potentially harmful materials that might enter our bodies.  And, the thick outer tunic has to be able to withstand the pull exerted by the ciliary body upon it.  The outer tunic portions include the:

Cornea:   must be transparent (but still thick) in order for light to pass through.  Note that it is the cornea that is worked on for Lasik surgery.
Sclera:   provides for extrinsic muscle attachment
Optic nerve:  Cranial nerve II, carries the retinal ganglion cell axons back to the brain

    The middle tunic has some extremely important functions.  It is best to describe the function for each middle tunic component individually...

choroid coat:  This is the portion of the middle tunic that covers the entire posterior aspect of the eyeball (underneath the retina).  The choroid coat is FULL of dark pigments.  Dark pigments absorb light, and do not reflect light well; you will see in the web page about light that if unused light was not absorbed, it would bounce around within our eyeballs and give us inappropriate information about our world.   So, the choroid coat prevents any unuseful light from being interpreted as visual information.
ciliary body:  This structure is composed of muscles, processes, and ligaments arranged in a ring around the lens.  This ring, kind of like a donut in shape, has been cut in the drawings above, as if you ripped a donut in half to dunk it.
    The ciliary body muscles pull on the lens via the suspensory ligaments.   Their pulling action causes the lens to change shape (from a long, thinnish oval to a more rounded or convex oval).  You will need to think about that to really understand it... because most of us would automatically think that if the ciliary muscles contracted, that would pull on the lens and draw it out into a thinner, longer shape.   But that is a misconception based in a lack of understanding of the anatomy of the eye.
    Let's consider one of the ciliary muscles... it runs in a ring around the lens, totally encircling the lens.  When this circular muscle contracts, the ring gets smaller, like:
o arrowrtblue.gif (2035 bytes)  o.   Notice that when the ring gets smaller, the space inside it gets smaller, too.   And, remember, the lens hovers inside because of tension on the suspensory ligaments from the ciliary body.  So, when the ciliary muscles contract, the tension on the suspensory ligaments decreases, and the lens is pulled LESS.
    The relaxation of the lens (which you will see allows for close-up vision) is called accomodation.  This is what we examined in lab when we did the "near-point accomodation" experiment.
iris:   The iris is the colored portion of the eye when we look at someone's eyes.  It projects from the ciliary body medially, and in front of the lens.  The iris divides the region containing the aqueous humor (the anterior segment) into two portions:  anterior chamber and posterior chamber.
    Smooth muscle within the iris is controlled by the ANS to adjust pupillary dilation/constriction.  The smooth muscle is arranged in both a circular and a radial pattern within the iris.  The circular smooth muscle is innervated by the parasympathetic nervous system; contraction of the circular smooth muscle causes pupillary constriction, like in the light reflex.  The radial smooth muscle is innervated by the sympathetic nervous system; contraction of the radial muscles cause pupillary dilation.

    The inner tunic is the retina.  There is another, separate web page that describes the retina, the structure containing the sensory receptors for light.

Other Structures Associated with the Eyeball:

    The eyelids-- these structures are composed of four layers of tissue.  A list of those four from the most anterior to the most posterior are:  skin, muscle, connective tissue, and conjunctiva.  The eyelids are also called the palpebrae.

    The conjunctiva requires a little more explanation.  The conjunctiva not only covers the innermost surface of the eyelid, but it also covers the outermost surface of the exposed sclera (just not the cornea).  The conjunctiva is a thin membrane that contains secretory epithelium.  Its secretions serve to help lubricate the eye for eyelid closure.   When this conjunctiva gets irritated, it is called conjunctivitis, which is also more commonly called "pink eye."

   Lacrimal apparatus-- this apparatus is composed of a lacrimal gland, lacrimal ducts, and a lacrimal sac.  The whole purpose of this apparatus is to produce tears to moisten and lubricate the eye.   Part of the tear secretion also contains the enzyme "lysozyme," which is an important tear component for fighting off bacterial infections that may try to enter through the eye.

    Tears are secreted from the lacrimal gland, which is located lateral and superior to the eye.  The tears are released onto the sclera of the eye and have to travel all the way medial and inferior to the eye in order to exit through the lacrimal ducts.  This constant flow of tears across the eye keep it clean.  The two lacrimal ducts impinging on the eye are called the superior and inferior canaliculi in your lecture book, and are called the superior and inferior canals in your lab book; let's use the lab book terminology here.  Tears enter these canaliculi and drain into the lacrimal sac.  From the lacrimal sac, tears then drain through the nasolacrimal duct into the nasal cavity.  When we cry, more tears enter our eye than can drain quickly enough, and the tears overflow; the increased flow of tears into our nasal cavity cause the runny nose associated with crying.

   Extrinsic eye muscles-- these were covered entirely in lab.

Go on and learn about the light, and then the retina!