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