Since the eye is globular, it's customary to refer to its structures with reference to 1) its optical axis; and 2) its imaginary central point. Thus, to speak of a structure as being "inner" or "outer" means it's nearer to (or farther away from) the physical central point of the spherical eyeball. Throughout this lesson, this convention will be used in describing the layers of the retina, the portions of the choroid, and so on.
Similarly, the structural asymmetry resulting from the placement of the lens and the retina defines the "front" and "back" of the eye.
A section of the eyeball taken parallel to the optical axis is a "horizontal" section; one at right angles to the optical axis is a "vertical" section.
A horizontal section through the eye, showing the three tunics. The apparatus for focusing and production of aqueous humor are also visible, though at this magnification details are not evident. This is a dorso-ventral view of the right eye, with the optical axis running from left to right. Note that the rectus muscles to rotate it left and right insert onto the surface of the sclera. The optic nerve's epineurial covering is continuous with the sclera; and also with the meninges of the brain. In this image only the innermost portion of the retinal tunic is easily seen; but the choroid portion of the uveal tunic and the external corneoscleral tunic are also represented around the margins of the globe.
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