382 Neuroanatomy: Draw It to Know It
The Eye
Here, we will draw the eye. First, we will draw the outer
and middle layers of the eye and then we will draw the
inner layer of the eye — the retina. Begin with the cornea,
which is the anterior portion of the eye’s outer layer. Th e
cornea is avascular and transparent to optimize the pas-
sage of light. Its contour, smoothness, transparency, and
refractive index all play an important role in focusing
light on the retina, and they all demand a healthy
endothelium and epithelium.
Next, show that where the cornea ends, the outer layer
becomes the sclera. In contrast to the cornea, the sclera is
opaque and blocks the transmission of light. We refer to
the portion of the sclera we can see as the “white of the
eye”; conjunctiva covers it. Posterior to the conjunctiva,
the six extraocular muscles insert into the sclera. Note
that both the cornea and sclera comprise a fi brous histol-
og y that gives the outer eye a semi-elasticity and high
tensile strength to allow it to endure the extraocular
muscle forces placed upon it and to protect the eye from
physical harm.
Now, draw the biconvex lens. Like the cornea, the lens
is transparent and serves to focus a target on the retina.
Th e cornea and lens bend the target’s light rays so that
they strike the retinal area of maximal visual acuity: the
fovea. Th is light ray manipulation is called optic refrac-
tion. Unfortunately, opacities, called cataracts, commonly
develop in the lens and produce hazy vision, reduced color
intensity, increased glare, and worsened visual acuity.
In front of the lens, draw the iris, and show it in coro-
nal view, as well. Indicate that the open region within the
center of the iris is the pupil. Th e pigmented epithelium
of the iris blocks light transmission and funnels light
through the pupil. Th e iris forms an adjustable dia-
phragm that opens and closes based on the illumination
demands of the eye. In bright light, show that parasym-
pathetically innervated, circumferentially arranged, iris
sphincter muscles contract and constrict pupil size;
whereas, in darkness, sympathetically innervated, radi-
ally arranged, pupillary dilator muscles activate and
widen pupil size (also, see Drawing 23-6).
Lateral to the iris, draw the ciliary body, and posterior
to it, draw the choroid. Indicate that these three struc-
tures form the middle layer of the eye, called the uvea.
Th e choroid is a thin, brown, highly vascular layer sand-
wiched between the sclera and retina; it nourishes the
retina and removes heat produced during phototrans-
duction, which is the process wherein the photorecep-
tors transform light into neural signal.
Th e functions of the ciliary body are twofold. First,
show that it anchors suspensory ligaments, collectively
called zonule, which stretch the lens and alter its refrac-
tive power. As mentioned, the refraction of light adjusts
where a visual object falls on the retina, and the adjust-
ment for near or far objects is called accommodation.
Accommodation for near objects occurs from relaxation
of the zonule. During far vision, the ciliary bodies are
relaxed, the zonule are stretched, and the lens is fl at-
tened. During near accommodation, the ciliary bodies
contract (ie, shorten), which relaxes the zonule and
rounds the lens (ie, thickens it), and the near object is
brought into focus.
To demonstrate this principle for yourself, do the fol-
lowing. Extend your right index fi nger to represent a
relaxed ciliary body. Th en make a V with your left hand’s
thumb and index fi nger and touch their tips to the ends
of the relaxed ciliary body. Th e V represents the taut
zonular fi bers. Imagine that your left hand is the lens;
feel the pull of the zonule on your left hand and imagine
the lens being fl attened. Th en, contract the ciliary body
(ie, collapse your stretched right index fi nger); the zonule
fold in on themselves, which causes your left hand to
relax, and you can imagine the lens rounding.