380 Neuroanatomy: Draw It to Know It
Know-It Points
The Eye
■ Th e outer layer comprises the cornea and the sclera.
■ Th e middle layer comprises the iris, choroid, and
ciliary body.
■ Th e inner layer is the retina and, from inner to outer,
lies the nerve fi ber layer, the synaptic and cell body
layers, and the photoreceptor cell segment layer (the
rods and cones).
■ Th e lens is transparent and focuses a target on the
retina.
■ 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.
■ Ciliary epithelia actively secrete aqueous humor,
which is reabsorbed at the iridocorneal fi ltration
angle into the canal of Schlemm.
■ Th e vitreous chamber contains vitreous humor,
which helps maintain the eye’s shape.
■ Th e retina transitions into optic nerve where it exits
the eye, posteriorly, at the lamina cribrosa.
■ In the center of the optic nerve head sits the optic
cup: a white-appearing hole through which the
central retinal vessels (artery and vein) emanate.
■ Th e optic disc is the pink-colored ring of nerve tissue
that surrounds the optic cup.
■ Impaired axoplasmic transport at the lamina cribrosa
results in optic disc swelling (disc edema).
■ Th e fovea lies in the center of the retina and is the
area of highest visual acuity.
■ Th e optic nerve head corresponds to the blind spot.Visual Pathways: Axial View
■ Th e right visual fi eld projects to the temporal left
hemiretina and the nasal right hemiretina.
■ Th e left temporal hemiretina projects ipsilaterally to
the left lateral geniculate nucleus.
■ Th e right nasal hemiretina sends crossing fi bers
through the optic chiasm to the contralateral lateral
geniculate nucleus (the left lateral geniculate nucleus).
■ Th e left lateral geniculate nucleus sends optic
radiations to the left occipital cortex.
■ Th e optic projections between the retina and the
optic chiasm are the optic nerve.
■ Th e projections between the optic chiasm and the
lateral geniculate nucleus are the optic tract.Visual Pathways: Sagittal View
■ Th e superior visual world projects to the inferior
portion of the retina.
■ Th e inferior visual world projects to the superior
portion of the retina.
■ Central vision comprises the majority of the lateral
geniculate body and lies posterior.
■ Peripheral vision comprises the minority of the lateral
geniculate body and lies anterior.
■ Th e superior optic radiation bundle projects to the
superior primary visual cortex.
■ Th e inferior optic radiation bundle projects to the
inferior primary visual cortex.
■ Injury to an optic radiation disrupts visual perception
of a single visual quadrant, called quadrantanopia.
■ Cortical representation of central (or macular) vision
lies in the posterior calcarine sulcus and occupies a
large cortical area relative to its small retinal expanse.
■ Cortical representation of peripheral vision lies in the
anterior calcarine sulcus and encompasses a small
cortical area relative to its broad retinal expanse.