Page 182 - Edited - Webster HEAD AND NECK - part 1

HN 148

THE EYES AND ORBITS

Veggio co' bei vostri occhi un dolce fume
Che co' miei ciechi gia veder non posso.
(Michelangelo)

Get thee glass eyes
And, like a scurvy politician, seem
To see the things thou dost not.
(William Shakespeare, King Lear)

The visual system begins at the eyes. Like the ear (and even, to a lesser extent, the somatosensory
system) a neural transduction mechanism is placed with a special non-neural structure which itself transforms
the light. In the case of the visual system, the eye produces a focused image on the transducers and controls
the intensity of the signal. Further, by means of the extraocular muscles, the eyeballs provide a mechanism for
shifting the transducers around to direct them at various parts of the environment.

THE BONY ORBIT AND ADNEXA OCULI

The eyes cannot function properly without various accessory organs - the "the adnexa oculi". Medical
students must know about these.

In humans, the bony orbit is a pyramidal box, with the apex posteriorly (HN 144, Fig. C). The two
medial walls (alongside the nasal cavity) are parallel, the two lateral walls at right angles to each other. The
eyeballs face approximately directly forwards, looking through the roughly quadrangular openings of the
orbits.

The optic (second cranial) nerve (which is not really a nerve at all, but a CNS tract, since the retina is
part of the central nervous system, and the myelinogenic cells of the optic nerve are oligodendroglia and not
Schwann cells ) leaves the back of the eyeball, surrounded by all three meningeal layers, including
1
subarachnoid space (HN 163) and enters the middle cranial fossa through the optic canal (HN 92; 113; 114;
140; 143). The arterial blood supply of the interior of the eyeball (and its venous drainage) enter within the
nerve. Pathologically increased intracranial pressure is transmitted to the eyeball in the subarachnoid space,
occluding the thin-walled vein and causing congestion of the interior of the eye (which is accessible to non-
invasive inspection through the cornea by means of an ophthalmoscope).

The eyeball is attached to the bony orbit by six extraocular skeletal muscles (HN 156, Fig. C; 157 et
seq.), four of which are supplied by the oculomotor (third cranial) nerve, one by the trochlear (fourth cranial)
nerve and one by the abducent (sixth cranial) nerve. Eye movements are co-ordinated into fixed patterns by
the CNS, so that the direction of movement of one eyeball determines the direction of movement of the other
and vice versa (see Neuro notes, Vol. I, pp.147-148, 153, and Vol. III, pp.7-9). Weakness of any one extraocular
muscle for any reason, causes the eyeball to take up an "off-axis" (see Neuro notes, Vol. I, p.156, and Vol. III,
pp.11-14) position - a squint or strabismus.

The posterior surfaces of the eyelids and anterior surface of the eyeball are covered by the conjunctiva
(HNN 168). Over the transparent part of the eye this is known as the corneal epithelium - a non-keratinising
stratified squamous epithelium, up to five cell layers thick. The entire conjunctiva is provided with free nerve
endings derived from axons of the ophthalmic (note: "ophthalmic" - pronounced "off-thalrnick" - not
opthalmic) division of the trigeminal (fifth cranial) nerve (HN 196 et seq.). This somatosensory innervation of
the conjunctiva is important (a) in generating the blink and teaxxproducing reflexes in response to small
foreign bodies and other irritants; and (b) because the vesicular eruption of herpes zoster ("shingles") - Neuro

1 Note: Hence the optic nerve is subject to demyelinating disorders of the CNS - such as multiple sclerosis - and NOT to those of
the PNS.

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