Page 183 - Edited - Webster HEAD AND NECK - part 1

HN 149

notes. Vol. III. pp. 19 & 37 - of this division of the trigeminal nerve occupies an area including the conjunctiva
and corneal epithelium. which thereby becomes prey to secondary bacterial infections (and scarring). The
corneal epithelium regenerates extremely well and quickly: a small abrasion (known as a "corneal erosion")
heals within 24 hours. This is important for maintaining transparency of the cornea (see HN 150). The
epithelial layer continues beyond the cornea, where it constitutes the conjunctiva proper. It becomes thicker,
and continues onto the posterior surfaces of the eyelids: the absenc. of keratin is testified by the pink colour,
due to the underlying capillary beds. The conjunctiva! epithelium contains goblet (mucus producing) cells -
which accounts for the globules of inspissated mucus which collect at the comers of the eyes.The space
between each lid and the eyeball is a blind pocket, known as the (upper and lower) conjunctival recess or
fornix (HN 168) The gap between the lids, through which we look, is the palpebral fissure (HN 167, Fig.
A)

The upper and lower eyelids contain a dense collagenous connective tissue "skeleton" (the tarsal
plate) stretched like a crescentic sheet across the upper and lower parts of the orbital opening (HN 156, Fig. A;
167. Fig. 8) A circular skeletal muscle (the ·'orbicularis oculi") is included in both lids, forming a sphincter
to close them (HN 167: 248). The sphincteric action we know as "blinking" - an important protective reflex
(HN 185) - is also responsible for spreading tears ,1cross the conjunctiva. If this muscle is paralysed the lower
lid falls away from the eyeball “bloodhound fashion" and the cornea becomes vulnerable to desiccation and
bacterial infection (HN 246 et seq.). An additional muscle runs from the roof of the orbit into the upper lid
and serves to lift it (the "musculus levator palpebrae superioris" - the "lifter-of-the-upper-lid-muscle") (HN
158. Fig. (a): 167. Fig. (C): 168) This muscle is peculiar - it is in larger part skeletal muscle (supplied by the
oculomotor - third cranial - nerve): and in smaller part smooth muscle (supplied by the sympathetic system -
post-synaptic cell bodies in the superior cervical ganglion). [Ophthalmologists commonly refer to the smooth
muscle component as "Muller's muscle" - a not very useful eponyr'. since Müller's name is also attached to two
other orbital/ocular muscles. If you must have a name for this omponent of levator palpebrae call it the superior
tarsal muscle. Paralysis of either of these muscle components results in a drooping upper lid - a ptosis (HN 190:
192)].

Along the margins of the lids are rows of stout hairs - the eyelashes. (There are usually two rows for
each lid.) The hair follicles are provided with modified sweat and sebaceous glands, but more importantly,
each lid is provided with a row of about 30 tarsal (Meibomian) glands. These are large sebaceous glands
embedded in each tarsal plate (HN 168). Their openings can readily be seen immediately behind the eyel.; hes
(look at those of a friend, or your own in a mirror). As for the rest of the skin, their secretion (sebum) is fatty.
In the eye it acts, by surface tension, to inhibit the overflow of tears beyond the lid margins; and by similar
means to promote the spread of fluid over the conjunctiva, and also (since it floats on the water of tears - how
poetic - isolating the aqueous phase from the air) to inhibit evaporation. (Bacterial infection of either the
glands associated with eyelash follicles or of a tarsal gland is a stye: styes in the latter category are painful
because the inflammatory swelling is restrained by the connective tissue of the tarsal plate and thus the
pressure rises markedly. Tarsal glands so affected commonly become encysted - a ''Meibomian cyst" or
"chalazion".)
In each orbit, lateral and superior to the eyeball, is the lacrimal (tear) gland, a serous gland about the
size and shape of an almond kernel, opening through multiple dusts into the upper conjunctival recess (HN
156. Fig. B: 169: 168). [Secretomotor fibres are parasympathetic from the superior “salivatory” nucleus via
the facial nerve. What, then, are the pathways involved in (a) the “eye watering” response to a foreign body on
the conjunctiva: and (b) weeping as an emotional phenomenon? - see HN 185). The secretion of the lacrimal
glands - tears - contains a bacteriocidal lysozyme: tears are also important for gaseous exchange for the
corneal conjunctiva. which is normally avascular, and for maintaining corneal translucency (HN 150). There
is a steady resting secretion of tears (increased by emotional states, grit in the eyes etc.). The liquid is picked
up through two small apertures (the lacrimal puncta - singular “punctum”), one at the medial (inner) end of
each lid, on the apex of a lacrimal papilla. (That on the lower lid is easily seen in a mirror - just pull down
the lid and you will see the opening on the summit of what appears to be a little hillock - the papilla - about 5
mm from the medial end of the palpebral fissure.) The muscles of the eyelids keep the punctae opposed to the
conjunctiva, and tears are taken up by ‘‘capillary action”. They are then transported to the lacrimal (tear) sac,
in the orbit just below and medial to the eyeball and in the bony lacrimal fossa (HN 156). From the lacrimal sac.
tears pass through the lacrimal (tear) duct to the nasal cavity (HN 169; 205). (Hence the “stuffy nose”
caused by a good bout of weeping.)

\NewCMedPhysSc\10 HN 148 Eyes&Orb.