Page 186 - Edited - Webster HEAD AND NECK - part 1

HN 152

and ciliary body in parasympathetic paralysis (when the ciliary muscle and constrictor pupillae are relaxed) the
scleral sinus collapses. The importance of this will be dealt with later (see: Aqueous Humour - HN 152).

3. The Lens and Contents of the Chambers of the Eye.

Cornwall Out, vile jelly!
Where is thy lustre now!
Gloucester. All dark and comfortless.
(William Shakespeare, King Lear)

The Lens and the Mechanism of Accommodation.
The lens is a biconvex structure, slung immediately behind the iris by fine connective tissue threads
(the suspensory ligament of the lens) between the equator of the lens and the ciliary body (HN 163; 164 Figs.
A & B). The space between the lens and the iris is the posterior chamber. Three features of the lens are of
crucial importance.

First, like the cornea and for the same reason the lens is avascular. Its metabolic requirements are taken
care of by the aqueous humour.

Second, it is cellular, the individual cells (or “lens fibres”) elongated and arranged in a complex
geometry. New cells are added at the periphery throughout life, the oldest, which are anuclear, forming the
central core. The nuclei of the outermost “fibres” tend to be located at the equator (presumably because they
there will interfere least with the passage of light although the outermost covering of the lens (beyond the
capsule - see below) is in fact a cuboidal epithelium. The major content of the cells is a protein known as
crystallin. They also contain an unusual metabolite of tryptophan (3-hydroxykyurenine glycoside, if you must
know) which accumulates with age and accounts for the increasing brown colour of the lens. This metabolite is
largely responsible for the strong absorptive properties of the lens in the UV region probably contributing to
reduction of chromatic aberration. [The refractive performance of the eye is such that at the extreme short
wavelength end of the visible spectrum (violet) the rays are focused well in front of the retina: filtering out
“blue-violet” might minimise the interference from this less well-focused image by reducing its intensity.]
Disturbance of the protein content of lens fibres reduces transparency: this can be either temporary, in the case
of transient disturbances of local ionic concentrations; or permanent, as in over-exposure to infra-red radiation
(“glass-blowers’ cataract”), or in metabolic disorders (such as diabetes mellitus), or from unestablished causes.
The condition is known as cataract (treatable only by removal of the lens).

Thirdly, the lens is elastic, a property conferred by the constituency of its cells - the “lens fibres” -
and the elasticity of the acellular (connective tissue) lens capsule, deep to the investing simple cuboidal
epithelium. Elasticity is fundamental to the mechanism of accommodation used by mammals (as well as by
most birds and reptiles). Instead of moving the lens backwards and forwards (as in a camera, and as is done by
all other vertebrates) the focal length is changed by changing the curvature. In order to reduce the focal length
(and thereby to accommodate for near vision) the ciliary muscle (stimulated by parasympathetic fibres in the
oculomotor - 3rd cranial - nerve) contracts, thereby reducing the circumference of the ciliary body and
slackening the suspensory ligament of the lens. The lens, by its inherent elasticity, therefore takes up this slack
by assuming a more nearly spherical shape. When the ciliary muscles relax, the reverse sequence occurs. An
important weakness of the “passive” component of the accommodation mechanism is its dependency on the
continuing elasticity of the lens throughout life. This will be discussed later.

4. The Non-cellular Contents of the Eyeball

The Aqueous Humour: Glaucoma
This liquid, its composition resembling CSF, fills the anterior and posterior chambers. It is secreted by
the internal epithelium of the ciliary body into the posterior chamber, circulates through the pupil into the
anterior chamber and is reabsorbed through the trabecular network into the scleral sinus (and so to the venous
blood - see unfletched arrows in HN 164. Fig. A). The “half-life” of aqueous humour is about 30 mins (i.e.

\NewCMedPhysSc\10 HN 148 Eyes&Orb