Page 404 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 389

The special senses 389

opaque posterior portion, the sclera. Peripherally, the cornea is continuous
with the sclera at the sclerocorneal junction. The sclerais a tough, fibrous mem-
brane which is responsible for the maintenance of the shape of the eyeball
and which receives the insertion of the extraocular muscles. Posteriorly, it is
pierced by the optic nerve, with whose dural sheath it is continuous.

The vascular coat
This is made up of the choroid, the ciliary body and the iris.
The choroid is a thin but highly vascular membrane lining the inner
surface of the sclera. Posteriorly it is pierced by the optic nerve and anteri-
orly it is connected to the iris by the ciliary body.
The ciliary body includes the ciliary ring, a fibrous ring continuous with
the choroid, the ciliary processes, a group of sixty to eighty folds arranged
radially between the ciliary ring and the iris and connected posteriorly to
the suspensory ligament of the lens, and the ciliary muscles, an outer radial
and inner circular layer of smooth muscle responsible for the changes in
convexity of the lens in accommodation and supplied by parasympathetic
fibres transmitted in the oculomotor nerve (III).
The iris is the contractile disc surrounding the pupil. It consists of four
layers:
1◊◊an anterior mesothelial lining;
2◊◊a connective tissue stroma containing pigment cells;
3◊◊a group of radially arranged smooth muscle fibres — the dilator of the
pupil (supplied by the sympathetic system) and a circular group, the pupil-
lary sphincter (supplied by the parasympathetic fibres in the oculomotor
nerve);
4◊◊a posterior layer of pigmented cells which is continuous with the ciliary
part of the retina.

The neural coat
The retina is formed by an outer pigmented and an inner nervous layer, and
is interposed between the choroid and the hyaloid membrane of the vitre-
ous. Anteriorly, it presents an irregular edge, the ora serrata, while posteri-
orly the nerve fibres on its surface collect to form the optic nerve. Its
appearance as seen through an ophthalmoscope is shown in Fig. 272. Near
its posterior pole there is a pale yellowish area, the macula lutea, the site of
central vision, and just medial to this is the pale optic disc formed by the
passage of nerve fibres through the retina, corresponding to the ‘blind
spot’. The central artery of the retina emerges from the disc and then divides
into upper and lower branches; each of these in turn divides into a nasal
and temporal branch. Histologically, the retina consists of a number of
layers but from a functional point of view only three need be considered: an
inner receptor cell layer — the layer of rods and cones — an intermediate
layer of bipolar neurones, and the layer of ganglion cells, whose axons form
the superifical layer of optic nerve fibres (Fig. 255).