The globe is an approximately spherical structure made up of three layers (Figure 61.1).
The globe is an approximately spherical structure made up of three layers (Figure 61.1):
Sclera, the dense, fibrous outer layer that provides structure and protection to the eye. There are two gaps in the sclera – one anteriorly for the cornea and one posteriorly for the optic nerve.
Choroid, the middle vascular layer whose main role is the supply of nutrients to the sclera and retina.
– Rods are the most numerous photoreceptor and are extremely light sensitive. Rods perform their visual function mainly in dim light, but cannot distinguish between different wavelengths of light.
– Cones primarily function in bright light. There are three types of cones, each responding to the wavelength of a different primary colour (red, green and blue), resulting in colour perception.
– Retinal ganglion cells relay signals from the rods and cones to the brain: their axons form the optic nerve. The optic nerve (cranial nerve II) then transmits visual information to the occipital lobe of the brain (via a synapse in the lateral geniculate nucleus).
The globe is divided anatomically into three chambers:
Anterior chamber, the space between the cornea and the iris;
Posterior chamber, the triangular space between the iris, the lens and the ciliary body;
Vitreous chamber, the space behind the lens.
The three chambers contain two intraocular fluids:
Figure 61.1 Anatomy of the globe of the eye.
The blood supply to the globe is from a single source – the ophthalmic artery. Arterial blood enters the globe through branches of the ophthalmic artery: the central retinal artery, the anterior ciliary arteries and the posterior ciliary artery. Venous drainage of the eye is via the central retinal vein and vortex veins.
In addition to the visual information carried by the optic nerve:
Sensory innervation of the globe takes place through the ophthalmic division of the trigeminal nerve (cranial nerve V1): the lacrimal branch innervates the conjunctiva and the nasociliary branch innervates the cornea, sclera, iris and ciliary body.
Parasympathetic pre-ganglionic neurons originate in the Edinger–Westphal nucleus of the brainstem and travel along the outside of the oculomotor nerve (cranial nerve III) to the ciliary ganglion. From here, post-ganglionic neurons travel in the short ciliary nerve to innervate two muscles: the sphincter pupillae of the iris, resulting in pupil constriction (meiosis), and the ciliaris, changing the shape of the lens (accommodation).
Sympathetic pre-ganglionic neurons originate from the T1 nerve root and synapse in the superior cervical ganglion. From here, the post-ganglionic sympathetic neurons ascend on the outside of the internal carotid artery and enter the orbit along with the ophthalmic division of the trigeminal nerve to supply the dilator pupillae muscle of the iris, resulting in pupil dilatation (mydriasis), and the superior tarsal muscle of the upper eyelid.
Motor innervation of the extraocular muscles is provided by the oculomotor, trochlear and abducens nerves (cranial nerves III, IV and VI, respectively). The lateral rectus muscle (which brings about eye abduction) is innervated by the abducens nerve, whilst the superior oblique muscle (which results in eye intorsion) is innervated by the trochlear nerve. The remaining extraocular muscles (superior and inferior rectus, medial rectus and inferior oblique) are innervated by the oculomotor nerve. The facial nerve (cranial nerve VII) innervates the main upper eyelid retractor, the levator palpebrae superioris.