The ciliary zonule is a thickened part of the vitre- ous membrane which is in contact with the poste- rior surface of the ciliary process [Figs. 12.1, 12.4, 12.8]. At the inner margin of the ciliary processes, it splits into two layers. The posterior layer is ex- tremely thin and lines the hyaloid fossa. The an- terior layer is thicker and forms the suspensory ligament of the lens. It is attached principally to the anterior surface of the capsule of the lens a short distance from its margin, to the margin of the lens, and the posterior surface close to the margin. [Figs. 12.1, 12.8]. The multiple attach- ments of the suspensory ligament to the lens re- sult in the formation of a multilocular, fluid-filled space around the margin of the lens—the zonular space.
The suspensory ligament holds the lens firmly in the hyaloid fossa. When the eye is at rest, the suspensory ligament maintains a certain degree of
Fig. 12.8 Section through the anterior part of the eyeball.
Lens Posterior chamber Ciliary
zonule Ciliary
process Vitreous
Pigmented layer of retina Ora serrata Sclera
Ciliary part of retina Ciliary M.
Conjunctiva Sinus venosus sclerae Pectinate
ligament Iris and iridial part of retina Cornea
Anterior chamber Pupil and sphincter M.
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The eyeball
DISSECTION 12.6 Lens Objective
I. To study the lens and the capsule of the lens.
Instructions
1. Incise the anterior surface of the lens with a sharp knife, and apply a little pressure to the margin with the finger and thumb. This will extrude the body of the lens and allow the capsule to be studied separately.
2. Compress the body of the lens between the finger and thumb, and note that the central part (nucleus) is firmer than the remainder. The lens is made up of a number of concentric laminae, which can be easily peeled off after the lens has been hardened in alcohol.
CLINICAL APPLICATION 12.2 Corneal transplant Corneal transplant, or keratoplasty, is a procedure where
diseased corneal tissue is surgically removed and re- placed by healthy donor corneal tissue harvested from a recently deceased individual. Corneal transplants can be
‘penetrating’ when the whole thickness of the cornea is
replaced, or ‘lamellar’ when the graft involves only a few layers of the cornea. The fact that the cornea is avascular, devoid of lymphatics, and has relatively few antigen- presenting cells makes it immunologically privileged and less likely to be rejected than other organ transplants.
CLINICAL APPLICATION 12.1 Corneal reflex Testing for the corneal or blink reflex is part of neuro-
logical examination. It is especially useful in patients with head injury to assess the integrity of the brainstem.
Stimulation of the cornea of one eye with a wisp of cot- ton causes both eyelids to blink. This reflex blink protects the eye from foreign bodies. Stimulus of the cornea of one side causes a direct (same side) and consensual (op- posite side) blink response.
The afferent (sensory) limb of the reflex arc is me- diated through the nasociliary branch of the ophthal-
mic division of the trigeminal nerve, which supplies the cornea. The efferent (motor) limb is through the temporal and zygomatic branches of the facial nerve which supply the orbicularis oculi. An absent corneal reflex may be due to sensory neuropathy affecting the trigeminal nerve, a lesion in the seventh cranial nerve, or a brainstem injury. Failure to blink on one side indi- cates seventh nerve palsy on that side. Failure to blink on both sides usually indicates a sensory loss of the tested side.
CLINICAL APPLICATION 12.3 Central retinal vessel occlusion Obstruction of either the central retinal artery or the vein
can result in acute-onset loss of vision due to ischaemia of the retina. Sometimes, only a branch of the vessel may be occluded, resulting in patchy loss of vision.
Study question 1: what is the branching pattern of the central retinal artery? (Answer: the artery divides into su- perior and inferior branches, and each of these, in turn, divides into nasal and temporal divisions.)
Study question 2: the central retinal artery is typically an end artery. What does this mean? (Answer: end ar- teries are the sole supply of arterial blood to a territory.
They do not anastomose with other arteries. As such, if
an end artery is occluded, there is complete deprivation of blood to the area supplied by it.)
Increased pressure in the subarachnoid space tends to compress the central vein of the retina and cause disten- sion of its tributaries in the retina. These distended veins can be seen with the ophthalmoscope.
The central artery of the retina and its branches are blood vessels which can be examined in the living, and signs of vascular pathology can be observed first hand.
You should take every opportunity to use an ophthal- moscope and examine the normal retina and blood vessels.
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Lens
CLINICAL APPLICATION 12.4 Convergence–accommodation reflex Failure to bring both eyes to focus on an object leads to
double vision. To test this, hold up your finger about 30 cm from your nose, and look past it at a distant object (do not focus on the finger). In this situation, the finger appears double and out of focus. Now slowly bring your eyes to focus on the finger. The two images move to- gether and come into focus as a single object.
The ability to change focus from distant objects to near objects is dependent on convergence and accom- modation. Convergence occurs when the two medial
recti work together and ensure that the image of the finger falls on corresponding parts of the two retinae.
Accommodation occurs when the ciliary muscles con- tract, the ciliary zonule and lens are brought forward, the suspensory ligament of the lens is relaxed, and the lens becomes more spherical, bringing the near object to fo- cus on the retina.
The two actions are linked together in the convergence–accommodation reflex and are associ- ated with constriction of the pupil.
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chapter 13
Organs of hearing and equilibrium
Introduction
The ear contains the auditory apparatus and the organs for balance and equilibrium. It is readily di- visible into three parts: the external ear, the middle ear, and the internal ear.