Glaucoma is a disease in which there is greatly increased intra-ocular tension. This increased tension is caused by obliteration of the filtering angle, preventing the escape of effete aqueous humour, which thus accumulates. It occurs generally in old persons, and is associated with marked cupping of the optic disc, which is surrounded by a glaucomatous ring. Iridectomy is frequently performed, with a view to restoring the filtering angle, and thus reducing the pressure.
The lens is situated between the portion of the eyeball containing the aqueous humour in front and the portion containing the vitreous behind. The vitreous humour, enclosed in its hyaloid envelope, presents anteriorly a deep concavity, the fossa patellaris, into which the posterior surface of the lens fits. The lens is held in position by its suspensory ligament, derived from the zonule of Zinn (q.v.), and is enclosed in a transparent capsule. This capsule separates it from the aqueous humour, and when it is ruptured by trauma the fluid enters the lens, causing swelling and opacity (traumatic cataract). The posterior surface of the lens is more convex than the anterior, which, however, becomes increasingly convex, owing to the elasticity of the lens, when the capsule is relaxed by the action of the ciliary muscle in accommodation for near objects. This elasticity of the lens diminishes with age, as does likewise the refractive power, and hence the near point (the nearest point from which it is possible to converge the rays upon the retina) becomes more distant. This condition is known as presbyopia, and requires for its treatment a convex lens. In later life diminished energy of the ciliary muscle is also a factor. The axis of the lens runs from before backwards through the centre of the pupil, and its extremities are termed the anterior and posterior poles of the lens, while the equator of the lens is its peripheral circumference. Cataract is the only disease of the lens. It consists of an opacity, which may be partial or complete. The partial forms are generally congenital, while the complete are most frequently due to senile changes, but also arise from diabetes and trauma, especially where the anterior capsule of the lens has been penetrated. The treatment is generally extraction of the affected lens, a strong convex lens being subsequently employed to correct the hypermetropia.
The vitreous body, consisting of a transparent gelatinous material, occupies the portion of the eyeball between lens and retina, which constitutes about four-fifths of the whole. In shape it is roughly spherical, save for the patellar fossa in front, and it is enclosed in the hyaloid membrane, which is in contact with the internal limiting membrane of the retina, and adherent to it at the optic entrance. In front of the ora serrata the hyaloid membrane becomes thickened and constitutes the zonule of Zinn. The zonule presents radiating ridges alternating and fitting between those of the ciliary processes. The ciliary ridges are adherent to the fossae of the zonule, but the ciliary fossae are not adherent to the zonular ridges, lymph spaces intervening. As it approaches the equator of the lens the zonule splits into two layers-an inner, which covers the anterior portion of the vitreous and presentsvthe fossa patellaris, and an outer stronger layer, which blends with the front of the lens capsule near the equator and constitutes the suspensory ligament of the lens. The suspensory ligament is fenestrated, and through the gaps in it the fluid in the anterior chamber can communicate with the canal of Petit (which is a sacculated lymph space surrounding the equator of the lens, situated behind the suspensory ligament), and also with the fossae between the ciliary ridges. A lymph channel, the hyaloid canal of Stilling, which represents the foetal hyaloid artery, runs from the optic papilla, through the vitreous, to the posterior surface of the lens. Sometimes the hyaloid artery persists, but as a rule special treatment is necessary to render even the canal of Stilling visible. The vitreous may be affected by extension of inflammatory processes from other parts ; by suppurative processes from penetrating wounds or the lodgment of foreign bodies ; by haemorrhages, and by undue fluidity. It may shrink from the retina, detachment of the retina frequently following. Musccb volitantes are frequently complained of by myopics, and are due to minute remains of embryonic tissue in the vitreous, and motes are also frequently seen in commencing cataract.
The eyeball derives its blood-supply from branches of the ophthalmic division of the internal carotid. These consist of (1) central artery of the retina, which supplies the retina, and anastomoses slightly at its margin with (2) the short ciliary arteries, some eight in number, which pierce the sclerotic near the optic nerve, and, breaking up into a capillary plexus, supply the chorioid ; (3) the two long ciliary arteries, which, piercing the sclerotic to the outer side of the optic nerve, run forward to the base of the iris, anastomose with the anterior ciliary, and form the circulus arteriosus major, which supplies the ciliary muscle, and send branches into the iris to form the circulus arteriosus minor ; (4) the anterior ciliary arteries, small twigs from the muscular and lachrymal branches of the ophthalmic, which penetrate the sclerotic near the corneal junction, and anastomose with the posterior ciliary. They supply the conjunctiva and the plexus round the circumference of the cornea. This plexus is normally invisible, but in iritis it forms a pink circumcorneal zone of fine, closely-set, nearly parallel vessels.
The canal of Schlemm is a circular venous channel embedded in the corneo-scleral junction, which communicates with the anterior chamber by the spaces of Fontana. Its blood is removed by the anterior ciliary veins, which fall into the venae vorticosae.
The venae vorticosa, some four or five in number, are the chief veins of the eyeball. They run in the outer layer of the chorioid (external to the arteries), and pierce the sclerotic near the equator of the globe, to fall into the ophthalmic vein.
The nerves of the eyeball are derived from the nasal branch of the ophthalmic of the fifth, which sends in two long ciliary nerves, and also supplies a root to the ciliary ganglion (sensory). The motor root of the ganglion is supplied by the third nerve, and there is also a sympathetic root. From the ganglion some twelve short ciliary nerves pass to supply the various coats of the eyeball, the third nerve enervating the ciliary muscle and the circular (contractor) fibres of the iris, and the sympathetic the radial or dilating fibres of the iris.
In inflammatory affections of the globe pain is frequently referred to both upper branches of the fifth, affecting the circumorbital, nasal, and temporal regions, and the upper jaw and teeth, and is accompanied by profuse lachrymation and blepharospasm from communications between the fifth and seventh nerves. Irritation of the nasal branch of the fifth also frequently leads to watering of the eye.
The pupil is contracted by the third nerve, and dilated by the cervical sympathetic, these effects being involuntary, except in so far as they can be brought into action by accommodation. Accommodation is required for near objects, the lens being rendered more convex by the action of the ciliary muscle, acted on by the third nerve, while the iris contracts. One can therefore make the iris contract by accommodating for a near object. In locomotor ataxy the reflexes are lost, and therefore the pupil will not contract to light, as such contraction is reflex, but it will contract on accommodation for near objects, as accommodation is voluntary. This condition is known as the Argyll-Robertson pupil.
The pupil is contracted in normal sleep, in those following occupations necessitating close attention to small work (weaver's eye), in conditions where the brain is engorged with blood, in coma, in bleeding under the tentorium, and in small lesions of the pons. It is also contracted in irritation of the third nerve or paralysis of the cervical sympathetic. Certain drugs also cause contraction, eserine acting locally, morphia probably both locally and generally, and chloroform and alcohol (coma) generally. The effect of chloroform on the pupil is of particular importance surgically, as, when the pupil is contracted and fixed the reflexes are abolished, and one may safely operate. The alcoholic pupil is contracted, but not fixed, as it will slowly dilate on stimulation of the patient.
The pupil is dilated in nightmare, generally in blindness, in anaemia of the brain, faintness, and concussion. It is also dilated by paralysis of the third nerve or irritation of the cervical sympathetic. Atropine causes dilatation by acting locally. The pupil becomes dilated and fixed when chloroform is pushed too far, and, speaking generally, it dilates in all cases where death is impending.
The orbital cavity, consisting of a strong bony margin anteriorly, but of very thin, delicate bones internally, is subject to fractures and to affection by some tumours. The orbital plate of the frontal is frequently broken by extension of fractures of the vault, and by thrusts of sharp-pointed instruments into the eyeball, the anterior lobes of the brain being sometimes thereby involved. Effused blood in such cases may cause proptosis, and ultimately find its way forward under the conjunctiva and under the eyelids. It is distinguished from black eye by its much slower appearance, its patchiness, and by its not being circumscribed at the orbital margins. The inner orbital wall may be similarly fractured, and, if the nasal mucous membrane be torn, may give rise to emphysema of the orbital tissues, while in fracture of the inferior orbital wall the superior maxillary nerve is apt to be damaged, and the antrum of Highmore opened into. The outer and upper margin of the orbit is the most common site of dermoid cysts. Ivory exostoses sometimes occur about the orbital margin, giving rise to displacement of the eyeball, with proptosis. Sarcomata springing from the orbit itself, or invading it from the antrum of Highmore, the base of the sphenoid, or the temporal or zygomatic fossae, have similar effects. The contents of the orbital cavity consist of the eyeball, optic nerve, muscles, nerves, and vessels ensheathed in fatty tissue and fascia.