and to the superior maxilla in the case of the internal ligament, while th6 limbs are attached to the upper and lower tarsal plates. The internal tarsal ligament, or tendo oculi, is the stronger, and passes to its insertion in front of the lachrymal sac, while it may send a posterior limb round the lachrymal sac to be inserted into the lachrymal bone. The septum orbitale, which forms a diaphragm separating the superficial from the deep structures of the orbit, is a membranous expansion from the periosteum of the margin of the orbit into the eyelids. In the upper eyelid it blends with the tendon of the levator palpebra? superioris, and in the lower with the tarsal cartilage. Externally it forms the external tarsal ligament, while internally it diverges from the internal tarsal ligament, and passes behind the lachrymal sac to be inserted into the lachrymal bone.
Congenital defects of the eyelids are uncommon. Epican-thus is the term applied to a semilunar fold of skin extending over the inner canthus from the upper to the lower eyelid ; ankylo-blepharon is a fusion of the two eyelids ; coloboma, a vertical fissure, sometimes associated with a corneal dermoid. Chalazion is a granulomatous tumour which occurs in the substance of the eyelid, generally the upper.
The conjunctiva is a delicate mucous membrane, covered with epithelium, which lines the inner surfaces of the eyelids and the front of the globe. The reflections from the eyelids to the globe are called the superior and inferior fornices. Only the epithelial layer covers the front of the cornea, the connective tissue layers ceasing at the corneal margin. The caruncle at the inner canthus is formed from conjunctiva, and just external to the caruncle is the plica semilunaris, a fold of the membrane which disappears on full external rotation of the eyeball, to permit of which movement it exists. It corresponds to the nictitating membrane of birds. The conjunctiva is but loosely attached to the eyeball, and use is made of this fact in plastic operations for the relief of symble-pharon, in which, owing to injury, the eyelids become adherent to the eyeball. This laxness readily permits of oedematous swelling of the conjunctiva (chemosis) occurring, as well as of subconjunctival hemorrhage from rupture of the unsupported vessels.
The lachrymal and two palpebral branches of the ophthalmie branch of the internal carotid supply the conjunctiva with blood, while it is supplied by four nerves : above, the supratrochlear ; outside, the lachrymal ; inside, the infra-trochlear (all derived from the ophthalmic division of the fifth) ; and below, by the palpebral branches of the superior maxillary of the fifth.
The conjunctiva is subject to both acute and chronic inflammation. In the acute form the conjunctiva becomes deeply congested, the congestion generally being arrested at the corneal margin, where the normal conjunctival vessels terminate. In iritis a zone of circumcorneal inflammation is also seen, but the individual vessels cannot be distinguished ; whereas in conjunctivitis they are not only readily distinguishable, but can be moved with the conjunctiva on the cornea by manipulation of the lower eyelid.
Trachoma, or chronic granular conjunctivitis, is most often met with among the poor, and frequently gives rise to entropion. Pterygium is a triangular vascularized thickening of conjunctiva, occurring most frequently to the inner side of the cornea, upon which its apex abuts and over which it may grow.
The lachrymal gland, which, occupying the fovea lachrymalis of the frontal bone, lies at the upper and outer part of the orbit behind the conjunctiva, secretes the tears which serve to keep the conjunctiva moist, and to remove small particles which may have settled on the surface. The gland consists of two lobes, separated by a process of Tenon's capsule. The orbital lobe is the larger and more important ; the smaller is known as the palpebral portion. Excision of the gland may be performed for tumour or intractable epiphora (overflow of tears on to the cheek), sufficient moisture being subsequently secreted by small detached lobules. The gland is supplied by the lachrymal of the fifth, stimulation of which, either directly or reflexly through particularly the nasal branches, causes profuse lachrymation.
The tears are conveyed from the gland to the surface of the conjunctiva by numerous small ducts which pierce the conjunctiva at its reflection on to the upper eyelid. From thence they are carried across the front of the eyeball by gravity, capillarity, and the frequent blinking action of the eyelids, to the inner angle, where, save when the secretion is excessive, they enter the lachrymal canaliculi through the lachrymal puncta on the eyelids. The canaliculi, about ^ inch long, run first vertically, the upper upwards and the lower downwards, then horizontally inwards and finally open close together or by a single opening into the lachrymal sac. The lachrymal sac occupies the lachrymal groove, lying upon the orbital septum (which is attached to the crest of the lachrymal bone), the lachrymal bone, and a portion of the nasal process of the superior maxilla, while in front it is crossed above by the tendo oculi, but below is comparatively superficial, lying under the skin and orbicularis muscle. Its junction with the lachrymal canal is the narrowest portion of the common apparatus. The nasal duct continues from the sac through an osseous canal, the lachrymal canal, formed by the lachrymal and ascending process of the superior maxilla above and the inferior turbinate and superior maxilla below, to open by a small aperture in the mucous membrane of the inferior meatus of the nose. Its direction downwards and backwards is indicated by a line from the commencement of the lachrymal canal to the first upper molar. The lachrymal passages are liable to inflammatory infection from the conjunctiva and the nose, which parts may infect one another through this channel. Inflammation and injury of the duct tend to cause blocking and consequent overflow of tears (epiphora). Sometimes also the lachrymal sac becomes inflamed and distended (dacryocystitis), which may even go on to suppuration, the abscess pointing below the tendo oculi.
The ocular muscles, with the exception of the inferior oblique, take origin from a common tendon, which nearly surrounds the optic foramen. The inferior oblique arises from the antero-nasal aspect of the orbital floor, and then, passing backwards and outwards under the rectus inferior, turns up between the rectus externus and the eyeball, to be inserted into the posterior temporal aspect of the eyeball. The superior oblique passes forwards between the recti superior and internus to the inner side of the anterior margin of the orbit, where it passes through a pulley, and is then directed down and outwards beneath the tendon of the rectus superior, to be inserted a little above and anterior to the inferior oblique into the temporal aspect of the eyeball between the superior and external recti, and midway between the cornea and entrance of the optic nerve.
The internal and external recti, having median insertions, rotate the eye upon its true vertical axis inwards and outwards respectively. The superior and inferior recti, being inserted external to the median position, rotate the eyeball upon a horizontal axis, which, instead of being strictly coronal, is directed backwards and outwards. When acting alone, therefore, in addition to turning the eyeball up and down, they also turn it to the nasal side.
The superior and inferior oblique muscles rotate the eyebalj round a horizontal axis which is likewise not coronal, but is inclined forwards and outwards nearly at right angles (75 degrees) to the axis of the recti muscles. The oblique muscles accordingly turn the eyeball down and up, and at the same time turn it toward the temporal side. Hence, to obtain a movement of the eyeball directly upwards, the superior rectus and inferior oblique act together, the inward tendency of the oblique being corrected by the outward tendency of the rectus, and similarly, in moving the eyeball directly downwards, the inferior rectus and superior oblique act together.