.Vitreus, corpus vitreum, is a transparent jelly-like substance consisting of an exceedingly delicate meshwork, the interstices of which are filled out by fluid. It has the transparency of water—may be felt in a basin of water but not seen. It contains no blood vessels, no nerves, and depends for its nourishment on adjoining structures, principally the ciliary body: A canal beginning at the front of the papilla extends to the lens — Cloquet's canal. The fibrillae making up the stroma of the vitteus have their origin just anterior to the ora ser rata, on the orbicularis ciliaris. Diseases of the ciliary body readily affect the vitreus.
The crystalline lens lies in a della—fossa pa tellaris — on the anterior surface of the vitreus. (Fig. 1). It is a transparent body, having the shape of a biconvex lens. The centre of the anterior surface is designated the anterior pole, a similar point on the posterior surface of the posterior pole. The lens capsule is a typical glass membrane, like that of Descemet or the lamina vitrea of the chorioid. Lining the inner surface of the anterior capsule is a layer of cuboid epithelial cells. At the equator of the lens these elongate to form the lens fibres. Those layers of the lens substance next to the capsule comprise the cortex. At the centre is a harder, denser mass, the nucleus. The lens grows larger throughout life tc make room for the ever-increasing number of lens fibres within its body.
Posteriorly the lens is held to the anterior border layer of the vitreus by a firm union in the shape of a ring—Ligament= hyaloideo, oapsnlare. The zonular fibres are the main sup port of the lens. They are structureless, non nucleated fibres, clear as glass. They are firmly attached to the ciliary body on the one side and to the anterior and posterior surface of the lens, near the equator, on the other side. (Fig. 1).
The cornea and lens are the media of the eye principally concerned in the transmission of light to the retina and in the formation of images upon it. The rays of light striking the ,cornea are refracted to pass through the pupil. The lens now brings them to a focus on the retina, producing an inverted image. The lens by the action of the cilliary body adjusts itself to focusing objects at different distances — accommodation. When the ciliary muscle con tracts the lens increases its convexity and shortens its focus, as in the act of reading. At about 45 years of age the lens, owing to the sclerosis of its fibres, begins to lose its elasticity. This is the condition of presbyopia -- old sight. In myopia the focal image is
formed in front of the retina, usually due to too great length of the antero-posterior diam eter of the globe. In hyperopia the eyeball is too short. Astigmatism is the condition in which the rays of light do not converge to a point on the retina. It is ordinarily due to inequality of curvature of the cornea (or lens).
By the field of vision we mean the space in which one can see, while steadily gazing at a point in the direct line of vision. The field does not extend regularly in all directions. It reaches farthest toward the external side, where it extends over 90°. The field for colors grows smaller in the following order: blue, red and green. Defects in the visual field are termed scotomata.
The orbit is the pyramidal cavity in which the globe lies. Its walls are formed by seven bones of the face; namely, frontal, sphenoid, ethmoid, nasal, lacrimal, superior maxillary, and palate bones. These bony walls separate the orbit from the following cavities: the in tracranial, the frontal, the nasal, and the antrum of Highmore. Diseases of the sinuses frequently involve the orbit. The anterior opening of the orbit is its base. Here the walls become thickened into a strong bony rim margin of the orbit — to defend the eyeball against injury. At the upper margin of the orbit is the supra-orbital notch for the passage of artery and nerve of the same name. At the lower margin is another notch for the infra orbital artery and nerve.
The globe is lodged in the orbit in a cushion of fat, and is held in place by connective tissue, the ocular muscles, and the eyelids. The periosteum of the orbit extends over its margin anteriorly to form the fascia tarso-orbitalis. Surrounding the posterior two-thirds of the eyeball, the connective tissue of the orbit be comes condensed into a capsule (Tenon's capsule above mentioned). (Fig. 2).
Posteriorly the orbit has three apertures: (1) optic foramen for the optic nerve and ophthalmic artery; (2) superior orbital fissure, opening into the middle fossa of the skull and transmitting nerves for the ocul‘r muscles and the first branch of the trigeminus; (3) the inferior orbital fissure, connecting the orbit with the temporal fossa and serving for the passage of the second branch of the trigimin?l nerve. Near the apex of the orbit is the ciliary ganglion, for the supply of the ciliary muscle and iris. Abnormal protrusion of the eye is exophthalmus. Absence of the bulb is anoph thalmus.