The circular area in the inner opening of the optic nerve canal, corresponding to the area of the lamina cribrosa, is called the disc or papilla. This space, consisting of conducting fibres only, receives no impressions, and is known as Mariotte's blind spot. The cupping or hollow ing of the disc may be physiological, or patho logical as in glaucoma. An arterial ring— Circle of Zinn surrounds the optic nerve en trance into the sclera.
The ciliary body, with a breadth of 5 or 6 mm., forms a girdle around the eyeball, cor responding to the middle zone. It is divided into a flat, broader posterior part — orbiculus ciliaris — which is succeeded anteriorly by a system of some 70 processes—corona ciliaris. Between the ciliary processes are the ciliary valleys.
In the equatorial region, in the supracho rioidal lathillae smooth muscle fibres make their appearance, developing anteriorly into the ciliary muscle — or muscle of accommodation. The outer bundles of the muscle run in a meri dians' direction. This is replaced inwardly by a system of circular bundles — Mueller's muscle. In myopia the meridianal bundles are more developed; in hyperopia the circular bundles.
A layer of blood vessels covers the inner surface of the muscle, being a direct continua tion of the chorioid. Over this vascular layer the lamina vitrea of the chorioid and the pig ment epithelium spread out. The innner sur face of the ciliary body is lined by a single layer of epithelium—which represents the retina. This ciliary epithelium is everywhere very uneven, due to tiny elevations and depres sions to which it clings — the whole arrange ment forming the reticultun of Heinrich Muel ler. Lastly, the membrana limitans interns comes forward to clothe the ciliary epithelium, just as it clothes the retina posteriorly. The ciliary epithelium secretes the aqueous and has largely to do with the nourishment of the lens and vitreous.
The iris has its insertion on the anterior surface of the ciliary body. (Fig. 1). Severed from connections it has the form of a circular plate — like the diaphragm in optical instru ments. The outer margin is the ciliary border. The circular opening in the middle is the pupil. This latter is located slightly to the nasal side.
The anterior surface is divided by . a zig-zag line 1.5 mm. from the pupillary margin, into an inner, pupillary zone, and an outer ciliary zone. Trabeculae.of connective tissue containing ves sels traverse the iris, converging toward the centre, like the spokes in a wheel. Depressions between the trabeculae are termed crypts. In the ciliary zone running circularly are the con traction furrows, corresponding to the creases in the palm of the hand. The lesser arterial circle of the iris is situated under the zig-zag line. It is formed by an anastomosis from the radiating vessels which enter the root of the iris. These vessels are derived from the greater arterial circle of the iris, which is located in the vascular layer of the ciliary body. (Fig. 3). The greater arterial circle is in turn derived from an anastamosis of branches from the two long posterior ciliary arteries. When the stroma is delicate the sphincter muscle may be seen as a whitish band, 1 mm. in width, immediately adjoining the pupillary margin.
The stroma bearing the blood vessels and comprising the bulk of the iris is a continuation of the chorioid, by way of the vascular layer of the ciliary body. The pigment epithelium re mains the same; while the ciliary epithelium takes on pigment; so that the structures poste rior to the iris are protected from the light by two strata of highly pigmented cells. The sphincter and the dilitater muscles are of ectodermic origin formed by a differentiation of the pigment epithelium. Other muscles of the body are of mesodermic origin. The posterior surface of the iris has a very dericatei system of radiating and circular marlcings.
The iris varies greatly in individuals, When the stroma is loose and sparse and contains few chromatophores the result is the blue iris, because the black pigment epithelium through the delicate veil anteriorly appears bluish— the blue eye. When the stroma is heavy and the chromatuphores numerous we have the brown iris,- here the chromatophores deter mining the color. Aside from the color, there is, therefore, an anatomical difference— the blue iris being the light thin one; the brown iris the heavy thick one.