This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1881. Excerpt: ... CHAPTER XVIII. THE EYE. The organ of sight is Composed of the eyeball and various accessory structures, such as the eyelids, lachrymal gland, muscles, etc. The eyeball is composed, in the first place, of a dense, firm, spheroidal connective-tissue envelope, whose anterior transparent portion, the cornea, is more convex than the posterior opaque segment, the sclerotic, and differs somewhat from it in structure; the sclerotic is pierced posteriorly by the optic nerve. Within the sclerotic lies a vascular tunic, the choroid, formed of several layers of tissue, and thrown anteriorly, just behind the sclero-corneal junction, into numerous longitudinal folds, called the ciliary processes. An extension from the ciliary processes passes forward, constituting the iris, which is a perforated vascular connective-tissue and muscular curtain, suspended behind the cornea, and connected peripherally, near the sclero-corneal junction, with a connective-tissue structure called the ligamentutn pectinatum. Passing backward from the ligamentum pectinaturn, between the ciliary processes and the sclera, and attached posteriorly to the choroid, is a muscle having the form of a flattened ring, thickest in front, called the ciliary muscle. The direction of the muscle-cells in the ciliary muscle, which are of the smooth variety, is in part meridional or oblique, in part circular. The ciliary processes and muscle form together the greater part of a structure known as the ciliary body. The retina, the innermost of the layers forming the wall of the eyeball, spreads out from the point of entrance of the optic nerve over the inner surface of the choroid. At about a third of the distance back from the front of the eye, the nerve-elements of the retina cease in a wavy line, called the o...