SENSE OF SIGHT. 215 



cells continuous with those covering the posterior surface of the cornea; the 

 posterior surface is lined by a limiting membrane bearing pigment epithelial 

 cells continuous with those of the choroid. The various colors which the iris 

 assumes in different individuals depend upon the quantity and disposition of 

 the pigment granules. 



The muscle-fibers of the iris, which are of the non-striated variety, are 

 arranged in two sets the sphincter and dilatator. 



The sphincter pupillce is a circular, flat band of muscle-fibers surrounding 

 the pupil close to its posterior surface; by its contraction and relaxation the 

 pupil is diminished or increased in size. The dilatator pupillce consists of a 

 thin layer of fibers arranged in a radiate manner; at the margin of the pupil 

 they blend with those of the sphincter muscle, while at the outer border they 

 arch to form a circular muscular layer. 



The ciliary muscle is a gray, circular band, consisting of unstriped muscle- 

 fibers about y^ of an inch long running from before backward. It is at- 

 tached anteriorly to the inner surface of the sclerotic and cornea, and poste- 

 riorly to the choroid coat opposite the ciliary processes. At the anterior 

 border of the radiating fibers and internally are found bundles of circular 

 muscle-fibers, constituting the annular muscle of Miiller. The ciliary muscle 

 thus consists of two sets of fibers, a radiating and a circular, both of which are 

 concerned in effecting a change in the convexity of the lens in the accommo- 

 dation of the eye to near vision. 



The retina forms the internal coat of the eye. In the fresh state it is a 

 delicate, transparent membrane of a pink color, but after death soon becomes 

 opaque; it extends forward almost to the ciliary processes, where it terminates 

 in an indented border, the or a serrata. In the posterior part of the retina, at 

 a point corresponding to the axis of vision, is a yellow spot, the macula lutea, 

 which is somewhat oval in shape and tinged with yellow pigment. It presents 

 in its center a depression, ihefovea centralis, corresponding to a decrease in 

 thickness of the retina; about JQ of an inch to the inner side of the macula is 

 the point of entrance of the optic nerves. The arteria centralis retince pierces 

 the optic nerve near the sclerotic, runs forward in its substance, and is dis- 

 tributed in the retina as far forward as the ciliary processes. 



The retina is remarkably complex, consisting of ten distinct layers, from 

 within outward, supported by connective tissue. These are as follows 

 viz.: 



1. Membrana limitans interna. 



2. Fibers of optic nerve. 



3. Layers of ganglionic corpuscles. 



4. Molecular layer. 



