240 HUMAN PHYSIOLOGY. 



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

 are arranged in two sets the sphincter and dilatator. 



The sphincter pupilla is a circular, flat band of muscle-fibers sur- 

 rounding 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 T ^ of an inch long running from before back- 

 ward. It is attached anteriorly to the inner surface of the sclerotic 

 and cornea, and posteriorly 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 accommodation 

 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 ora 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, the fovea centralis, corresponding to a decrease 

 in thickness of the retina ; about -fa of an inch to the inner side of 

 the macula is the point of entrance of the optic nerves. The arteria 

 centralis retina pierces the optic nerve near the sclerotic, runs for- 

 ward in its substance, and is distributed 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. 



5. Internal granular layer. 



