THE TUNR 



the ciliary processes and iris, forming the pars ciliaris retinae and pars iridica retinae 

 already referred to. This forward prolongation consists of the pigmentary layer 

 of the retina together with a stratum of columnar epithelium. The retina is soft, 

 semitransparent, and of a purple tint in the fresh state, owing to the presence of a 

 coloring material named rhodopsin or visual purple; but it soon becomes clouded, 

 opaque, and bleached when exposed to sunlight. Exactly in the center of the 

 posterior part of the retina, corresponding to the axis of the eye, and at a point 

 in which the sense of vision is most perfect, is an oval yellowish area, the macula 

 lutea; in the macula is a central depression, the fovea centralis (Fig. 879). At the 

 fovea ceritralis the retina is exceedingly thin, and the dark color of the choroid is 

 distinctly seen through it. About 3 mm. to the nasal side of the macula lutse 

 is the entrance of the optic nerve (optic disk}, the circumference of which is slightly 

 raised to form an eminence (colliculus nervi optici) (Fig. 880) ; the arteria centralis 

 retinse pierces the center of the disk. This is the only part of the surface of the 

 retina which is insensitive to light, and it is termed the blind spot. 



Lamina cribrosa Colliculus nervi optici 



_ ___ __ _ Poms opticus 



Retina 



Choroid 



Sclera 



Posterior 



short ciliary 



artery and 



vein 



Pial sheath 



Arachnoid 

 sheath 



Dural sheath 

 Intervaginal spaces 



Bundles of 

 optic nerve 



Central artery and 

 vein of retina 



FIG. 880. The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section. (Toldt.) 



Structure (Figs. 881, 882). The retina consists of an outer pigmented layer and an inner 

 nervous stratum or retina proper. 



The pigmented layer consists of a single stratum of cells. When viewed from the outer surface 

 these cells are smooth and hexagonal in shape; when seen in section each cell consists of an outer 

 non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends 

 as a series of straight thread-like processes between the rods, this being especially the case when 

 the eye is exposed to light. In the eyes of albinos the cells of this layer are destitute of pigment. 



Retina Proper. The nervous structures of the retina proper are supported by a series of non- 

 nervous or sustentacular fibers, and, when examined microscopically by means of sections made 

 perpendicularly to the surface of the retina, are found to consist of seven layers, named from 

 within outward as follows: 



1. Stratum opticum. 



2. Ganglionic layer. 



3. Inner plexiform layer. 



4. Inner nuclear layer, or layer of inner granules. 



5. Outer plexiform layer. 



6. Outer nuclear layer, or layer of outer granules. 



7. Layer of rods and cones. 



1. The stratum opticum or layer of nerve fibers is formed by the expansion of the fibers of the 

 optic nerve ; it is thickest near the porus opticus, gradually diminishing toward the ora serrata. 





