STRUCTURE OF THE RETINA 



637 



At the pars ciliaris retinae, the retina 

 is represented by a layer of columnar 

 cells, derived from the fusion of the 

 nuclear layers which are in contact with 

 the pigment layers of the retina and con- 

 tinued over the ciliary processes. 



Pigment Layer. This layer, which 

 was formerly considered part of the 

 choroid, consists of cells which cover 

 and entirely surround the outer limbs 

 of the rods and cones. 



Blood-vessels oj the Eyeball. The 

 eye is very richly supplied with blood- 

 vessels. In addition to the conjunc- 

 tival vessels, which are derived from the 

 palpebral and lachrymal arteries, there 

 are at least two other distinct sets 

 of vessels supplying the tunics of the 

 eyeball, i, The vessels of the sclerotic, 

 choroid, and iris, and 2, the vessels of 

 the retina. The first are the short and 

 long posterior ciliary arteries which 

 pierce the sclerotic in the posterior half 

 of the eyeball, and the anterior ciliary 

 which enter near the insertions of the 

 recti. These vessels anastomose and 

 form a very rich choroidal plexus; 

 they also supply the iris and ciliary 

 processes, forming a very highly vas- 

 cular circle round the outer margin 

 of the iris and adjoining portion of 

 the sclerotic. The distinctness of 

 these vessels from those of the con- 

 junctiva is well seen in the difference 

 between the bright red of blood-shot 

 eyes (conjunctival congestion), and the 

 pink zone surrounding the cornea 

 which indicates deep-seated ciliary con- 

 gestion. 



The central artery of the optic nerve 

 enters the retina from the center of 

 the optic disc and sends out branches 

 over the retinal cup lying in the nerve 



FIG. 450. Diagrammatic Section of the 

 Macula Lutea and Fovea Centralis. 2, 

 Layer of nerve fibers; 3, layer of multi- 

 polar cells; 4, internal molecular layer, 

 composed of intertwining arborescent proc- 

 esses; 5, layer of bipolar cells, or internal 

 granular layer; 6, external molecular layer, 

 composed of intertwining arborescent proc- 

 esses; 7, nuclei of epithelial cells, or ex- 

 ernal granular layer; 8, frillwork formed 

 by processes from fibers of Miiller, often 

 called the "external limiting membrane "; 

 9, layer of rods and cones; 10, layer of 

 pigment epithelium. 



