550 THE NERVOUS SYSTEM. 



nervous layer of the retina, later passing into the retina and 

 ending in a capillary plexus external to the inner nuclear layer. 

 During fetal life a small artery passes forward through the 

 vitreous to the posterior surface of the capsule of the lens. In 

 the fovea centralis there are no blood-vessels, and very few in the 

 rest of the macula lutea. 



Minute Structure of the Pars Optica Retince (Figs. 329, 330, 

 331). The pars optica of the retina consists of ten layers, arranged 

 in the following order, beginning with the most internal /. e. t with 

 the layer next to the hyaloid membrane which encloses the vitreous 

 humor and ending with the most external, the pigmentary layer 

 which is contiguous to the lamina vitrea of the choroid : 



1. Membrana limitans interna; 



2. Nerve-fiber layer ; 



3. Ganglionic layer ; 



4. Inner molecular layer ; 



5. Inner nuclear layer ; 



6. Outer molecular layer ; 



7. Outer nuclear layer ; 



8. Membrana limitans externa ; 



9. Layer of rods and cones; 

 10. Pigmentary layer. 



1 . Membrana Limitans Interna. This is a transparent mem- 

 brane, and a part of the supporting connective tissue of the retina, 

 or the fibers of Muller, in connection with which it is again 

 referred to (p. 553). 



2. Nerve-fiber Layer. This is sometimes described as the fibrous 

 layer. It is composed, however, of nerve-fibers, and not of so-called 

 fibrous tissue. The fibers of which it is composed are those of the 

 optic nerve, and, as this enters the eye from behind, these fibers 

 must pass through all the layers, excepting the membrana limitans 

 interna, in order to reach the nerve-fiber layer. It will be re- 

 membered that the fibers of the optic nerve pass through the 

 lamina cribrosa of the sclerotic ; at this part of their course their 

 medullary sheaths disappear and they become simple axis-cyl- 

 inders, and as such they traverse the choroid and the retina until 

 they reach the nerve-fiber layer, where they radiate from the point 

 of entrance and terminate, some in the cells of the third or gan- 

 glionic layer, while others pass through the third and fourth layers 

 and terminate in the fifth or inner nuclear layer. The nerve-fiber 

 layer is thickest at the point of entrance of the optic nerve, and, 

 gradually becoming thinner, ends at the ora serrata i. e., anterior 

 to this, nerve-fibers are not found in the retina. Or this may be 

 stated in another way, by saying that nerve-fibers are found in the 

 pars optica retinae, but not in the pars ciliaris retinae. 



3. Ganglionic Layer. This is called also vesicular layer and 

 layer of nerve-cells, and consists of a single layer of large ganglion- 



