STRUCTURE OF THE EYE. 357 



RETINA. 



The retina extends from the papilla of the optic nerve to the pupillary 

 border of the iris, and is divisible into three parts; the pars optica retinae 

 includes all which is actually connected with the optic nerve and which 

 therefore is sensitive to light. It covers the deeper portion of the optic 

 cup, ending near the ciliary body in a macroscopic sharp, irregular line 

 bounding the ora serrata. The pars ciliaris and the pars iridica retinae 

 are the rudimentary layers covering the ciliary body and iris respectively. 



The pars optica retinae in a fresh condition is a transparent layer 

 colored reddish by the " visual purple." In sections it presents many layers 

 arranged as seen in Fig. 411, the cells of which are related to one another 

 as in the diagram, Fig. 412. The outer layer of the optic cup forms the 

 pigmented epithelium of the retina, which consists of a simple layer of six- 

 sided cells. Toward their outer surface (that next the chorioid, where the 

 nucleus lies) they are poor in pigment, whereas in their inner portion they 

 contain numerous rod-shaped (1-5 // long) brown granules of the pigment 

 "fuscin." In albinos the pigment is lacking. From the inner surface of 

 the pigmented epithelium numerous processes extend between the rods and 

 cones. 



The visual cells, which are found along the outer surface of the inner 

 retinal layer, are of two sorts, rod cells and cone cells. In both, the nucleus 

 is found in the inner half of the cell, and the outer non-nucleated half 

 projects through a membrane, the membrana limitans externa. This 

 causes the visual cells to appear divided into two layers, their nucleated 

 parts beneath the limiting membrane constituting the outer nuclear layer 

 (or outer granular layer) , and the non-nucleated parts outside of the mem- 

 brane forming the layer of rods and cones. 



The rods are four times as numerous as the cones. They are regularly 

 placed so that three or four rods are found between every two cones (Fig. 

 411). The rods are elongated cylinders (60 JJL long and 2 fi thick) con- 

 sisting of a homogeneous outer segment in which the visual purple is found 

 exclusively, and a finely granular inner segment. In the outer third of 

 the inner segment there is said to be an ellipsoid, vertically striated struc- 

 ture (which in some lower vertebrates is very distinct). The portion of the 

 rod cells below the limiting membrane is a slender thread, expanding to 

 surround the nucleus which is characterized by from one to three trans- 

 verse bands. Beneath the nucleus the protoplasm again becomes thread- 

 like and terminates in a small club-shaped enlargement without processes 

 (Fig. 412). 



The cones likewise consist of an outer and an inner segment. The 



