6 4 6 



TEXT-BOOK OF PHYSIOLOGY. 



8. The layer of ganglion cells. 



9. The layer of nerve-fibers. 



Modern histologic methods of research have made it possible to reduce the 

 retina, exclusive of the pigment cells, to three successive layers of nerve-cells, 

 supported by a highly developed neuroglia, forming what has been termed the 

 fibers of Miiller. These nerve-cells are as follows: 



1. The visual cells. 



2. The bipolar cells. 



3. The ganglion cells. 



The relation of these nerve-cells one to another and to the supporting neuro- 

 glia tissue and the manner in which they unite to form the above-mentioned 

 layers are schematically shown in Fig. 305. 



The pigment layer* is composed of 

 hexagonal cells. Though formerly 

 described as forming a part (the inner 

 layer) of the chorioid, these cells belong 

 embryologically to the retina. From 

 their retinal surf ace delicate pigmented 

 processes extend into and between 

 the rods and cones. On exposure to 

 light these procesess elongate and 

 push themselves between the rods. 

 In the dark they retract and withdraw 

 into the cell-body. 



The visual cells which form the 

 layer of rods and cones are of two varie- 

 ties, the rod-shaped and the cone- 

 shaped. 



The rod-shaped visual cell con- 

 sists of a straight elongated cylinder 

 extending through the entire thickness 

 of Jacobson's membrane and a fine 

 fiber containing a nucleus, which, 

 after piercing the external limiting 

 membrane, passes into the outer molec- 

 ular layer, where it terminates in a 

 spheric enlargement. The outer por- 

 tion of the rod is clear and homo- 

 geneous, though containing a pigment 

 known as visual purple or rhodopsin; 

 the inner portion of the rod is slightly granular. 



The cone-shaped visual cells also consist of two portions, a conic portion 

 situated in Jacobson's membrane between the rods, and a fine fiber, contain- 

 ing a nucleus, which, after piercing the external limiting membrane, passes 

 into the outer molecular layer, where it terminates in a fine tuft. The 

 inner portion of the cone is thicker than the rod and rests on the limiting 

 membrane; the outer portion tapers to a fine point and is known as the 

 cone-style. The cones, as a rule, are shorter than the rods. The proportion 

 of rods to cones varies in different parts of the retina, though there are on 



FIG. 305. CROSS-SECTION OF THE RETINA 

 FROM A MAMMAL. A. Layer of rods and 

 cones. B. Visual cells (outer granules). 

 C. Outer molecular layer. E. Bipolar cells 

 (inner granules). F. Inner molecular layer. 

 G. Ganglion cells. H. Layer of nerve- 

 fibers, a. Rods. b. Cones, e. Bipolar 

 rod. f. Bipolar cone. r. Lower ramifica- 

 tion of a bipolar rod. f. Lower ramification 

 of a bipolar cone, g, h, i, j, k. Ganglion 

 cells in various stages, branching from F. 

 x, z. Bipolar contact of rods and cones, t. 

 Muller's supporting fibers. S. Centrifugal 

 nerve-fibers. (After Ramon y Cajal.} 



