122 



BIOPHYSICAL CHARACTERISTICS OF THE EYE 



III— 12). The radiant energy passes through them in the following 

 order to reach the photoreceptor layer of rods and cones bounded on 

 the external side by the pigment epithelium lying adjacent to the 

 choroid: (1) the internal limiting membrane, (2) the optic nerve fiber 

 layer, (3) the ganglion cell layer, (4) the inner plexiform layer, (5) the 

 inner nuclear (bipolar cells) layer, (6) the external plexiform layer, 



, ... (7) the outer nuclear layer, (8) the external 



io [p,'(*)..° . | limiting membrane, (9) the photoreceptor 



layer, and (10) the pigment layer. The first 

 six layers are grouped as contained in the 

 cerebral portion and the last four in the neu- 

 roephithelial portion of the structure. 



From a biophysical point of view the chief 

 interest centers on the visual cells (9) and 

 pigmented epithelium (10) as the basic ele- 

 ments concerned with the interception of the 

 radiant energy and the use of the absorbed 

 energy to excite the nerve impulses that are 

 propagated along the nerve fibers. 



8- 



fr 



\ 



Retinal Receptor Mechanism 



The rod and cone visual cells are radially 

 packed in a shell lying between the external 

 limiting membrane and the pigmentary 

 epithelium (10). Their photosensitive seg- 

 ments are turned radially outward with 



Fig. 111-12. Grouping of the neurons in the human 

 retina into functional systems. The incident light 

 passes through the following layers: 1. Internal 

 limiting membrane next to the vitreous humor. 2. 

 Layer of optic never fibers. 3. Layer of ganglion cells 

 which receive the nerve impulses from the bipolar cells 

 above them. 4. Inner plexiform layer. 5. Inner 

 nuclear or bipolar cell layer. 6. External plexiform 

 layer. 7. Outer nuclear layer. 8. External limiting 

 membrane. 9. Photoreceptor layer. Cones and rods 

 mixed. Cones resemble flasks with narrow necks. 

 They are about one-sixth shorter than the slender, 

 nearly cylindrical rods. Rods about 2 microns thick 

 and 60 microns long. The nerve impulses are sup- 

 posed to originate in them as a result of the absorption 

 of the incident radiant energy. 10. Pigment layer. 

 Single pigmented cell, vertical section, hexagonally 

 packed. Color, dark brown. 



