THE SENSE OF SIGHT 



683 



1. The layer of pigment cells. 



2. The layer of rods and cones, or Jacobson's layer. 



3. The external limiting membrane. 



4. The outer nuclear or granular layer. 



5. The outer molecular or reticular layer. 



6. The inner nuclear or granular layer. 



7. The inner molecular or reticular layer. 



8. The layer of ganglion cells, 

 o. 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 

 fibers of Muller. 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 neu 

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

 layers are schematically shown in Fig. 289. 



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- 



& $ S?3 rsj&.&T4sx 



Of Jacobson's membrane and a fane ^ inner granu i es ). F. Inner molecular layer. 



fiber containing a nucleus, which, 



FIG. 289. CROSS-SECTION OF THE RETINA 

 FROM A MAMMAL. A. Layer of rods and 



^GangUon ceU, 

 n . f ^ 





Lower ramifica . 



containing 



after piercing the external limiting f ^ ^^ ^ 



membrane, passes into the outer mo- tion of a bipolar rod. I 



lecular iayer! where it terminate , a ^ ^ Lgl 



spheric enlargement. 1 he outer poi ^ z Bipolar contact of rods and cones t. 



tinn of the rod is clear and homo- Muller's supporting fibers. S. Centrifugal 



geneous, though containing a pigment nerve-fiber,-^ , <W 

 known as visual purple or rhodopsm; 



