NORMAL HISTOLOGY. 



which it is developed. The fundamental divisions of the retina are: (i) the 

 external lamella, the pigmcntcd layer on the outer surface; and (2) the 

 internal lamella, which includes the remaining layers of the retina. The 

 inner lamella may be subdivided further into the neuroepithelial and the 

 cerebral layers. Sections of the retina (Fig. 397) show under the microscope 

 from without inwards the following layers: 



I. OUTER LAMELLA OF OPTIC 

 VESICLE 



II. INNER LAMELLA OF OPTIC 

 VESICLE 



Pigmented layer 



> Pigmented layer 



f 2. Layer of rods and cones ") Neuro- 



3. Layer of bodies of visual cells, or \ epithelial 



outer nuclear layer 



4. Outer plexiform layer 



5. Layer of bipolar cells, or inner 



nuclear layer 



6. Inner plexiform layer 



7. Layer of ganglion-cells 



8. Layer of nerve-fibres 



J layer 



1 Cerebral 

 layer 



Fibre of Miiller 



To these nervous layers must be added two delicate membranes, (i) the 

 membrana limitans internet, which bounds the inner surface of the retina, 



and (2) the membrana lim- 

 itans externa, which lies be- 

 tween the outer nuclear 

 layer and the layer of rods 

 and cones. These mem- 

 branes represent the termi- 

 nal portions of the support- 

 ing neurogliar fibres, or 

 fibres of Miiller. 



The pigmented layer, 

 formed of deeply pigmented 

 cells, constitutes the most 

 external layer of the retina 

 and represents the outer 

 wall of the foetal optic vesi- 

 cle. It is composed of 

 hexagonal cells, from 12- 

 1 8 ^ in diameter, the proto- 

 plasm of which is loaded 

 with fine, needle-shaped 

 crystals of pigment (f?ts- 

 ciri). The outer portion of 

 the cells is almost free from 

 pigment and contains the 

 nucleus. From the inner 

 border fine protoplasmic 

 processes extend inwards 



Internal limiting: 

 membrane 



Ganglion cell 



Fibres of Miiller 



Inner plexiform 

 layer 



Bipolar nerve- 

 cells 



Blood-vessel 



Outer plexiform 

 layer 



Layer of visual 

 cells 



Nucleus of cone- 

 cell 



Pigment layer 



FIG. 397. Section of human retina, near posterior pole of 

 eyeball. X 230. 



between the rods and cones 

 of the neuroepithelial layer, 

 and under the influence of 

 light the pigment particles 



wander into these processes and thus surround the percipient elements. 



The layer of rods and cones, although usually described as a distinct 

 stratum, is onlv the highly specialized outer zone of the layer of visual cells 



