THE NERVOUS TUNIC. 



1463 



1.7 mm. in its vertical diameter. At its centre is often seen a well-marked excavation, 

 the optic cup, from the bottom of which emerge the blood-vessels which supply the retina. 

 Being insensible to visual impulses, the optic entrance corresponds to the "blind-spot." 

 Structure of the Retina. The retina is composed of nervous elements which 

 are supported by a specialized sustentacular tissue or neuroglia. Morphologically it 

 must be considered as composed of two lamellae, which correspond to the outer and 

 inner walls of the optic vesicle (page 1482) from which it is developed. These fun- 

 damental divisions of the retina are : ( i ) the external lamella, the pigmented 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, made perpendicularly to its surface 

 (Fig. 1220), show under the microscope from without inward the following layers : 



I. OUTER LAYER OF OPTIC 

 VESICLE 



II. INNER LAYER OF OPTIC 

 VESICLE 



I'' 



LAYERS OF THE RETINA. 

 Pigmented layer 



> Pigmented layer 



2. Layer of rods and cones 



3. Layer of bodies of visual cells or 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 



Neuro- 

 epithelial 

 layer 



Cerebral 

 layer 



FIG. 1219. 



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

 limitans interna, which bounds the inner surface of the retina, and (2) the membrana 

 limitans externa, which lies between the outer nuclear layer and the layer of rods 

 and cones. These membranes represent the terminal portions of the supporting neu- 

 rogliar 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 vesicle. 

 It is composed of hexagonal cells, from .012 .018 mm. 

 in diameter, the protoplasm of which is loaded with fine, 

 needle-shaped crystals of pigment {fuschi). The outer 

 portion of the cells is almost free from pigment and con- 

 tains the nucleus. From the inner border fine proto- 

 plasmic processes extend inward between the rods and 

 cones of the neuroepithelial layer. Under the influence 

 of light, the pigment particles wander into these processes 

 and, under such conditions, the pigmented cells may 

 remain attached to the retina when the latter is separated 

 from the choroid. Ordinarily, the pigmented layer ad- 

 heres to the choroid and, hence, was formerly considered 

 to be a part of that membrane. The pigmented cells are 

 separated by a distinct intercellular cement substance and in some of the lower 

 animals contain colored oil droplets and particles of a highly refracting myelin-like 

 substance (inyeloid granules of Kiihne). 



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

 is only the highly specialized outer zone of the layer of visual cells and, therefore, 

 constitutes the outer portion of the neuroepithelial division of the retina. It is com- 

 posed, as its name indicates, of two elements, the rods and the cones, which are the 

 outer ends of the rod and cone visual cells. They are closely set, with their long 

 axes perpendicular to the surface of the retina. The rods far outnumber the cones, 

 except in the fovea centralis, in which location cones alone are found. In the macula 

 each cone is surrounded by a layer of rods ; elsewhere the cones are separated by 

 intervals occupied by three or four rods. 



The rods of the human retina (Fig. 1221) have an elongated, cylindrical form, 

 and measure approximately .060 mm. in length and .002 mm. in diameter. Each rod 



Pigmented cells from outer layer of 

 retina ; surface view. X 250. 



