VISUAL TISSUES 



257 



vts. 



Some explanation of the real relations existing between these different 

 layers is highly necessary. Figure 227 is a diagram which explains in 

 part what has already been discovered. As in other retinas the rods and 

 cones are the visual 

 cell-organs. The 

 visual cells them- 

 selves form a thick 

 layer below the 

 external limiting 

 membrane, and 

 their nuclei form 

 the outer nuclear 

 layer. The bodies 

 of the cone cells 

 are thicker than 

 those of the rod 

 cells, and their 

 bases are more 

 greatly expanded. 

 The line of these 

 bases is the mem- 

 brane-like line be- 



9-c. 



w./. 



sup. c. 



FIG. 227. Diagram of some of the known elements of the retina 

 in man. Compare with Fig. 225. vis.c., layer of visual cells 

 whose perceptory organs form the rod and cone layer and whose 

 nuclei and processes form the outer nuclear layer and Henle's 

 layer; nv.c., layer of bipolar and amakrine nerve cells whose 

 nuclei form the inner nuclear in ordinary preparations; g.c., 

 ganglion cells of the ganglion cell layer; nv.f., nerve fiber layer; 

 sup.c., supporting or neuroglia cells. (From " STOHR'S Text-book 

 of Histology " by LEWIS.) 



tween the inner 

 reticular layer and 

 Henle's layer of 

 fibers. 



The other cellular layers represent a number of different kinds of 

 nerve cells whose processes make various intricate connections. The 

 use of these connections is obscure and not really known. All that can 

 be said is that the various cells act in some way together and correlate the 

 various impulses in a way that prepares them to represent the image to 

 the brain. Some of these nerve-cell relations are shown in the diagram 

 represented by Figure 227. 



There are sustentacular elements in the retina of man which extend 

 from the fiber layer to the rods and cones, and serve to hold all together. 

 These are known as the radial fibers, and two of them are pictured in the 

 diagram. They are neuroglia elements. 



The connective tissue, muscle, and other histological details of the 

 outer parts of the eyeball as well as of the iris and sclera cannot be 

 taken up here. Many blood vessels penetrate all parts of the eye, in- 

 cluding the retina, to supply its various needs. 



Technic. Eye preparation presents the greatest possible variety 

 of technic, owing to the very great variety in the different kinds of eyes. 



