THE INTERNAL COAT THE NERVOUS TUNIC 



653 



membrane. Both rods and cones are hexagonal or. nearly circular in 

 transection. The slight intervals between the neighboring elements and 

 the processes of the pigment epithelium are occupied by a homogeneous 

 fluid, probably a somewhat modified lymph. The rods far outnumber 



FIG. 554. FROM A MERIDIONAL SECTION OP A CHILD'S EYE, SHOWING THE LAYERS 

 OF THE RETINA AT A POINT MIDWAY BETWEEN THE MACULA LUTE A AND THE 

 ORA SERRATA. 



1, pigment layer; 2, bacillary layer; 8, external limiting membrane (indistinct); 4, 

 outer nuclear layer; 6, outer reticular layer; 7, inner nuclear layer; 8, inner reticular 

 layer; 9, ganglion cell layer; 10, nerve fiber layer; 11, internal limiting membrane; 

 a, choroid coat. Hematein and eosin. Photo. X 400. 



the cones; three to four rods generally appear between two successive 

 cones. The total number of cones in the human retina has been esti- 

 mated at 70,000,000 ; that of rods at 130,000,000. 



3. The external limiting membrane (membrana Umitans externa) 

 consists of the flattened and amalgamated extremities of the sustentacular 

 cells (Muller's fibers), which form the chief supporting tissue, the neu- 



