648 



THE EYE 



been bleached by exposure to light. Possibly these changes possess a 

 stimulant action upon the neuro-epithelial elements. 



2. The rod and cone layer (bacillary layer) consists of a series 

 of columnar elements which are disposed in a palisade-like manner, and 

 whose narrow extremities are embedded in the surface of the layer of pig- 

 ment epithelium. The rod and cone layer contains elements of two dis- 

 tinct types, the rods and the cones, specialized receptors of the rod and 

 cone visual cells,, and very similar to each other in their structure. Each 



FIG. 548. ISOLATED ROD AND CONE VISUAL CELLS OF THE PIG. 



o-e, cones; f-i, rods; za, outer segment; zi, inner segment of the cone, the latter 

 consisting of an ellipsoid, ze, and a more or less elongated neck, ra; zk, cone nucleus; 

 zi, cone fiber; sa, outer, and si inner segment of the rod; sk, rod nucleus; sf, rod 

 fiber. (After Kolliker.) 



rod and each cone visual cell consists of two distinct portions, the outer 

 of which, alone, lies in the bacillary layer; the inner portion is included 

 in the outer nuclear layer of the retina. The outer portion is cytoplas- 

 mic, and its broad base rests upon the external limiting membrane; the 

 inner portion is narrow, nucleated near its center, and extends entirely 

 through the outer nuclear layer. 



THE EODS. The outer, cytoplasmic, or bacillary portion of each 

 rod visual cell consists of a somewhat thickened spheroidal base, the inner 

 segment, and an outer filamentous extremity, the outer segment. These 

 two segments are quite as distinct in fresh unstained tissue as in fixed 



