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THE LAYEES OF THE KETINA 581 



instant of death, the pigment granules accumulate in the irregular 

 processes of the cells which surround the rods and cones, the outer 

 or basal portion of the cell being relatively free from pigment. In 

 an eye which is shaded from the light, or in one removed in com- 

 parative darkness, the pigment has apparently retracted until it 

 lies entirely within the body of the cell. Even under these condi- 

 tions the extreme base of the cell frequently presents a narrow zone 

 which is relatively free from pigment. Similar changes in the dis- 

 position of the pigment undoubtedly occur in the living eye under 

 the influence of exposure to varying degrees of light. 



The function of this pigment and of the peculiar changes in its 

 disposition is still somewhat speculative, but it may, without doubt, 

 be safely asserted that these phenomena are concerned with the 

 renewal of the visual purple of the rods and cones after the same 

 has 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 pigment epithelium. The rod and cone layer contains 

 elements of two distinct types, the rods and the cones, which are 

 nevertheless very similar to each other in their structure. Each 

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

 which, alone, lies in the bacillary layer ; the inner segment is 

 included in the outer nuclear layer of the retina. The outer seg- 

 ment is cytoplasmic, 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 Rods. The outer, cytoplasmic, or bacillary portion of each 

 rod consists of a somewhat thickened base and an outer filamentous 

 extremity. These two portions are quite as distinct in fresh un- 

 stained tissue as in fixed and stained preparations, the distinction 

 being due to the fact that the inner segment of each rod, while 

 finely granular and easily stained, is also singly refractive ; the outer 

 segment, on the other hand, not only stains with difficulty but is 

 doubly refractive. The outer, therefore, under all conditions ap- 

 pears bright and lustrous as compared with the isotropic inner 

 portion. 



The outer segment contains the vistial purple or rhodopsin 

 which, during life, is rapidly bleached by exposure to light, and is 

 as rapidly renewed through the agency of the pigment epithelium. 



