THE SENSE OF VISION. 



775 



Rods. 



Cones. 



d, are due to shadows of the large retinal vessels. During this determination 

 it is of course necessary that the head should occupy a fixed position with 

 regard to the paper. This condition can be secured by holding firmly between 

 the teeth a piece of wood that is clamped in a suitable position to the edge of 

 the table. The diameter of the blind spot, as thus determined, has been found 

 to correspond to a visual angle varying from 3 39' to 9 47', the average 

 measurement being 6 10'. This is about the angle that is subtended by the 

 human face seen at a distance of two meters. Although a considerable por- 

 tion of the retina is thus insensible to light, we are, in the ordinary use of the 

 eyes, conscious of no corresponding blank in the field of vision. By what 

 psychical operation we " fill up " the gap in our subjective field of vision 

 caused by the blind spot of the retina is a question that has been much dis- 

 cussed without being definitely settled. 



The above-mentioned reasons for regarding the rods and cones as the light- 

 perceiving elements of the retina seem sufficiently conclusive. Whether there 

 is any difference between the rods and the cones with regard to their light- 

 perceiving function is a question which may be best considered in connection 

 with a description of the qualitative modifications of light. 



The histblogical relation between the various layers of the retina is still 

 under discussion. According to recent observations of Cajal, 1 the connection 

 between the rods and cones on the one 

 side and the fibres of the optic nerve 

 on the other is established in a man- 

 ner which is represented diagram- 

 matically in Figure 235. The pro- 

 longations of the bipolar cells of the 

 internal nuclear layer E break up into 

 fine fibres in the external molecular 

 (or plexiform) layer C. Here they are 

 brought into contact, though not into 

 anatomical continuity, with the termi- 

 nal fibres of the rods and cones. The 

 inner prolongations of the same bipolar 

 cells penetrate into the internal molec- 

 ular (or plexiform) layer F, and there 

 come into contact with the dendrites 

 coming from the layer of ganglion-cells 

 G. These cells are, in their turn, con- 

 nected by their axis-cylinder processes 



it, j.i uu e J.L i- TII FIG. 235. Diagrammatic representation of the 



With the fibres Ol the OptlC nerve. Ihe structure of the ret ina (Cajal): A, layer of rods 



bipolar Cells which serve as connective and cones ;.B, external nuclear layer ; C, external 



.... , , , , . molecular (or plexiform) layer; E, internal nu- 



ImkS between the rods and the OptlC clear ] ayer . Fj internal molecular (or plexiform) 



nerve-fibres are anatomically distin- la y er = > l& ? er of ganglion-ceils ; H, layer of 



iii/ T IT \ nerve-fibres. 



guishable (as indicated in the diagram) 



1 Die Retina der Wirbelthiere, Wiesbaden, 1894. 



