68 THE VERTEBRATE RETINA 



each fiber represents a single, unsummated cone. Obviously, summation 

 is very great even in the human retina — and the human eye is built, 

 better than most, for 'sharp' vision! 



Another important cause of the haziness of rod vision is the dilatation 

 of the pupil. To have only the rods in action, the illumination must be 

 dim — below the threshold of stimulation of the relatively insensitive 

 cones. The pupil opens to let in more light, which permits the rods to 

 continue in action but, incidentally, has two unfortunate effects: the 

 'depth of focus' of the eye is reduced, and the periphery of the lens 

 comes into play with its detrimental effect upon the quality of the 

 optical image. There is nothing the retina can do about it, and twi- 

 light vision here suffers another loss in resolution for which the in- 

 dividual rods should not be blamed. In animals whose eyes are built for 

 moonlight, this factor may be negligible or absent since the lens is then 

 large, and the whole area of its surface exposed by the widened pupil 

 is probably optically 'good'; but the retinal summation factor is still pres- 

 ent in such animals, and indeed in far greater degree than in ourselves. 



Retinal Factors in Sensitivity — The differences between rod- and 

 cone-vision with regard to sensitivity are, like the acuity-differences, 

 caused by three factors. They are not unrelated to the acuity-differences, 

 and in the case of sensitivity two of the factors reside in the visual cells 

 themselves and only one is extrinsic. The sensitivity-promoting factors in 

 the rod mechanism are: (a) the size of the outer segment; (b) the 

 extent of summation; and (c) rhodopsin. 



The business end of a rod or cone is its outer segment. It is in this 

 part of the cell, nearest the pigment epithelium and thus farthest from 

 the source of light, that the light effects chemical changes which initiate 

 the impulse that travels down the length of the cell and, if it is strong 

 enough, evokes a nerve-impulse in the associated bipolar. By and large, 

 rod outer segments tend to be long cylinders whereas cone outer seg- 

 ments are shorter (Figs. 22-26) ; and while these may be as thick through 

 at their bases as rod outer segments, they taper more or less and may 

 even be quite pointed at their tips. Hence the names originally applied 

 to the two types of cells, though the human cone outer segment is now 

 known not to be at all conical when properlv preserved. 



If a geometrical cone and a cylinder have the same area of base and 

 the same height, the cone then has only one-third of the volume of the 

 cylinder. Here is an important intrinsic reason why, other things being 



