164 EXPERIMENTAL PHYSIOLOGY. 



corresponding to them are nearer the centre of the eyeball and each cell sends a 

 connecting fibre inward into the outer nuclear layer of the retina, where it is con- 

 nected with the dendrites of one of the first relay cells, the bipolar cells. The rods 

 have slightly thickened inner, and straight slender outer, limbs. They are con- 

 nected with their nuclei by fine thread-like processes. The cones, which are 

 shorter than the rods, are also divided into inner and outer limbs. Their outer 

 limbs are shorter and sharper than those of the rods; the inner ones are much 

 thicker and connected each by a substantial process with its nucleus, which lies 

 nearer to it than do those of the rods. The dendrites of the bipolar cells, which are 

 the next units on the path of the impulse, are connected by short thick processes 

 with their cell bodies, the nuclei of which make up the inner nuclear layer. 

 The axones of these cells are short. They end in the inner molecular layer, 

 where they establish connection with the dendrites of the second relay cells, the 

 giant cells. It is the axones of the giant cells which, passing over the surface, 

 make the inner layer of the retina and join together at the optic disc to go out as 

 the optic nerve. In all parts of the retina except the fovea centralis the bipolar 

 cells connect each with several fibres from rod-cells or ccne-cells, and are in turn 



FIG. 46. Diagram of the structures in the retina. 



several of them connected with one giant cell, so that the number of nerve elements 

 represented in the optic nerve is a good deal less than the number of cells in the 

 retina, and one fibre in the nerve may carry impulses which arise from any one or 

 from all of a number of rods or of cones. The histology of the fovea is different 

 in this as in some other respects from that which we have described. It consists 

 only of the light-sensitive cells and the fibres which connect them with the first 

 relay cells. These, together with the giant cells to which they lead, are swept 

 aside from the fovea itself and, piled up around its margin, cause the slight 

 thickening of the retina which surrounds it. The sensitive cells in the fovea 

 are all cones, no rods are to be found, and they are each of them connected through 

 a different bipolar cell to a single giant cell and so to a separate fibre of the optic 

 nerve. The arrangement affords some histological basis for the acuteness of 

 vision which we have in the fovea, since each cone has a separate path to the brain. 

 It is found that two points in the external world may be distinguished as separate, 

 provided that the angle by which they are separated at the eye is 5 minutes or 



