7cS4 PHYSIOLOGY. 



The rod-fiber is a continuation at its inner end of the rod. The 

 fiber contains the rod-nucleus. 



Cones. Both rods and cones are closely set like a palisade over 

 the whole extent of the retina, between the external limiting mem- 

 brane and the pigmentary layer, except at the macula lutea, where 

 there are only cones. The smallest angular distance at which points 

 can be separately distinguished is 50 seconds, with which the size 

 of a retinal image is 3.65 micromillimeters. This size coincides 

 closely with the diameter of the cones at the fovea, which are about 

 3 micromillimeters. 



Fig. 335. Hexagonal Cells from the Pigment Layer of the Retina of a 

 Rabbit. ( BALL. ) 



The cones, like the rods, consist of two segments, an inner and 

 an outer. The cones are shorter than the rods. The outer seg- 

 ment of the cone has cross striations. The inner segment is much 

 thicker and shorter than the rod, and is rounded. The ellipsoid of 

 the cone is larger than that of the rod, and lies in the peripheral 

 part of the inner segment. A cone-fiber is a continuation of the 

 cone. Between each cone there are usually two or three rods, show- 

 ing the greater abundance of rods. The external limiting mem- 

 brane is a product of the Miiller fibers, the sustentacular tissue of 

 the retina. In the vicinity of the ora serrata the nerve-fiber and 



