THE SENSES. 



703 



nally. This is, however, practically a layer made up simply of bipolar 

 nerve-cells with prolongations more or less long which run to the ex- 

 ternal surface of the retina and there form a series of hodies known as 

 the rods and cones. 



1. TJie rods 'and cones are really a kind of secretion from the pro- 

 toplasm of the bipolar cell beneath, and are not distinct nerve-cells. 

 They consist of bodies more or less alike, which extend up through the 

 external limiting membrane from the cells beneath. 



Fig. 417. Transverse section of a mammalian retina. A, Layer of rods and cones; B, bodies 

 of visual cells (external granular); C, external molecular layer; E, layer of bipolar cells (internal 

 granular); F, internal molecular layer; $, layer of ganglionic cells; //, layer of optic-nerve fibres; 

 a, rod; 6, cone; c, body of the cone cell; d, body of the rod cell; e, bipolar rod cells; /, bipolar cone 

 cells; (7, /i, i,j, fc, ganglionic cells ramifying in the various strata of the internal molecular zone; 

 r, inferior arborization of the bipolar rod cells, connecting with the ganglionic cells; r^ inferior ar- 

 borization of the bipolar cone cells; t, epithelial or Miiller cells; x, point of contact between the 

 rods and their bipolar cells; z, point of contact between the cones and their bipolar cells; s, centri- 

 fugal nerve-fibre. (Cajal.) 



The Mods. Each rod (fig. 417, a) is made up of two parts, very differ- 

 ent in structure, called the outer and inner limbs. The outer limb of the 

 rods is about 30/* ^5- inch long and 2/* broad, is transparent, and doubly 

 refractive. It is said to be made up of fine superimposed discs. It re- 

 sembles in some ways the myelin sheath of a medullated nerve. It 

 swells up on exposure to light, and is part of the layer in which the 

 pigment called visual purple is found. The inner limb is about as 

 long but slightly broader than the outer, is longitudinally striated at 

 its outer and granular at its inner part. Each rod is connected by 

 a fine hair-like process to a nerve-cell in the external granular layer be- 

 low (figs. 417, d\ 417A, 2). 



The Cones. T&ach cone (fig. 417, c), like the rods, is made up of two 

 limbs, outer and inner. The outer limb is tapering and not cylindrical 

 like the corresponding part of the rod, and about one-third only of its 



